Growth periodicity in Pinus sylvestris L., with special reference to the effect of day-length

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Ü-HUWi'H PüiKlODlUlTY Ih PINUb üYiN&üThTü h . , WITh SPiCGIAL KJliiMjrüiilMÜJi; Tu Thüi üJT’ü'üiCT Üj?‘ JJÂY-iüiiNLfTh.

T hesis su b m itted fo r th e degree of Doctor of Philosophy in th e U n iv e rs ity o f nondon by P.F.WAhililNÜ, B . S c . hay, 1950.

ProQuest Number: 10098954

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OUWTEwTB. Page. PAPÏ I - liMÏKÜJJÜUl'IÜli.

1

PAuT 11 - hÜi4,Ax, UÏUx,E UP urtUViTln lb PlbUB ùYxi/EÜTBIB.

5

1.

te rm in a tio n of Ueeci.

5

2.

p i r s t - y e a r seeax ing s.

5

3.

tro w th ol' snoou in secona ana l a t e r years.

8

4.

tro w th of buas.

y

0.

Boot-growth.

11

6.

Ganibia,i a .c tiv ity .

15

7.

Summary of annual growth in P . s y l v e s t i s . 16

PAKÏ I l l - PhOTUPEnlujjlSlv Ib PlbUS SYx,7ESTxiIS. A. I n tr o a u c tio n .

18. 18.

1 . t e n e r a i a s p e c ts o f photoperiodism in woody s p e c ie s .

18.

2 . Photoperioaism in th e genus p in u s .

22.

3 . scope of p re s e n t work.

25.

B. B xperim entai.

26.

1. photo perio aism in i i r s t y ear seeaxings of P. s v x v e s t,ris . 26. 2. E f f e c t of d a y -le n g th on shoot growth a f t e r the f i r s t y e a r.

83.

3. E f f e c t o f d ay -le n g th on th e d u ra tio n of cambial a c t i v i t y .

121.

4. E f f e c t of d a y -le n g th and tem p eratu re on th e b reaking of aorm ancy.'

142.

page 5. PMT I f .

The mechanisni of photoperioaisiri in Pinus s v i v e s t r i s .

145

PACTÜP3 GÜiTiWuiulAür TM Ai\ihlJAn CYOïH OF OkuWTH J im P. üYufAüïulO. 157

BIBuIOOPAPriY. APPBNBIX I . APPAWBlX I I .

i6b

PAHT 1.

IMTkÜPUQTXUM. In re c e n t y ears th e re has been a g r e a t re v iv a l of

i n t e r e s t among p la n t p h y s io lo g is ts throughout the worla in problems associated, with growth and development in p la n ts . Most o f t h i s re c e n t work has been c a r r ie d out w ith annual specie; since i t i s g e n e ra lly considered more convenient to work w ith such p la n ts .

For t h i s reason our knowledge of the physiology

of growth in woody species f a l l s sh o rt of th a t for herbaceous p la n ts .

There can be no aoubt, of course, th a t many of the

r e s u l t s obtained from the study of herbaceous p la n ts hola also fo r woody sp ecie s.

For example, evidence w ill be brought

forward below to support the view t h a t the miechanism of photoperiodism i s the same fo r both woody and herbaceous sp ecies S im ila rly , i t has been shovm (Uzaja, 1934) th a t auxin i s p resen t in th e growing shoots of c e r ta in woody sp ecies, j u s t as in herbaceous p la n ts . N evertheless, th e re are many fe a tu re s of growth p e c u lia r to woody species and not found in ty p ic a l herbaceous p la n ts . For example, growth of the shoot in herbs i s normally term inated by flower formation, whereas in most woody species th e annual cycle of growth in a high proportion of the growing p o in ts i s u ltim a te ly term inated not by a flower bud, but by a v eg etativ e r e s ti n g bud.

In v e s tig a tio n of fe a tu re s of growth

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1

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Great B r it a in are (1) tem perature; (2) l i g h t i n t e n s ity ; (3) le n g th of day. The probable importance of tem perature and l i g h t i n t e n s i t y in a f f e c tin g the annual cycle of growth in woody species has long been recognized, and in th e p ast th e se, to g e th e r w ith r a i n f a l l , have g e n e ra lly been considered to be th e o v e r-rid in g f a c to r s .

The discovery, however, th a t such

processes as th e d u ratio n of extension growth and time of le a f f a l l in woody sp ecies may be a f f e c te d by le n g th of day under experimental con d itions (Gamer ana A llard, 1923) r a is e s the question as to how f a r t h i s fa c to r i s im portant in a f f e c tin g the annual rhythm of growth in n a tu re .

Very l i t t l e d e f in ite

inform ation on t h i s s u b je c t i s a v a ila b le a t p re se n t, and i t was th e re fo re decided to carry out an in te n s iv e study o f the annual cycle of sev eral growth processes in a sin g le species, w ith s p e c ia l referen ce to the r o le o f photoperiodic e f f e c ts in the annual cycle of grov/th.

For t h i s purpose the sp ecies,

Pinus s v l v e s t r i s n , appeared to have sev eral advantages, v i z .; (1)

Various o th er species of the genus Pinus were already known to show marked photoperiodic e f f e c ts (Bogdanov, 1931, Kramer, 1936, J e s t e r à Kramer, 1939), and th e re were grounds fo r expecting th a t P. s v lv e s tr i s vrould also prove to be 'p h o to p e rio d ic ' (Sylven, 1942).

(2) I t i s

a species which i s indigenous to Great B r ita in ( a t

l e a s t in th e North, and i s n a tu r a lis e d in southern England). - 3 -

Hence observations w ith re s p e c t to th e annual cycle of growth under n a tu ra l c lim a tic co n dition s could be regarded as 'norm al' for t h i s sp ecies. An in te n s iv e study of the annual cycle of growth, in the l i g h t of more re c e n t advances in the physiology of development, does not appear to have been made prev io usly for a wooay species, although i t has been aone fo r various herbaceous p la n ts , inclu d in g the onion ( e .g . Heath, 194b) and c e r ta in c e re a ls ( e .g . Purvis and Oregory, 1937). Before the experimental work i s described, a b r i e f account of th e normal cycle of growth in Pinus s v l v e s t r i s w ill be given, since a knowledge of t h i s is e s s e n tia l before an attem pt i s made to discover the fa c to rs which determine i t .

PAHT II.

THE

mmi&h ÜYGlE ÜF QPüVl'H IN PINUS

SYlYESTRIS.

Although pinus s v l v e s t r i s i s commonly used as a 'ty p e ' sp ecie s fo r th e te ach in g of elem entary botany, th e re i s very l i t t l e a c c u ra te ana ü e t a ile ü inform ation as to i t s various phases o f growth, e i t h e r in textbooks or elsew here.

The follow ing

account i s based upon th e perso n al o b serv atio n s of th e w r ite r over a p e rio d of th re e y ears on experim ental p ls n ts grown in p o ts and on t r e e s of a l l ages growirg under n a t u r a lis e d co n d itio n s a t Oxshott Heath, Surrey, which was v i s i t e d on numerous occasions throughout t h i s p erio d ,

ifhere th e

o b serv atio n s of o th e r a u t h o r i t i e s are drawn upon, t h i s i s duly in d ic a te d . 1.

G-erm.ination o f the seed. The r ip e seed i s normally shed from mature cones during

dry, fin e weather in e a rly sp rin g , u s u a lly in karch (Groom, 19U7). In f o r e s tr y p r a c t ic e th e cones are normally c o lie c te a in w in ter, and s to re d in a warm, dry p la ce, which brings about opening of th e cone s c a le s and r e le a s e s the seed,

such seed i s normally

SOTO th e same sp rin g , w ithout any p r i o r ' s t r a t i f i c a t i o n ' and normally about 90% o f th e seed germ inates. Emergence o f th e s e e d lin g occurs in l a t e A p ril, or e a rly liay, and. i s ' e p i g e a l ', th e t e s t a being c a r r ie d up on th e t i p s of the cotyledons. 2.

F i r s t - v e a r s e e d l i ngs.

Ihe emergence ol th e cotyledons, of which th e re are -

5

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4-9,

is followed by th e p roduction of li n e a r ' ju v e n ile '

le a v e s, of th e type well Known in c o n ife rs (Chamberlain, 193b). The i n i t i a l p e rio d of le a f-fo rm a tio n i s not a,t f i r s t accompanied by any elo n g atio n of tlie in tern o d es, so th a t a " r o s e tte " stage i s formed.

This l a s t s fo r about s ix weeks, a f t e r which

extension of the in tern o d es commences between the leaves a t th e ba.se o f th e r o s e t t e .

From then onwards, th e re i s a

continuous ex ten sio n o f successive in tern o d es between the b a sa i leaves of th e r o s e t t e , and t h i s process keeps pace w ith th e continued production of new leaves a t th e c e n tre , so t h a t a r o s e t t e of leaves continues to e x i s t in th e a p ic a l region throughout growth.

This p a tte r n of behaviour, in which a

r o s e t t e stag e precedes th e exten sion of the shoot has also been observed by th e w r ite r in seed lin g s o f o th e r c o n ife rs , e.g . Picea s it c h e n s is, narix spp. , Oearus a.tiantica» etc. G-rowth of th e shoot, w ith continuous le a f-fo rm a tio n and in tern o d e extension, continues u n t i l l a t e August or e a rly September under favourable c o n d itio n s, and then ceases w ith th e formation of a term in al r e s tin g -b u d a t th e cen tre o f the a p ic a l r o s e t t e of le a v e s.

The a p ic a l r o s e t t e remains p re se n t

throughout the p erio d of dormancy.

The appearance of normal

f i r s t - y e a r seed lin g s i s shown in Figure 1.

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'V

E’i g . i .

Mature f i r s t - y e a r s e e d lin g s .

'

In th e development o f tn e ro o t-sy stem o f f i r s t - y e a r , • se e d lin g s, a long r a d i c l e i s formed which soon g iv e s r i s e to secondary r o o ts .

These secondary r o o ts bear fin e t e r t i a r y

ro o ts which p o t e n t i a l l y are capable o f g iv in g r i s e to m ycorrhizal ' s h o r t - r o o t s ' i f in f e c tio n o c c rs.

Under favourable

c o n d itio n s, m ycorrhizal ro o ts f i r s t appear in J u ly or e a rly August and continue to in c re a se in number throughout the autumn.' Growth of primary and secondary ro o ts a lso continues u n t i l October or November.

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3.

GrrQgth_oi . .the .shoot .in the secona and l a t e r vaarR. A fter the formation of a term inai r e s tin g bua a t the end

of the f i r s t seaso n 's growth the p la n ts remain dormant u n t i l th e follow ing spring.

Towards the end of Icarch or e arly April

elongation of th e r e s ti n g buds may be d etected in young tr e e s (o r some weeks l a t e r in older t r e e s ) .

In second-year seedlings,

however, the i n i t i a l stages of extension of the new shoots c o n s is t of elongation of the internodes betweeen the ju v e n ile leaves forming th e term in al r o s e tte of the f i r s t y e a r 's growth. K• Thus we have the unusual occurrence of a lapse of s ix months or : more between the formation of the leaves and the extension of ^ the a s s o c ia te d in tern o d es.

f u r th e r growth of the shoot in

second year seedlings c o n s is ts of the extension o f the internodes between the scale leaves o f the restin g -b u d , the whole process occupying a period o f 6 - B weeks.

When t h i s period of

extension has been completed th e re i s no fu rth e r growth by the continued a c t i v i t y of the ap ical meristem, as in oth er species e.g . Robinia pseudacacia (Büsgen, 1929).

Extension growth of

the annual shoot in Pinus s v lv e s tr i s th e re fo re c o n s is ts so lely of expansion of i n i t i a l s already l a i d down in the bud, (Wight, 1933) and is completed by the end of hay or e a rly June.

Thus,

a f t e r the f i r s t year of growth the number of nodes formed by the annual shoot i s predetermined by the number o f sc a le -le a v e s formed in the term inal r e s tin g bud in the previous year. "

8

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B y.the time t h a t extension ol th e shoot has been completed, tne g reen le av es (borne in p a i r s on dwarf shoots in th e a x i l s of th e s c a le -le a v e s ) have a t t a i n e d a le n g th of approxim ately only 1 cm.

These leav es grow by means o f a b asal

meristem, which remains a c tiv e long a f t e r extension o f th e shoot has been completed.

Toisky (1913) showed t h a t in S.Russia, the

leav es o f P. s v l v e s t r i s continue growth in le n g th u n t i l August, and th e w r ite r has found t h a t in Southern England growth continues u n t i l about th e l a s t week in August (se e page 91 below). 4.

G-ro'wth o f buds. Inform ation on th e growth of the r e s t i n g buds i s very

s l i g h t and th e follow ing o b serv atio n s of th e w r ite r are only of t

a p relim in ary n a tu re .

According to l i g h t (i9 3 3 ), by November

th e r e i s v i s i b l e w ith in th e obvious r e s t i n g bud o f P. s v l v e s t r i s a minute rudim entary r e s t i n g bud o f th e next season.

While

c e r ta in evidence d escrib ed below tends to confirm t h i s o b serv atio n , i f p u rely v e g e ta tiv e buds o f young t r e e s are d is s e c te d in w in ter, a continuous s e r ie s of s e a le -le a v e s may be found, r i g h t to th e minute rudim entary s c a le s of the apex i t s e l f . Because of t h i s c o n tin u ity in th e s e r ie s i t becomes im possible to dra.w any v i s i b l e d i s t i n c t i o n between th e s c a le s which w ill form th e term in al nodes of the next s e a s o n 's shoot and those which w ill form th e lowermost s c a le s o f next se a so n 's r e s ti n g buds.

IXiring the elo n g atio n o f th e young shoot in th e sprin g,

however, the l i m i t s o f the new r e s t i n g bud soon become - 9 -

re c o g n iz a b le , fo r th e p a ire d le av es of th e dw arf-shoots grow out of th e a x i l s o f tn e s c a le - le a v e s a t an e a r ly sta g e except a t th e extreme apex;

i t i s l a t e r seen t h a t th e in te rn o d e s of

t h i s l a t t e r reg io n do not extend and t h a t th e a p ic a l s c a le s are th e lowermost o f th o se o f th e new buds. Hegular in s p e c tio n o f t h i s new te rm in al bud from June onwards shows a slow in c re a s e in s iz e f o r s e v e ra l months.

The

g rad u al in c re a s e in th e s iz e of th e buds i s shown in Figure 2, which g iv e s th e mean le n g th o f th e te rm in a l buds of second y e a r se e d lin g s grown in th e open ground, th e d a ta being o b ta in ed by ta k in g samples from about 30 p la n ts on th e su c c e ssiv e d a te s .

Û4

15/9 23/9

Fig. 2.

Grrowth in le n g th o f te rm in a l buds.

D is se c tio n of th e buds a t su ccessiv e i n t e r v a l s shows t h a t th e growth c o n s is ts not only of an in c re a s e in s iz e of th e dw arf-shoot rudim ents in th e a x i l s of th e s c a le s , b u t a lso o f th e continued form ation and growth of new s c a le s and dw arf-shoot in itia ls .

E lo ng ation of th e buds con tin ues even a f t e r l e a f -

growth has ceased, bu t by th e end of September appears

p r a c t i c a l l y to have ceased.

I f th e te rm in a l buds o f female-cone-

b e a rin g branches are d is s e c te d , i t i s f o u n ^ h a t th e rudim entary female cones o f th e follow ing sp rin g ere j u s t a i s c e r n i b l e e a rly in September,

how i t i s w ell Known t h a t th e female cones are

borne a t th e extreme apex of tn e annual shoot, so t h a t in buds of t h i s ty pe, th e re can be no f u r t h e r foirniation o f new i n i t i a l s once th e female cone i n i t i a l s have been formed a t th e beginning o f September. 5.

Hoot-growth. No systema.tic study of ex ten sio n growth in th e ro o ts of

Pinus s v l v e s t r i s i s known to th e a u th o r, but acco rdin g to Wight (1933) "th e r o o ts of P. s v l v e s t r i s show two p erio d s of growth in le n g th .

The f i r s t i s e a r ly in th e season (hay-June) and th e

l a t t e r a t th e end o f August and th e beginning of September.

In

th e former p e rio d th e p ro d u ctio n of fib ro u s ro o ts seems to predom inate, though th e r e i s also some growth in le n g th o f the s to u t ro o t le a d e r s .

These grow in le n g th most v ig o ro u sly during

th e l a t e r p e rio d o f grow th."

These conclusio n s were a p p aren tly

based upon o b se rv a tio n s o f a " q u a lit a ti v e " n a tu re made upon th e ro o ts o f t r e e s growing under n a tu r a l c o n d itio n s .

Under such

co n d itio n s, however, th e g e n e ra l p i c t u r e i s com plicated by v a r ia tio n s in s o i l —w ater co n te n t.

O bservations o f a p r e 1imi nary

n a tu re on ro o t-g ro w th were c a r r ie d out by th e w r i t e r on p la n ts growing under b o th c u ltu r e ana f i e l d c o n d itio n s . -

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As Brown (1915)

has p o in te d o ut, in c o n if e r s , w ith in a space o f 1 cm. o f th e ro o t t i p a brown c o lo u ra tio n appears, "so t h a t new growth can be d e te c te d w ithout any a i f f i c u i t y .

Furthermore, a f t e r

c e s s a tio n of growth in th e autumn, t h i s brown m antle approaches n e a re r th e r o o t - t i p , so n ear, in f a c t , t h a t one can be reaso n ab ly sure as to th e presence o f new growth". O bservations on ro o t-g ro w th were c a r r i e d out in 1948 by t r a n s p l a n t i n g th i r d - y e a r se e d lin g s from po ts to ' root-o b serv atio n b o x e s ', provided w ith s lo p in g g l a s s - s i d e s .

These boxes were

12 inches in le n g th ana aepth ana 10 inches in w iath a t th e top. The g la s s was normally Kept covered by means o f m etal sh e e ts which could be f i t t e d on the o u ts ia e end removed as n ecessary f o r o b se rv a tio n .

The p la n ts were t r a n s f e r r e d to th e boxes in

A p ril, and a t t h i s d ate i t could be seen t h a t a small amount o f new growth had been made by a few r o o ts .

The " b a ll" o f r o o ts ,

as removed from th e p o t, was kep t i n t a c t and placed in th e s o i l in th e boxes so t h a t i t r e s t e d a g a in s t th e g la s s on one s id e . Since the o ld b a l l of r o o ts was s t i l l c l e a r l y a i s t i n g u i s h a t l e , any new growth in to th e surrounüing s o i l coula also be c l e a r ly seen. (F ig u re 3).

Four 'p o t s ' of p la n ts were t r a n s f e r r e d to

each box, o f which th e r e were s ix i n a l l .

O bservations were

made a t freq u en t i n t e r v a l s throughout th e growing season u n t i l th e end o f October.

Under th e se c o n d itio n s i t was found t h a t

very l i t t l e ro o t-g ro w th o ccu rred d uring A pril and th e f i r s t two weeks of l a y , when shoot growth was ta k in g p la c e . -

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A few new

r o o ts were formed, by some of th e groups o f p la n ts , b u t in ma.riy cases new ro o t-g ro w th was e n t i r e l y a b s e n t.

I h r in g th e second

h a l f o f May, however, as shoot growth was te rm in a tin g , ro o t a c t i v i t y became very g e n e ra l in a l l groups o f p l a n t s .

This

a c t i v i t y involved the form ation o f long secondary r o o ts from which f i n e r branches b e a rin g m ycorrhizal s h o o t-ro o ts a ro se , and was m aintained, w ithout any observable s la c k e n in g ^ u n til w ell in to October.

F i g .3.

R o o t-o b serv atio n box.

An e n t i r e l y comparable p ic tu r e was o b ta in ed by in s p e c tin g th e r o o t a c t i v i t y o f pot-grown p la n ts throughout th e same period. An attem pt was made in 194b to o b ta in (^w.antitative d a ta ~

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on th e normal growth of th e r o o ts by digging up a t 2 -weekly i n t e r v a l s second-year se e d lin g s growing in th e open ground in th e n u r s e r ie s o f th e F o re s try Commission a t Wareham, D orset. U n fo rtu n a te ly th e p e n e tr a tio n of th e r o o ts in to a hard 'p a n ' from which they could not be removed i n t a c t made i t im possible to c a rry out th e o r i g i n a l i n t e n t i o n o f o b ta in in g q u a n t i t a t i v e d a ta on th e amount of root-grov/th, b u t s u f f i c i e n t r o o ts were e x tr a c te d to confirm f u l l y th e ' q u a l i t a t i v e ' r e s u l t s o b tain ed from th e o b se rv a tio n boxes, v iz . t h a t th e r e i s lim ite d ro o t a c t i v i t y u n t i l e x ten sio n of th e shoot has ceaeed, a f t e r which ro o t-g ro w th occurs a c t iv e l y u n t i l th e autumn. F in a lly , o b se rv a tio n s were c a r r i e d out a t r e g u la r i n t e r v a l s on th e r o o ts of mature t r e e s growing a t Uxshott, Surrey, by ex cav atin g to a depth o f se v e ra l in ch es, g.nd tr a c in g th e r o o ts to t h e i r a p ic e s, which ?/ere then examined fo r evidence o f r e c e n t new growth.

In s e l e c t i n g s i t e s fo r ex cavation o f th e

r o o ts , as f a r as p o s s ib le p o s itio n s where th e s o i l was m oist were chosen.

On 25th A p ril th e long r o o ts showed 3-4 cms. of

new growth and new s h o rt (m y co rrh izal) r o o ts were p re s e n t. Prom e a r ly fcay u n t i l mid-June, however, th e re was very l i t t l e tr a c e of ro o t a c t i v i t y .

From 29th June u n t i l th e autumn ro o t

a c t i v i t y remained continuous in s i t u a t i o n s where th e s o i l was m oist. Ihus although Wight' s o b se rv a tio n s t h a t ro o t-g ro w th -

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becomes markedly a c tiv e in l a t e kay and June i s confirmed, under co n d itio n s o f a r t i f i c i a l c u ltu r e no evidence could be o b ta in e d of any re d u c tio n in a c t i v i t y u n t i l th e ena o f th e autumn.

I t seems very p rob ab le t h a t th e ap paren t rhythmic

a c t i v i t y observed by Wight was due to v a r i a t i o n in soi l - w ater co n ten t. No evidence was found in second y ear se e d lin g s o f th e d i f f e r e n t i a t i o n observed by Wight between th e r e l a t i v e a c t i v i t y of " fib ro u s ro o ts " (by which i s presumably meant l a t e r a l ro o ts o f h ig h er o rd ers) and " ro o t le a d e r s " , a t d i f f e r e n t tim es of th e growing season. O ccasional sig n s of r o o t a c t i v i t y may sometimes be d e te c te d in pot p la n ts o f h. s v i v e s t r i s during th e v /inter, but such a c t i v i t y i s g e n e r a lly only s l i g h t ,

ko reover, in p la n ts

kept under fav ou rab le tem perature c o n d itio n s in a greenhouse throughout th e w in te r th e r o o ts as w ell as th e shoots remain com pletely dormant.

Under n a tu r a l tem p erature c o n d itio n s ,

fre q u e n t sig n s o f renewed ro o t a c t i v i t y are g e n e r a lly ev id en t in March, b e fo re any e x te r n a l sig n s of shoot growth are ap p aren t. As alread y d escrib ed , however, ro o t-g ro w th does not become f u l l y a c tiv e u n t i l th e end of M^ay or e a r ly June, when th e new shoots have appeared. 6.

Cambial a c t i v i t y . The annual cy cle of cambial a c t i v i t y in Pinus s v l v e s t r i s -

15

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was follow ed in d e t a i l by V/ight fo r b o th shoot and r o o t. Commencement of cambial a c t i v i t y in th e sp rin g , both in th e trun k and in th e buds, precedes th e breaking of th e buds.

The

most vigorous growth in th e trunk occurs from lV,.ay to e a rly J u ly .

A fte r t h i s , slow b u t continuous cambial a c t i v i t y

co n tin u es u n t i l th e beginning of October. Cambial a c t i v i t y in th e ro o ts s t a r t s some weeks l a t e r than in th e shoot, and then commences uniform ly along th e le n g th of th e r o o t.

Cambial a c t i v i t y in th e th in n e r d i s t a l p a r ts of

th e r o o ts i s over com paratively e a r ly and growth ceases f i r s t a t th e r o o t a p ic e s .

In th e proximal p a r t s of th e ro o ts

cambial a c t i v i t y con tin u es almost as long as t h a t in th e shoot. The o b se rv a tio n s of th e w r ite r (page 124) confirm th e o b se rv a tio n s of Wight w ith r e s p e c t to th e d u ra tio n o f cambial a c t i v i t y in th e shoot. 7.

Summary o f th e annual growth cycle in P. s v l v e s t r i s . The annual cy cle of growth and development ( a f t e r th e

f i r s t y e a r ) o u tlin e d above, i s summarised in Table I. I t i s seen t h a t growth of th e shoot commences w ith th e o nset of more fav o urable c o n d itio n s in th e s p rin g .

E xtension

growth of the shoot ceases in 6-8 weeks, but l e a f growth continues fo r much lo n ger by th e a c t i v i t y of a m eristem a t th e base of each l e a f .

Cambial a c t i v i t y i s m ain tained from s p rin g

u n t i l w ell in to October.

(Irowth of th e buds con tin ues from

e a r ly June u n t i l September. E xtension growth of th e r o o ts may - 16 -

commence e a r ly in the sp rin g , but th e p e rio d o f g r e a t e s t a c t i v i t y commences a t th e end of May or e a r ly June, and con tin ues w ell in to th e autumn.

Cambial a c t i v i t y in th e d i s t a l

p o rtio n s of th e ro o t continues fo r a lim ite d p e rio d only in June and J u ly . Table 1 .

Annual G-rowth Cycle in P. s v l v e s t r i s . Date of c e s s a tio n .

Phase o f growth.

Date of commencement.

Shoot. Cl) Extension growth.

A p r il/ e a r ly May.

end May/ e a r ly June

(2) deaf growth.

A p r il/ e a r ly May.

end August

(3) Cambial a c tiv ity .

A p r il/ e a r ly May.

October.

(4) Bud growth.

June.

September.

(1) E xtension growth.

March.

Oct/Nov.

(2) Cambial a c tiv ity .

June.

J u ly .

Remarks. Amount of growth predeterm ined by number of i n i t i a l s in th e bud.

Root. G re a te s t a c t i v i t y commences in May/June. A pplicable to distal r o o ts . Proximal p a r ts o f r o o ts as fo r shoot.

These o b serv atio n s agree c lo s e ly w ith th o se made by Brown (1915) and Kienholz (1934) on Pinus stro b u s and P. r e s in o s a . in N. America, except t h a t in th e se l a t t e r sp e c ie s ex ten sio n growth of th e shoot continues somewhat lo n g er ( u n t i l e a r ly August) than in P. s v l v e s t r i s . - 17 -

III.

PHüTOPjMÛDISM Ii\ PIMUa üYjjVjüüTPIü.

A. IMTPÜIUGTIÜH. 1.

ijenerai a sp ects o f photoperiodism in Woodv Hpeciea. The e x iste n c e o f a c a p a c ity fo r p h o to p e rio d ic response

in woody sp ecie s was f i r s t shown by G-arner and A lla rd fo r jnriodendron t u l i p i f e r a and Acer negundo in 1923. Since t h a t date a number o f o th e r in s ta n c e s of photoperiodism in woody sp ecie s have been re p o r te a .

This work has r e c e n tly been reviewed

by th e w r ite r (Wareing, 1949) and only th e main r e s u l t s need be summarised here.

A lto g e th er some 5U sp ecie s i n 30 a i f f e r e n t

g enera have so f a r been in v e s t ig a te d (Appendix 1) and th e se have a l l , w ithout exception, been found to e x h ib it some form of p h o to p erio d ic response, although not a l l th e v ario u s types of response are shown by every s p e c ie s .

In g e n e ra l, however,

i t may be s a id t h a t s h o rt photoperiods ( o f th e ord er o f 10 hours d u ratio n ) h asten the onset o f dormancy in se e d lin g s o f a l l sp ecie s so f a r in v e s tig a te d , w ith one exception ( Ouercus a lb a (Kramer, 1935)).

The "onset of dormancy" i s here taken to

includ e th e c e s s a tio n of ex ten sio n growth, w ith or w ithout th e form ation of a te rm in al r e s t i n g bud (acco rd in g to th e normal behaviour of th e sp ecie s in qu estio n ) and followed by l e a f f a l l in deciduous sp e c ie s. With lon g er ph o toperiods th e o n set o f dormancy i s delayed and in some sp ecie s e.g . Robi n i a ps eudacacia . -

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i.iriodendron t u l i p i f e r a , pinus ta e d a (Kramer, 1936), th e r e i s a c e r t a in c r i t i c a l le n g th of p ho to p erio d above which growth giay be m ain tain ed co n tin u o u sly fo r a t l e a s t 18 months.

In

o th e r s p e c ie s , however, e .g . P raxinus am ericana. Quercus b o r e a l i s (Kramer, 1936) and Acer pseudoplatanus ( w r i t e r , u n p u b lish e d ), th e r e appears to be no ' c r i t i c a l p h o to p e rio d ' above which growth may be m aintain ed i n d e f i n i t e l y , and in such s p e c ie s dormancy i s n o t a p p re c ia b ly aelayeü even under continuous l i g h t , as comparea w ith ' long h a y s ', alth o u g h s h o r t days b rin g h a ste n e d dormancy. In a t h i r d type of response i t i s found t h a t when s e e d lin g s are grovm under a s e r i e s o f d i f f e r e n t p h o to p erio d s, w ith in c re a s in g d a y -le n g th th e r e i s a c e r t a in optimum photo­ p e rio d fo r e x te n sio n of th e shoot, and a t d a y -le n g th s b o th below and above t h i s optimum., growth i s reduced e .g .

Pvrus

u s s u r i e n s i s . S a lix b ab v io n ica (koshkov, 1932). One of th e c h a r a c t e r i s t i c f e a tu r e s of photoperiodism in herbaceous p l a n t s i s t h a t the e f f e c t s are u s u a lly f u l l y evoked by q u ite low l i g h t i n t e n s i t i e s , so t h a t ' lon g-h ay ' e f f e c t s r e s u l t from supplem enting a. p e rio d o f n a t u r a l a a y lig h t o f, say, 10 hours by a f u r t h e r p e rio d of illu m in a tio n a t 10 fo o tcan d les.

Indeed, th e re i s good evidence t h a t th e ' s a t u r a t i o n '

i n t e n s i t y in some p la n ts i s only 1 fo o t- c a n d le or l e s s (Withrow and B enedict, 1936). -

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There i s e q u a lly good evidence t h a t l i g h t o f very low i n t e n s i t y e x e r ts profouna p h o to p e rio a ic e f f e c t s in woody sp e c ie s a ls o .

Thus h r amer (I'd 37) founa t h a t l i g h t o f only

1 fo o t-c a n d le i n t e n s i t y from s t r e e t - l i g h t s caused shoots of A.beli a a:r a n d i f l o r a to co n tin u e growing in th e autumn a f t e r s im i la r shoots under n a t u r a l d a y -le n g th s had become dormant. The q u e s tio n a r i s e s as to whether seasonal changes in le n g th o f day have any in flu e n c e on th e d u ra tio n of growth o f woody sp e c ie s under n a t u r a l c o n d itio n s .

The w r i t e r has p o in te d

out (Wareing, 1949) t h a t th r e e major c a te g o r ie s may be reco g nized among woody s p e c ie s , acco rdin g to th e d u ra tio n of th e growing p e rio d in r e l a t i o n to n a t u r a l d a y -le n g th c o n d itio n s , v i z . : (1) bpecies in which growth con tin ues throughout th e summer u n t i l September o r October.

I t has been shewn t h a t th e

d u ra tio n of growth in c e r t a i n sp ecie s of t h i s type i s u lt im a te ly c o n tr o lle d by the o n set of s h o r te r days in th e autumn e .g . hobin ia pseudaca c ia . u irio d en d ro n t u l i p i f e ra e tc . and t h i s i s no doubt t r u e f o r many o th e r sp ecie s which continue growing l a t e in th e season. (2) Species in which growth ceases in June, J u ly or e a r ly August, b e fo re th e r e has been any a p p re c ia b le f a l l in th e n a t u r a l le n g th o f day e .g . Acer p seu d o p latan u s. T i l l a euro p aea.

In such sp ecie s ex ten sio n growth occurs under

e f f e c t i v e l y co n sta n t d a y -le n g th c o n a itio n s , and ceases when -

20

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a c e r t a i n sta g e o f development hs,s been a t t a i n e a , o f which th e number of le av es formed on th e annual shoot may be u sed as an index.

I t i s e v id e n t t h a t in such sp e c ie s c e s s a tio n o f

growth cannot be c o n tr o lle d by any change in n a t u r a l d a y -le n g th c o n d itio n s . (3)

Species in which growth o f th e annual shoot i s r e s t r i c t e d

to expansion o f i n i t i a l s a lre a d y l a i d down in th e r e s t i n g bud. I t i s c l e a r tha,t in th e s e s p e c ie s , a f t e r the f i r s t y ear of growth, th e day le n g th c o n d itio n s cannot d i r e c t l y a f f e c t th e d u ra tio n o f th e ex ten sio n growth, or th e number of le av es developed.

Itany sp e c ie s of Pinus belong to t h i s categ ory ,

in c lu d in g Pinus s v l v e s t r i s ( p . 8 ) , a f t e r th e f i r s t y ear of growth,

n e v e r th e le s s , uhe d u ra tio n o f cambial a c t i v i t y in

P. s v l v e s t r i s has been shown to be a f f e c t e d by n a t u r a l dayle n g th changes in th e autumn (Wareing 1949 b ). The tim e o f l e a f - f a l l ' i n c e r t a i n sp ecie s i s a lso a f f e c te d by day le n g th c o n d itio n s in th e autumn and may be i n h i b i t e d i f th e n a t u r a l day le n g th i s a r t i f i c i a l l y len g thened ( Garner and A lla rd , 1923; LdatzAe, 1936). Thus th e e x te n t to which d a y -le n g th changes may c o n tro l th e g row th -cycle in woody sp e c ie s depends upon th e g ro w th p a t t e r n o f th e sp e c ie s in q u e stio n , but s h o rt-d a y e f f e c t s of some t y p e ^ c u r in th e autumn in sp e c ie s b elo nging to any of th e th r e e c a te g o r ie s d e sc rib e d above. -

21 -

2. Photop e r i oaism in th e genus P in u s. I’u rnin g to a c o n sid e ra tio n of what i s knovm re g a rd in g photoperiodism in th e genus p in u s , we must oraw a d i s t i n c t i o n between any e f f e c t s observea in f i r s t - y e a r seeaxings when extensio n growth i s not pre-üeterm ined, ana p o s s ib le e f f e c t s a f t e r th e f i r s t year when growth i s pre-aeterm in ea in many o f th e sp e c ie s. Bogdanov (1931) included seed lin g s of c e r ta in sp ecie s^ o f Pinus in h is experiments, in which i t was found t h a t sh o rt days hastened dormancy in a l l th e sp ecies s tu d ie d .

Bogdanov

also found t h a t races of pinus spp. from southern Kussia when grown in neningrad, tended to show two period s of ex tension growth in a s in g le season, in s te a a of the normal sin g le p e rio a . Kramer (1936) founa tnat- sh o rt aays hastenea aormancy in seed lin g s of P. ta e a a . wnereas unaer long aays ( id hour photo­ p eriod s) or continuous l i g h t i t couia be m aintained in growth throughout th e v a n te r.

(He does not sta,te whether or not growth

was continuous throughout t h i s p e rio d ,_ but h is i l l u s t r a t i o n s s u r e s t t h a t growth occurred as a succession of " f lu s h e s " ) . J e s t e r and Kramer (1939) in v e s tig a te d th e e f f e c ts of (a) s h o rt days ( 8 t hours), (b) normal days, (c) long days ( lb hours), S The w r ite r has been unable to r e f e r to th e o r ig in a l paper of Bogdanov, whose work i s known to him only through th e review of Gevorkiantz and Hoe (193b), and who do not give th e species a c tu a lly stu d ie d . -

22 -

and (d) continuous l i g h t , on a number of woody sp e c ie s, in c lu a in g P. re s in o s a . P. banksiana, P. e c h in a ta and P. c a r ib a e a . I t was found t h a t ’ s h o rt-d a y s ' brought about reduced growth in h e ig h t, whereas 'lo n g -d a y s ' caused in c re a se d h eig h t, as compared w ith 'normal d a y s '. Under continuous l i g h t , P. r e s in o s a and P. banksiana. re mained growing throughout th e w in ter.

No

in d ic a tio n was given as to whether d iffe re n c e s in h eig h t were due to d iffe re n c e s in number of nodes or le n g th of in tern o d e, and i t was not s ta t e d whether the prolonged growth of P. r e s in o s a and P. banksiana was continuous or occurred as a s e r ie s of " f lu s h e s " . Thus, although se v e ra l experiments have p rev io u sly been c a r r ie d out w ith vario u s sp ecies of P irn s , d e t a ile d accounts of the p r e c is e mode of growth under d i f f e r e n t day -len g th s exe not re p o rte d , and no attem pt nas been made to determine th e e f f e c t s o f n a tu ra l changes in day-len g th in the n o m a l cycle o f growth. A f u r th e r p h oto period ic e f f e c t in P. ta e d a was re p o rte d by P h i l l i p s (1941) who found t h a t dormant seed lin g s brought in to th e greenhouse in th e autumn could be induced to break dormancy immediately i f given supplementary lo w - in te n s ity illu m in a tio n (w ith re d l i g h t ) to extend th e n a tu ra l day le n g th to 18 hours, whereas s im ila r seed lin g s kept in th e greenhouse under normal days remained dormant u n t i l I'ebruary. Gustafson (1938) found t h a t p la n ts o f P. r e s in o s a which had been allowed to remsdn o u t-o f-u o o rs during th e w in ter broke - 23 -

dormancy in th e normsJ. way under n a tu r a l d ay -len g th c o n aitio n s in th e sp rin g , but p la n ts wnich naa been p r o te c te a from low tem peratures during th e w in ter snowed no new shoot growth unaer n a tu ra l sp rin g day le n g th s, but a id so i f th e p h o to -p erio d was in c re ase d to 16 hours.

From th ese o b serv atio n s i t i s evident

th a t although a p e rio d of low -tem perature exposure i s necessary fo r th e normal breaking of dormancy in sp ecies of P in u s, in the absence o f such exposure b reaking of dormancy may be brought about when th e photoperiod exceeds c e r ta in v alues. Although photoperioaism iias not p rev io u sly been demonstrated in P. s v i v e s t r i s . ayiven (lydkj iias arawn a t te n ti o n to the e a r l i e r work of b ch o tte (1900) ana iLngier ( l 9 i 3 ) , who found t h a t se e a lin g s of s t r a i n s of t h i s sp ecies from Worth Sweden showed reduced growth and ' s tu n tin g ' when grown a t more so u th erly l a t i t u d e s .

On th e b a s is of s im ila r r e s u l t s obtained

w ith d if f e r e n t g eo graphical races of Populus tre m u la . Sylven concluded th a t th e e f f e c t s described fo r P. s v i v e s t r i s were probably p h o to p erio aic e f f e c t s r e s u l t i n g from d iffe re n c e s in n a tu ra l le n g th of day when seed lin g s were tr a n s p la n te d from n o rth ern to southern l a t i t u d e s . d ang let ( 194k-3) has shown t h a t d ay -len g th i s probably a fa c to r in determ ining th e age a t which female rep ro d u ctiv e organs are f i r s t i n i t i a t e d in P. s v i v e s t r i s . and t h a t s t r a i n s from the n o rth of Sweden show a c c e le r a te d rep ro d u ctiv e -

24

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development when grown in more eouonerly l a t i t u d e s . The suggestion t h a t p. s v iv e s t r i s e x h ib its photoperioaism had thus already been put forward b efo re th e p re s e n t work was undertaken, but a c le a r dem onstration and a d e ta ile d a n a ly s is were s t i l l lack in g .

In th e se circum stances a thorough study

o f pho to p eriod ic phenomena in P. s v iv es t r i s promised to be of con sid erab le i n t e r e s t . 3. Scope of th e p resen t work . The in v e s tig a tio n s were c a r r ie d out during the years 1947 to 1949 in c lu s iv e , ana included o b serv atio n s on more than 2,500 in d iv id u a l experim ental p la n ts ,

experim ental work w ith

t h i s type of p la n t i s n e c e s s a r ily somewhat siov;, since the growing of a s in g le batch of f i r s t - y e a r seed lin g s occupies a whole growing season, while experiments w ith o ld e r p la n ts • involves growing them over a p erio d of 12 months or more b efore such experiments can be commenced, and here again only a. sin g le grow th-cycle can be obtained in any one season. The work was c a r r ie d out mainly a t Bedford College, London, but c e r ta in experiments, which involved th e c o n tro l of dayle n g th cond itio n s by hand-opera.tion of li g h t- p r o o f covers in the e a rly morning and l a t e evening over a p erio d of se v e ra l months, were more conveniently c a r r ie d out in the p r iv a te garden of the w r ite r , in 1947 and 1948 a t Teddington, Middlesex, and in 1949 a t Paynes Park, London.

-

25

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Although th e work was a i r e c t e a p r im a r ily towaras m e e l u c id a tio n o f th e r o l e o f photoperiodism in th e normal cy cle o f growth, s in c e so l i t t l e i s known re g a rd in g th e g e n e ra l f e a tu r e s of photoperiodism in woody s p e c ie s, th e in v e s t i g a t i o n s in clu d ed also experim ents on f i r s t - y e a r se e d lin g s designed p r im a r ily as a c o n tr ib u tio n towards an u n d e rsta n a in g of th e mechanism of photoperiodism in t h i s type of p la n t. B. BXPhIHIIlENTAl . 1.

Photoperiodism in f i r s t - y e a r se e d lin g s o f Pinus s v i v e s t r i s . Experiments on f i r s t y ear seedxings of Pinus s v iv e s td s

were c a r r ie d out in th e y ears 1947-1949.

As has a lre a d y been

p o in te d ou t, f i r s t - y e a r s e e d lin g s of P. s v i v e s t r i s d i f f e r in two im portant r e s p e c ts from o ld e r p la n ts o f t h i s sp e c ie s. (1)

The green leav es are ' ju v e n ile ' le av es borne on th e main

stem and are d eterm in ate in growth, as c o n tra s te d w ith th e p a ire d n eed les, .of in d e te rm in a te growth and borne on ' dwarf s h o o ts ', o f o ld e r p la n ts . (2)

The d u ra tio n o f growth of th e shoot in th e f i r s t yeax is

not predeterm ined by th e number of i n i t i a l s l a i d dovm in a r e s t i n g bud, as in th e o ld e r s ta g e s . In th e se two r e s p e c ts , th e r e f o r e , th e growth of f i r s t y ear se e d lin g s i s unique in th e l i f e - ni s to r y of an in d iv id u a l o f P. s v i v e s t r i s and hence any p h o to p e rio d ic e f f e c t s observed in th e se ed lin g s w ill n o t n e c e s s a r il y be eq u ally a p p lic a b le to th e l a t e r s ta g e s .

N e v e rth e le ss, because th e d u ra tio n o f -

-

growth i s n o t predeterm in ed in f i r s t - y e a r s e e d lin g s , they provide a convenient m a te r ia l f o r th e study o f th e e f f e c t o f d ay -le n g th on th e d u ra tio n o f growth in a woody s p e c ie s , s in c e r e l a t i v e l y la r g e numbers can be grown in p o ts in a small a re a , and, as w ill be shown, they prove to be h ig h ly s e n s i t i v e m ateri a l . 1 .1 .

Kxoeriment 1 . The f i r s t experim ent, conducted d u rin g th e summer of 1947

a t Teddington, M iddlesex, was designed to ex plo re as f u l l y as p o s s ib le th e e f f e c t s o f tv/o d i f f e r e n t le n g th s of p h o to p erio d on v a rio u s a s p e c ts o f grow th of th e r o o t and shoot i n f i r s t - y e a r s e e d lin g s o f P. s v i v e s t r i s . Methods.

The experim ental p l a n t s were grown i n

p o ts

c o n tain in g s o i l o b ta in e d from Wareham F o re s t, D orset, to which had been added 25/o by volume o f a compost (known as "G 5/10") used in n u r s e r ie s o f the F o re s try Commission and c o n ta in in g bracken r e f u s e , sp en t hops and d r ie d blood. The seed was su p p lie d by H.M .Forestry Commission and was s t a t e d to be d e riv e d from p a re n t t r e e s grown in F a s t A nglia. Seven or e ig h t seeds were sown in each p o t on 2 6 th May 1947. Germination follow ed r a p id ly ana on 15th June, by which d ate 50% o f th e s e e d lin g s had c a s t th e t e s t a , th e 'p h o to p e r io a ic ' tre a tm e n t was commenced.

F i f t y p o ts of se e d lin g s were p la c e d

in each of two l i g h t - p r o o f boxes c o n s tru c te d o f a wooden framev/ork and coverea w ith a p a te n t r o o f in g - m a te r ia l. - 27 -

Fach

box was p r o v i d e d w i t h a d e t a c h a b l e l i d w hich was removed f o r a s p e c i f i e d p e r i o d e a c h day.

One s e r i e s o f p l a n t s was exp o se d

from 6 a.m. to 4 p.m.

t h u s g i v i n g a 1 0 - hou r p h o t o ­

p e r i o d ( " S . Û . " s e r i e s ) and tlie o t h e r was exposed from 0 a.m. t o 8 p.m. (Gr.fc.T.), g i v i n g a 1 0 - ho u r p h o d o p e r io d ( " u . 1). " s e r i e s ) . From 6 t h August t h e n a t u r a l d a y - l e n g t h f e l l below lO h o u rs and s u p p le m e n ta ry i l l u m i n a t i o n was p r o v i d e d from a 1 0 0 - w a t t e l e c t r i c lamp su sp e n d e d 2 f e e t above t h e p l a n t s , g i v i n g an i n t e n s i t y o f i l l u m i n a t i o n a t t h e l e v e l o f t h e p l a n t s o f ab o u t 20 f o o t - c a n d l e s . The v a l i d i t y o f t h i s p r o c e d u r e i s d i s c u s s e d i n d e t a i l below.

Samples c o n s i s t i n g of th e p l a n t s from f iv e p o ts were taken from each s e r i e s fo r dry-weight d e te rm in a tio n a t i n t e r v a l s of 2 weeks throughout th e growing p e rio d .

The p l a n t s were

removed from th e p o ts , v^ashed f r e e of s o i l and k i l l e d by immersing in b o i l i n g 90‘/o alcohol** f o r b-lU minutes. were then d r ie d in an oven a t lüü°C fo r 24 hours.

The p l a n t s

The le n g th

of th e stem between th e cotyledons and th e apex, and th e number of leaves 1 cm. or more in l e n g th were determined f o r each plant.

(The a r b i t r a r y sta n d a rd o f 1 cm f o r l e a f - l e n g t h was

X All th e times of day quoted h e r e a f t e r r e f e r to G-.ki.T. xs The use of b o i l i n g alcohol f o r k i l l i n g purposes probably le d to some lo s s of weight due to th e s o l u t i o n of r e s i n i n the a lc o h o l, but as a l l p l a n t s were s i m i l a r l y t r e a t e d , fo r comparative purposes t h i s i s not im po rtan t. - 28 -

chosen because i t included, a i l mature leaves a,nd excluded only very young developing l e a v e s ) . hesults. Development of snoots.

Trie emergence oi tn e s e e d lin g s

was followed i n botii s e r i e s by a p e r i o a of growth i n which th e r e was a steady production o f le a v e s , not accompanied a t f i r s t by any d e t e c t a b l e elo n g a tio n of the stem, so t h a t th e le aves formed a c lo se " r o s e t t e " .

This piia.se continued fo r

approximately 6 weeks from th e time of emergence of th e s e e d li n g s .

There was no s i g n i f i c a n t a i f f e r e n c e in l e a f number

between th e two s e r i e s during t h i s p e r io a , b u t t h e r e was a n o t i c e a b l e d i f f e r e n c e i n th e le n g th of th e le a v e s , those of th e Ù.D. p l a n t s being markealy s h o r t e r tiian those o f - t h e u.D. s e r i e s (s e e below). By 27th J u l y ex ten sio n of th e lowest i n t e r n e a e s was d i s c e r n i b l e in a number o f p l a n t s o f th e

jj.D.

s e r i e s , ana from

t h i s date th e r e was a p e r i o a of continued stem ex ten sio n and l e a f formation.

In th e B.D. s e r i e s ex ten sio n of the i n t e r n e des

was not observable u n t i l about one week l a t e r than in th e L .D ., and th e le n g th of th e extended in te rn o d e s wa,s much l e s s in th e S.D. p l a n t s ( s e e below). The growth of th e shoot u l t i m a t e l y ceased w ith th e formation of a te rm in a l 'w i n t e r r e s t i n g bud' in b o th s e r i e s of plants.

In the

ù.D.

s e r i e s , however, dormancy commenced much ~ 2y -

e a r l i e r than in th e u.Û. s e r i e s , 5üyo of th e 3.D. p l a n t s showing te rm in al buds on 7 t h September, th e corresponaing date f o r th e ij.h. p l a n t s being h th ü ctober, so t n a t aormancy was ueiayea by 4 weeks under fü hour p n o top erioas.

The in te r n o d e s o f the te rm in a l r e g io n rem aineu unextenaed i n both s e r i e s , so t n a t a te rm in al r o s e t t e of le a v e s was formed, i n th e c e n tr e of which wa,s th e r e s t i n g bud. 1

Figure 4.

F i r s t y ear se e d lin g s grown under 10-hour ( l e f t ) and lb -h o u r ( r i g h t ) photoperiods r e s p e c t i v e l y .

The o b serv atio n s on stem e x te n sio n and l e a f - f o r m a tio n throughout th e growing p e r io a are summarised in Figures 5 and 6 , and the d a ta f o r th e mature p l a n t s i s giv e n in Table 2.

I t w ill

be seen t n a t th e f i n a l d i f f e r e n c e w ith r e s p e c t to t o t a l l e a f nuffiber between th e two s e r i e s vfas aue mainly to th e a i f f e r e n c e - 30 -

i n rinrstion of growth, t h e r e being l i t t l e d i i i e r e n c e between the r a t e s of l e a f p ro au ctio n in th e two s e r i e s .

lO O

40

80

50

30 20

TIME(WEEKS)

F i g . 5.

F ig . 6 .

s te m -le n g th s .

ueaf numbers.

Increases in Stem-length ana ^ e a f - number i n p l a n t s grown unaer iO-hour (open c i r c l e s ; ana lü -n o u r ( c l o s e s c i r c l e s ) p h o to perioas respectively. Table 2. No. S eries of plants

u a ta fo r mature s e e a lin g s .

Huration o f growth (days)

fcean number of le a v e s .

Fean l e n g t h of stem. ( cms.)

5 6 . 2 t 2.6*

1 . 4 1 t 0.08

0.3 2 + 0 .0 2

8 6 . 4± 2.5

4.20 f 0.20

0.60 t 0 .0 2

S.D.

25

84

L.D.

30

112

Length o f f i r s t 25 in te r n o d e s , (mm. )

X In t h i s and a l l l a t e r t a b l e s the Mean - Standard F r r o r are quoted. - 31 -

The a i f f e r e n c e i n mean le n g th of stem, nowever, was aue p a r t l y to th e a i f f e r e n c e i n the numoer of in t e r n o a e s (= number of l e a v e s ) , ana p a r t l y to tn e f a c t t n a t th e in t e r n o a e s were much s h o r t e r i n th e 3.1). p l a n t s .

Tnis i s shown by th e u a ta f o r th e

mean le n g th s of th e f i r s t 25 in te rn o d e s f o r each s e r i e s . I t i s seen t h a t th e mean in te r n o d e l e n g th in th e L.D. p l a n t s i s n e a r l y double t h a t in th e 3.D. p l a n t s . d i f f e r e n c e i s c l e a r l y seen in f i g u r e 4.

This

The d i f f e r e n c e in

l e a f - l e n g t h r e f e r r e a to above was m a in ta in e a tnroughout th e p e r io d of growth.

The l e n g t h of s u c c e s s iv e le aves on any one

p l a n t i n c r e a s e s u n t i l a maximum i s reach e a a t about th e 25th l e a f , and t h e r e a f t e r remains c o n s ta n t except i n th e a p i c a l reg io n .

Data fo r th e mean maximum l e a f - l e n g t h f o r each s e r i e s

are given in Table 3.

I t i s seen t h a t tn e le a v e s of a.D.

p l a n t s are s i g n i f i c a n t l y lo n g e r than tho se o f 3.D. te st, P

0 . 001 ) .

Table 3 . S e r ie s .

(w ith ' t '

Number of plants.

Maximum l e a f - l e n g t h . Number of leaves examined.

fcean l e a f - l e n g t h .

S.D.

24

65

2 .4 i 0.04 cms.

ii.D.

23

70

3.0 i 0 .04 cms.

In a fevf 3.D. p l a n t s i t was observea t n a t premature growth of ' dwaxf-shoots' b e a rin g p a i r s of le a v e s o c c u rre a in th e a x i l s of th e j u v e n i l e le a v e s . of th e ii.D. p l a n t s . - 32 -

This a i a n o t occur i n any

Thus unüer ' s iio rt-a a y s ' as a g a in s t ' long a a y s ', the follow ing a l f f e r e n c e s i n tn e growth of th e shoot were observeu:(1)

C essation o f growth occurred sooner;

( 2)

fewer t o t a l le av es were formed;

(3)

th e in te rn o d e le n g th was reduced;

(4)

th e l e a f - l e n g t h was reduced. The q u e s tio n as to whether th e se are t r u e 'p h o to p e r io d ic '

e f f e c t s , as opposed to e f f e c t s a r i s i n g from d if f e r e n c e s in amount of a s s i m i l a t i o n , i s considered below. Viftvp.lopment of h o o t s ,

o b serv atio n s on the development

of th e r o o t s were made tnrougnout th e p e rio d of the experiment. On 15th June, a t tn e commencement of th e experiment, the r o o ts c o n s i s t e d of simple unbranched r a d i c l e s .

By 13th J u ly

secondary r o o t s were g e n e r a l l y p r e s e n t, w ith a mean number of 15.3 per p l a n t on th e S.D. p l a n t s and 15.5 per p la n t on the ii.D.

' Short r o o t s ' were observable in both s e r i e s on 27th

J u ly , and by lO th August a few branched mycorrhizal s h o r t ro o ts were p r e s e n t.

The development of mycorrhizal r o o ts on the

samples taken on 7 t h September i s shown by th e d a ta of Table 4. Table 4. S eries.

Development of k v c o r r h iz a l r o o t s .

No. of p l a n t s examined.

No. o f p l a n t s showing mycorrhizas.

M an No. of mycorrhizal r o o t s per p l a n t .

S.D.

29

21

16

L.D.

30

17

22

- 33 -

Thus, up to th e time when the S.D. p l a n t s became dormant t h e r e were no c l e a r morphological d if f e r e n c e s in th e r o o t development of th e two s e r i e s . U n til th e on set of dormancy of th e shoot t h e r e was a c t i v e growth of both primary and secondary r o o ts in both s e r i e s , b ut a f t e r th e S.D. p l a n t s had become dormant a very s tro n g growth of th e primary r o o t s became apparent.

This e f f e c t was not

ob servable i n th e u.D. p l a n t s , but th e l a t t e r ü ia not become dormant u n t i l ü t h October, a f t e r vifhich a a te temperature and l i g h t c o n d itio n s were not so favourable fo r growth as they naa been following th e onset of aormancy in the S.D. p l a n t s . Unfortuna,tely no q u a n t i t a t i v e o b servation s were made on th e r e l a t i v e development of primary and secondary ro o ts during t h e a c t i v e growth p e rio a , but th e d a ta of Table 5 give such d a ta f o r the mature p l a n t s .

These d a ta in d i c a t e the g r e a t e r

development o f th e primary r o o ts i n the n.K. p l a n t s , whereas t h e mean l e n g t h of th e secondary r o o ts was g r e a t e r in the ii.D. plants.

These a l f f e r e n c e s are s i g n i f i c a n t (w ith ' t ' t e s t ,

P < Ü.Ü5). Table 0.

noot, le n g th s i n mature p l a n t s .

No. jjength of S e r ie s of primary r o o t . ________p l a n t s . ___ ( cms)

Mean le n g th of secondary r o o t s ( cms)._________

S.D.

14

46.4 t 2.65

9.0 3 1 0.72

L.D.

15

40.2 i 1.69

12.01 t 1.11

- 34 -

The r o o t systems of t y p i c a l i,.D. and S.D. p l a n t s are shoijvn in Figure 4. R e la tiv e growth of r o o t and s h o o t.

In order to

i n v e s t i g a t e whether d if f e r e n c e s in le n g th of day have any e f f e c t on the r e l a t i v e aevelopment of r o o t ana shoot in s e e d lin g s o f p. s v i v e s t r i s , th e in c r e a s e s in dry weight of th e s e organs were followea tiiroughout th e p e r io a of the experiment.

The a a t a are sumruarisea in Tab re 6 ana pigures 7 -9.

The p ercentag e a i f f e r e n c e i n totaJ. ary weight between the two s e r i e s i s not p r o p o r tio n a l to the a i f f e r e n c e in photoperioa, and on 24th August, before aormancy of the S.D. p l a n t s began to a c c e n tu a te th e d if f e r e n c e s between th e two s e r i e s , th e dry weights of th e o.D. p l a n t s (ex p ressed as a percentage of those of th e h.D. p l a n t s ) were as f o llo w s ;Totai p l a n t .

Shoots.

Hoots.

77.6yo

76.170

8ü.37o

The d if f e r e n c e s i n le n g th of stem betvmen the two s e r i e s on t h i s date were, however, g r e a t e r than the a i f f e r e n c e s in ary weights of shoot, the stem le n g th fo r th e S.D. p l a n t s being le s s than bOfo t h a t of th e u.D. p l a n t s . C o nsid eration of the dry weight curves fo r the r o o ts shows a steady in c re a s e in both s e r i e s throughout the p e rio d o f growth, th e ' espionential' phase l a s t i n g somewhat longer than fo r th e shoots.

I t w i l l be seen t h a t the g e n eral r a t e of dry

weight in c r e a s e in th e S.D. p l a n t s wa.s not n o tic e a b ly a f f e c t e d - 35 -

by th e o n s e t of aom ancy o f tn e shoot a u rin g th e l 2 t h weeK o f th e experiment. 80 2 280 2 160 200

120

160

o 120

80

80

60

40

40

, 20 TIME (WEEKS)

Weight oi" ütioots

jfïg.ü. Total dry weight.

^ 120

80 60 40 20

TIME(WEEKS)

j^lg.y. jury V/eignt oT Koots. Dry Weight Data f o r s e e a l i n g s grown unaer iu -h o u r (open c i r c l e s ) and 1 5 -hour ( c l o s e d c i r c l e s ) p h o to period s respectively.

- 36 -

T ab le 6.

urv-w eight a a ta .

Time f r o m _____________ 1.01 ^ day..____________ ^______ a h o r t _ d 2 %.

commence­ Total Shoots p l a n t (mgms) ment of experiment (mgms)

hoots (mgms)

Total p la n t (mgms)

Shoots (mgms)

Roots (mgms)

4.65

3.79

0.87

4.65

3.79

0.87

2 weeks

10.15

7.58

2.57

9.94

7.53

2.41

4 weeks

26.1

21.5+1.0

6 weeks 8 weeks

4 6 .8

86.5

31.9+1.6 6 0 . 6 +2 .1

10 weeks

142.9

0

6 .6

23.8

17.510.8

6 .3

14.9+0.6 25.9+1.2

37.0 08.4

25.3+1.1 39.0+1.9

11.740.6 19.440.9

95.6+Z.9

47.312.1

110.8

72.8+3.0

38.012.0

12 weeks

203.7 138.0+5.7

65.7+3.4

140.7

84.2 14.4

56.542.7

14 weeks

270.6 171.6±7.2

99.016.3

16 weeks

-

-

-

- 37 -

-

205.7

-

103.946.4

-

10L814-.3

The r e l a t i v e growtn o i snooo ana root, i s fr e q u e n tly measured by th e r a t i o : weight of shoot / weignt o f r o o t, t h i s being the so-caJ.led " s n o o t /r o o t r a t i o " .

This r a t i o was

c a l c u l a t e d fo r th e two s e r i e s of p l a n t s from the dry-weight da.ta on su ccessive sampling d a te s, and i s seen to vary throughout th e p e r io a of growth (Table 7 .) Table 7.

hhoot/poot h a t i o .

Date.

njjn

h.D.

15/6/47

4 .4

4 .4

29/6

3.U

3.1

13/7

3 .3

2.8

27/7

2.2

2.2

lu /8

2.3

2.Ü

24/8

2.Ü

1.8

7/9

2.1 1.7 1.7

1.5

21/9 5/lD

-

1.1

The value of th e s h o o t/r o o t r a t i o f a l l s p r o g r e s s iv e ly in both s e r i e s ana in the e a r l y stag es th e re i s no markea d i f f e r e n c e between them.

Tne g r e a t e r f a l l in the value of the

r a t i o for th e d.h. s e r i e s a f t e r 2 i t n August i s to be ezp ia in e a by the onset of aormancy in tn e snoot w m ie one r o o ts continued to grow a c t i v e l y . P e a r s a l l (1927) has shown t h a t fo r many p l a n t s th e re i s - 38 -

a l i n e a r r e l a t i o n between one iogaritJrjHis of th e siioot ana r o o t weight, v iz .

or

lo g 3 z k log H + G

where 3 = Hand

...................

(1)

shoot weight r o o t weight

k, C are c o n s ta n ts . Figure 10 shows th e r e s u l t s of p l o t t i n g log shoot-vfeight

a g a i n s t log roo t-w eig h t fo r th e two s e r i e s of p l a n t s of the p r e s e n t experiment.

O fO

05

05

iO

15

20

LOG. DRY WT.'* ROOTS

Fig. 10.

h e l a t i v e G-ro'Wth of 3hoot and Boot, in

10-hour - (open c i r c l e s ) ana 10-hour - ( c lo s e d c i r c l e s ) day s eed lin g s r e s p e c t i v e l y . I t i s seen t h a t the r u l e holas very c l o s e l y f o r both ij.D. and 3.0. p l a n t s auring the p e r i o a oi a c t i v e growth of th e shoot, but t h e r e i s d e v ia tio n fromi tn e l i n e a r r e l a t i o n in the l a t e r s tag es of the 3.0 . s e r i e s , aue of course, to the c e s s a ti o n of growth of th e shoot.

The r e g r e s s io n c o e f f i c i e n t s

for log shoot weight on log ro o t weight are found to be 0.8193 - 39 -

and

ü . 7494

fo r th e 'iorjg üay' ana ' s h o r t aay' s e r i e s

respectively.

Applying a ' t ' t e s t for th e a i f f e r e n c e in th e

values of th e se r e g r e s s io n c o e f f i c i e n t s , i t i s founa t h a t they do not a i f f e r s i g n i f i c a n t l y ( t = ü.88 corresponaing to a p r o b a b i l i t y le v e l of approximately Ü.4 fo r 10 aegrees of freedom). This i n a i c a t e s t h a t auring the perio d of a c t iv e growth th e r e l a t i v e balance of shoot and ro o t aevelopment i s u n a f f e c t e d by a i f f e r e n c e s in le n g th of aay, t h a t i s to say, th e value of k (given by th e slope of the curve) i s u n a f i e c t e a by the le n g th of the photoperioa. huxley (1932) ha,s p o in te a out tlia t on a i f f e r e n t i a t i o n of th e equation ( r ) we o b ta in aS dtd

k

an dt.h.

,....

Now ÛS ^ dh gjre th e r e l a t i v e r a t e s o f growth of shoot ana dtS dtH r o o t r e s p e c t i v e l y , ana hence equation (2) i n d i c a t e s t h a t th e re i s a co n stan t r a t i o between the r e l a t i v e r a t e s of growth of shoot and r o o t .

The r e s u l t s i n d i c a t e t h a t t h i s r a t i o i s

u n a f f e c t e d by le n g th of aay under th e co nditions of the experiment. 1.2.

Experimient 2 . bince the c o n a itio n s of Experiment 1 were such tha.t

th e d if f e r e n c e s between th e long day ana s h o r t üay p l a n t s might p o s s ib ly have been aue to a i f f e r e n c e s in photo s y n th e s is , a - 40 -

simple experiment was c a r r i e a out in 1948 to t e s t the e f f e c t of reducing the i n t e n s i t y of n a tu ra l a a y iig h t by duyo but keeping the le n g th of tne n a t u r a l photoperioa unchangea.

I'he

p la n ts grown under such co n aitions were comparea with (a) p la n ts grown in f u l l a a y iig h t and n a tu ra l aay le n g th and (b) p la n ts grown in f u l l d aylight but with a reauced photoperiod of 10 hours. At the same time a f u r t h e r s e r i e s of p la n ts was grown under n a t u r a l d a y - lig h t suppiementea by low i n t e n s i t y illu m i n a ti o n during the hours of darkness, so tha.t p la n ts were’ exposed to continuous l i g h t ,

aince the supplementary

illu m in a tio n was of such a low value (ED f . c . ) as to have a n e g l i g i b l e e f f e c t on a s s im ila tio n , any e f i e c t s observed coula be regarded as ' pho to p e rio aic' ana not aue to any appreciable changes in a s s im ila tio n . k e th o a s . The c u ltu r e methods were the same as those described in Experiment l .

The observa,tions on dry-weight were

discontinued, however, since i t had been found in Experiment 1 t h a t a d iff e re n c e in day-lengths haa no d e te c ta b le e f f e c t on the r e l a t i v e development of shoot ana ro o t, ana t h a t ary-weight da.ta r e l a t i n g to shoot growth addea l i t t l e to the information obtained from observations on leaf-number ana stem length. The s e e a was sown on Eüth A p r i l 1948, ana on emergence o f t h e s e e a l i n g s tney were a i v i a e a i n t o fo u r s e r i e s o f 12 p o t s each and exposed to a i f f e r e n t t r e a t m e n t s as f o l l o w s : -

- 41 -

S eries A. i'uii n a tu ra l aay iigh t ana normal aay iengins. Series B. i u i l n a tu ra l aayiign t but reaucea photoperiod of 10 hours, obtainea by covering the p la n ts from 4 p.m. to 6 a.m. ( O .k .f .) Series Ü.

Natural aayiigh t of reaucea i n t e n s i t y , obtainea by placin g over the p la n ts a wooden framework covered with several layers of cheese-cloth to give a l i g h t i n t e n s i t y equal to bO% of t h a t of f u l l daylight. The day-length conditions were ’normal*.

Series D. Natural daylight of normal day length supplemented during the hours of darkness by a r t i f i c i a l illu m in atio n of approximately 25 foot-candles a t the le v e l of the p la n ts . Observations were made to determine the aates on which 50fo of the p la n ts of each s e r ie s showed terminal r e s tin g buds. When a l l p la n ts had become dormant Lhe mean length of stem and the mean number of leaves were determined and from these data the mean internode lengths for the extended portion of the stem ( i . e . the portion below the terminal r o s e tt e ) were calculated. The data are summarised in Table 8.

The mean duration of growth

was obtained by determining the date on which 50% of the plan ts in any given s e r ie s showed terminal r e s t i n g buds. Representative pla n ts from each s e r ie s are shown in Figure 11. - 42 -

Ta b le 8.

Series

Data for mature plan t s of jiixperiment 2.

Do.of kean length T^ion+.c! plants of stem

uength of *in 1 tern eae

kean number of leaves

(rm.)

(cm)

kean duratlon of growth _ i o a i s J ____

47

3 .4 7 ^ 0 .1 3

Ü.86

49

Ü.69±0.Ü4

41

3.28 ±0.17

0.94

5 7 .3 + 2 .2

116

49

5 .9 0 ± 0 .2 3

1.37

5 3 .4 ± 1 .6

103

64.7± 2.0

116 89

-------

Fig. 11. R e p re s e n ta tiv e samples of the four series of plants of Experiment 2. (For d e s c r i p t i o n of tre a tm e n ts see p .42) Results.

The natural d ay len gth during the p e r io d of

the experiment (early kay to mia-September) ranged from 15 hours to 16i hours a t mid-summer ana then aiminisiied to approximately 13 hours by 12th September.

( Tne se times are from sunrise to

sunset and ao not include a pnotoperioa i cally active tw iiignt - 43 -

period of variab le d u ra tio n ) .

The aay-length conaitions for

s e r ie s A and (J may thus be aescrioea as *iong-aay’ .

irom tne

data i t i s seen th a t tne pla n ts of s e rie s A ana s e rie s b resembled the ' long-day' ana ' short-aay* p la n ts of Experiment 1. . The appearance of dwarf shoots bearing p a irs of leaves in the Ü.D. p la n ts was more prevalent than in the previous experiment. The p la n ts of s e r ie s C? which were grown under normal day-length but with reaucea l i g h t - i n t e n s i t y , closely resembled those of Series A with resp ect to duration of growth, le a f number and internode length, and were sharply contrasted with the plants of deries h, although the l a t t e r must have received a g r e a t e r quan t i t y of incident l i g h t .

Although th e p l a n t s of

Series C had s i g n if i c a n tl y fewer leaves than those grown unaer - f u l l day-light ( f o r ’ g* t e s t , p between u.u2 ana u.ui)., the length of tne internoaes was g re a te r in the shaded p lants, which thus showed s l i g h t ‘e t i o l a t i o n ^

This l a t t e r e ff e c t is

in marked contra st with the reduced internodes of short-day p la n ts , thus in d icatin g th a t the aifferences observed between the *long-day' and ‘short-aay' plants of Experiment 1 were not due to d ifferences in assim ilatio n , but were probably true photoperiodic e f f e c t s . The p la n ts of s e r ie s n, grown unaer continuous illum ina­ tio n , showed the following aifferences from the 'normal-aay' pla n ts of s e r ie s A:-

- 44 -

(1)

the mean auration of growtn was i3 aays snorter unaer continuous iillumination;

(2)

tne number of leaves was s ig n if i c a n tl y fewer; (witn *t* te s t, P < 0.00i);

(3)

the t o t a l length of stem, however, was g re a te r due to the very much longer internoaes under continuous illumination;

(d:)

there was no terminal *rosette* of leaves, nearly a l l the internodes, p r a c t i c a l l y to the apex of the stem, being f u ll y extended;

(b)

the l a t e r a l *awarf-shoots* grew out in the a x ils of the uppermost juvenile leaves, ana bore p a irs of neeaies, ju s t as had been notea in tne d.u. plants;

(6)

the terminal buds were very much smaller in size than those of the other bnree s e r ie s . I t should be noted th a t a l l these e ff e c ts were produced

by a r t i f i c i a l illum ination of only 2b foot-ca.ndies, and hence must be true *photoperioaic* e f f e c ts .

The significance of these

e f f e c t s w ill be fu rth e r discussed below. 1.3

lixperiment 3. evidence w ill be aaauced below in support of the view

t h a t photoperiodism in wooay species has e s s e n tia lly the same physiological basis as in nerbaceous pla n ts. , une of the c h a r a c t e r i s t i c features of photoperioaism in nerbaceous species i s t h a t i t i s the duration of u n b rp ^ n darkness which determines

whether or not flowering w ill occur unaer any given aay-length conditions, ana tn a t a snort *light* break auring tne aark period, completely moai fie s tne nature of tne response (see, for example, Gregory, 1^46),

Wnen one aark perioa i s in terru p ted

by a *light-break* so as to give two aark perioas of unequal length, then tne response appears to be aeterminea by the longer of tnese two dark periods (Harder and Bode, 1943).

On

the other hand, in te rr u p tio n of the photoperiod by a period of darkness aoes not appreciably a f l e c t tne time of flowering in herbaceous species.

As tnese e ff e c ts nave not previously

been reported for wooay species, the following experiment was c a r r ie d out to determine wnetner they are also observable with t h i s type of plant. kethods.

Tne culture conaitions were tne same as

those describea for tne preceding experiments.

Tne seea was

sown on 20th April 1948 ana on emergence tne seealings were divided into four s e rie s of eleven pots each, which were t r e a t e d as follows Se r i es A - Hxposea to a lb-hour aaily photoperiod from 6 a.m. to 9 p.m. using natu ral daylight supplemented as necessary by a r t i f i c i a l illumination from a lOu-watt filament lamp, giving an in te n s ity of approximately 20 f . c . a t the p la n ts . Ser i e s B - Exposed to an il-n o u r photoperioa from 6 a.m. to 0 p.m. using n atural aayiignt. - 46 -

Series C - Exposed to two perioas of l i g h t in each 24 hourperiod - (a) from 6 a.m. to i p.m. (b) from b p.m. to 9 p.m.

There were t m s two aaiiy perioas of

darkness, of 4 nours ana 9 nours auration respectiveiy, separated by l i g n t perioas of 7 nours ana 4 hours duration,

j'or the second l i g h t period from

5 p.m. - 9 p.m.) n a tu ra l daylight was supplemented by a r t i f i c i a l illum ination as for s e rie s A.

The

p la n ts of s e r ie s 0 were thus exposed to the same conditions as those of s e r ie s A? except for the dark period from 1 p.m. to b p.m. S eries h - Exposed to a photoperioa of l o t nours from 6 a.-m. to 4.30 p.m.

The dark period was in te rru p te d by

30 minutes of illu m in atio n a t 20 fo o t-c a n a les from 8.30 - 9,0 p.m., tnus giving two aark periods of 4 ana 9 hours' auration re s p e c tiv e ly .

The conditions

were thus the same as for s e rie s B, except for the ' lig h t- b r e a k ' during the dark period. The mean dates of dormancy were observed in each s e rie s and on m aturity the mean stem length, t o t a l leaf-number, and internode length were determined.

The data are summarised in

Table 9, and r e p r e s e n ta tiv e pla n ts from each s e rie s are i l l u s t r a t e d in Figure 12. h esults.

Tne data of Table 9 show t n a t the pla n ts of

s e r i e s A and B under 10 ana 11 hour photoperioas re s p e c tiv e ly , - 47 -

Table V.

Data for mature p l ants of Experiment

Series

No. of plants

A

52

4.62^0.21

1.07

66.1+ 2.1

119

B

47

i . o i i o.ob

0.62

38.2+ 0.9

86

G

54

5.69 t 0.20

1.00

79.1+ 1.8

135

D

50

3 . 5 4 t o . 11

0.99

49.3tlD 7

95

kean length of stem I cm)

nengtn of ’ internoae* (njfi)

kean numiber of leaves

kean auration of growth ( da.vs )

showea the type of response which can now be regarded as ty p ic a l for ’long-day* ana ’ short-day’ seeaiings of Dinas sy lv e s tr i s .

The in terru p tio n ol tne photoperioa by 4 nours’

dark in tne case of series U a i a not reauce the to t a l leaf number as comparée with series

a>.

but actually ré s u lté e in

a

longer auration of grov/th ana a g r e a ter to t a l leaf-number. This r e s u l t was unexpectea ana was tne subject of further in v e stig atio n which i s aescribea below. The plants of series D, which received a ’ lig h t-b reak ’ of only 30 minutes’ duration at low in te n s ity , showed a marked response to th is treatment, the t o t a l leaf-number being s ig n if ic a n tly g reater than th a t of series B. (for ’t ’ t e s t , P < 0.01) while the internode length was p r a c tic a lly equal to t h a t of series A. Thus, the in terru p tio n of the dark period oy a snort ’ li g h t break’ produced a response approaching th a t of plants - 4b -

grown under ^long-day* conaitions, as in short day herbaceous species.

Similarly the r e s u lts obtained when the photoperiod

was in te rru p te a with a period of darkness agree with the observation th a t in herbaceous species such interruption does not reduce the e ffe c t ol long pnotoperioas, tut/i!nïè\ennanc^ " •‘ rr'T... stji c 'long-day* e ffe c t (as snown Dy tne increased leaf-number; in P. s y lv e s tr is appears to be a new phenomenon.

F ig .12.

representative samples Irom the four series of plants of Experiment 3. ( For description of treatments see pages 46-47).

I t is of i n t e r e s t to compare the growth of series G and D which received the same to t a l period of lig h t (11 hours) and the same dark periods of 4 hours and 9 hours respectively, for whereas the number of leaves formed in series C was g re a te r than in series A the leaf-number of series D aid not - 49 -

a t t a i n th a t of se rie s A,I althou^i g re a te r than th a t of series B.J The difference between tne responses of series C and s e rie s D wouia appear to be aue to the nifference in amount of l i g h t received during the second period of illumination 4 hours of which p art was daylight in tne case of series C, ana only 50 minutes a t a low i n t e n s ity in the case of series D. This matter i s fu rth er discussed below. 1.4

liroeriment 4. Among the r e s u l t s of Experiment 2 and 3 in 1948 were

the observations th a t ( l ) the g r e a t e s t leaf-number is attained under a photoperiod of duration less than 24 hours ( i . e . continuous l i g h t ) ; (2) seedlings vhiich received a photoperiod in te rr u p te d by 4 hours darkness in addition to a 9*hour dark period each day a tta in e d a g re a te r leaf-number.than plants which received only one period of darkness of 9 hours'duration per day.

These two observations suggested th a t a high leaf-

number may a c tu a lly be favoured by a short period of darkness and tha.t i f two such periods are given in a single 24-hour cycle, then t h e i r e ff e c ts may be cumulative.

In order to t e s t th is

hypothesis two furth er experiments were carried out in 1949. In the f i r s t experiment seedlings were grown under various photoperiods, ranging from 12 hours to continuous lig h t, in order to determine under what duration of dark period the maximum leaf-number is attained. -

hu -

Metnoas.

The seeaiings were grown unaer the same

conditions as in the previous experiments,

uwing to tne very

poor germination ol seed sown in A p ril, a complete re-sowing ' had to be made on ih tn Lay l^4y.

emergence ol tne seeaiings

had been completed by IZtn June, on wnich date the plants were divided into nine se rie s ol lO pots each, of which seven series were used in the present experiment, and were exposed to the following photoperiods:Series 1. - 12-hour photoperiod; plants covered from 7 p.m. to 7 a.m. Se r ie s 2. - 14-hour photoperiod; plants covered from 9 p.m. to 7 a.m. Series 5. - 16 - hour photoperiod; plants covered from 1% hours a f t e r sunrise u n t i l darx.

irom middle ol August i t was

necessary to supply a r t i f i c i a l illum ination of 20 f. c. i n t e n s ity to supplement the natural daylight.

Tiie period

of I b i hours wa.s chosen, since there i s a tw ilig h t of approximately 30 minutes'duration before sunrise, during which the l i g h t in te n s ity i s s u f f ic ie n t to evoke a photoperiodic response. Ser i e s 4 . - 18 - hour photoperiod; plants not covered, but exposed to natu ra l daylength plus additional illumination at approximately 20 i . e . in the evening to give a t o t a l photoperiod of 18 hours from naif-an-dour before sunrise. - 01 -

Series ô, - 20-nour pJiotoperioa; plants expo sea to natural day-length, supplemented oy a r t i f i c i a l illumination from 3 a.m. u n t i l sunrise, and

from sunset u n t i l i i p.m.

Series 6. - 22-hour photoperioa; plants exposed to natural day-length supplemented dy a r t i f i c i a l illumination from sunset to sunrise, but covered from 5 p.m. to 7 p.m. Se rie s 7 . - 24-hour photoperiod; plants exposed to natural day-length, supplemented by a r t i f i c i a l illumination from sunset to sunrise. The duration of a r t i f i c i a l illumination for series 4 ana 5 was controlled by time-switches, otherwise the photoperioas were controlled by hand,

i t w i n de odservea th a t in the case

of s e rie s 1 - 3 the i u i i photoperiod consisted of natural daylight, except for a snort period of additional illumination from mid-August, to give a 16 hour photoperioa in the case of s e r ie s 3.

The plants oi series ^ - 7, on the other hand,

received the f u l l natural day, (except th a t the plants of s e rie s 6 were covered from 4 p.m. to 6 p.m.), supplemented by a r t i f i c i a l illum ination for varying periods. A number of plants of se rie s 4 and 5 had s t i l l not completed t h e i r growth by the end of October and hence these s e rie s were tra n s f e r re d to a greenhouse maintained at ib^C, on 31st October.

AH growth was completed by the end of hovember.

When shoot growth had ceased, determinations were made - 02 -

Data fo r Mature P la n ts o f Experim ent 4

T able 10 P h otop eriod

10

12

14

16

T o ta l stem le n g th (cms*) S

0 ,7 6 ± 0 .0 5

1 .0 6 + 0 .0 6

1 .5 0 ± 0 .0 9

1 ,7 8 ± 0 .1 0

3 6 .7 ± 1 .5 3

T o ta l no# o f le a v e s S

Leaves in te r m in a l r o s e t t e S '

No# o f extended - in te m o d e s X

18 4 .1 0 * + 0 .2 0

20

22

24

6 .3 2 + 0 .2 3

3 .9 1 ± 0 ,2 0

3 .6 8 + 0 .1 7

4 4 .9 ± 1 .9 5

5 3 .2 ± 2 .3

6 1 .6 ± 1 .3

7 1 .9 ± 2 .2

7 4 .7 ± 2 .7

5 1 .4 ± 2 .1

5 0 .6 ± 1 .6

-

2 9 .2

2 8 .8

1 9 ,5

7 .7

1 1 .5

1 1 ,6

-

2 4 .0

3 2 .8

5 2 .4

6 7 .0

3 9 .9

3 9 .0

cr ^ •

Stem -le n g th t o " r o s e t t e ” 5.

-

-

1 .1 2

1 .3 6

3 .7 5

6 .0 7

3 .7 4

3 .4 7

In te m o d e le n g th (mm#)

-

-

0 .4 6 6

0 .4 1 3

0*716

0 ,9 0 6

0 ,9 3 5

0 .8 8 9

1 0 /9

7 /1 0

29/10

4 /9

2 6/8

102

129

151

96

87

a

Mean d a te o f c e s s a t io n o f grovjth (50^ o f p la n ts )• Mean d u ra tio n o f grov