Growth Regulators in Flowering and Fruit Development

16 G r o w t h Regulators i n F l o w e r i n g and F r u i t Development L. C. LUCKWILL

Downloaded by FUDAN UNIV on January 19, 2017 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0037.ch016

Long Ashton Research Station, University of Bristol, England

Some of the earliest practical applications of growth regulators related to flowering and fruit development and many of the pioneers were American. Prominent amongst them were Felix Gustafson of Ann Arbor, Michigan, who in 1936 was the first to induce parthenocarpic fruits with auxins; Gardner, Marth and Batjer of the USDA who in 1939 pioneered the use of 1-naphthalene-acetic acid for pre-harvest drop control in apples - a method still in regular use today; and Clark and Kerns in Hawaii who used the same substance in 1942 to induce synchronous flowering in pineapples. In general, plant growth regulators mimic the action of genes and their special value and significance in the perennial fruit crops is that they enable us to do today what might take decades or even centuries to accomplish by conventional breeding techniques. Most pomologists would agree that i t would be highly desirable, and technically feasible, to breed an apple variety that would set fruit without pollination, that would not be subject to biennial bearing or need thinning, whose fruits would not drop from the tree before harvest, which could be readily propagated from cuttings and, above a l l , one that would partition a greater proportion of its assimilates into fruit, as opposed to vegetative growth. No one, however, is likely to embark on such an ambitious project because of the enormous time scale involved and the impossibility of predicting the needs of the industry that far ahead. So we are left with the alternative of using growth regulators to overcome the present genotype deficiencies of the crop - a subject which forms the central theme of this review. Flower Initiation The most obvious genotypic deficiency of the apple is the tendency to produce light and heavy crops in alternate years. 293 Plimmer et al.; Pesticide Chemistry in the 20th Century ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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T h i s phenomenon, w h i c h v a r i e s i n i n t e n s i t y i n d i f f e r e n t varieties, i s c h a r a c t e r i s t i c o f many o t h e r p e r e n n i a l f r u i t c r o p s i n w h i c h f l o w e r i n i t i a l s a r e l a i d down i n t h e summer o f t h e y e a r b e f o r e flowering. The s p a r s e f l o w e r i n d u c t i o n w h i c h a c c o m p a n i e s a h e a v y c r o p was f o r m e r l y a t t r i b u t e d t o d e p l e t i o n o f t h e c a r b o h y d r a t e and n i t r o g e n o u s r e s e r v e s o f t h e t r e e : but, whilst a l t e r n a t e b e a r i n g may h a v e d e v e l o p e d a s a means b y w h i c h t h e t r e e c o u l d c o n s e r v e i t s f o o d r e s e r v e s , t h e c o n t r o l mechanisms a r e c l e a r l y hormonal i n n a t u r e . L e a v e s promote f l o w e r i n d u c t i o n . A s i n so many b i o l o g i c a l p r o c e s s e s a b a l a n c e o f a p r o m o t e r and a n i n h i b i t o r seems t o be involved. I n t h e pome f r u i t s t h e f l o w e r p r o m o t i n g i n f l u e n c e comes f r o m t h e r o s e t t e o f l e a v e s s u b t e n d i n g t h e t e r m i n a l b u d i n which i n i t i a t i o n occurs. The g r e a t e r t h e t o t a l a r e a o f t h e s u b t e n d i n g l e a v e s , the g r e a t e r the chance of the bud becoming f l o r a l . I n p l a n t s where f l o w e r i n g i s i n d u c e d b y t h e p h o t o p e r i o d t h e r e i s s t r o n g evidence t h a t , a f t e r i n d u c t i o n by the c r i t i c a l dark p e r i o d t h e l e a v e s p r o d u c e a f l o w e r i n g hormone w h i c h , however, has n e v e r b e e n u n e q u i v o c a l l y i s o l a t e d and i d e n t i f i e d . Although f l o w e r i n g i s n o t i n d u c e d by p h o t o p e r i o d i n t h e a p p l e , i t i s p o s s i b l e t h a t t h e same o r a s i m i l a r t y p e o f s u b s t a n c e i s p r o d u c e d . A n o t h e r p o s s i b i l i t y , y e t t o be e x p l o r e d , i s t h a t l e a v e s f u n c t i o n o n l y i n d i r e c t l y i n f l o w e r i n d u c t i o n b y a i d i n g t h e movement i n t o t h e s p u r o f hormones c a r r i e d i n t h e t r a n s p i r a t i o n s t r e a m . The most l i k e l y t y p e o f hormones t o be i n v o l v e d h e r e w o u l d be t h e g r o u p o f s u b s t i t u t e d a m i n o p u r i n e s known as c y t o k i n i n s , p a r t i c u l a r l y zeatin (6-(4-hydroxy-3-methylbut-2-enyl)-aminopurine) and i t s r i b o t i d e , w h i c h a r e b e l i e v e d t o o r i g i n a t e i n t h e r o o t a n d a^e f o u n d i n r e l a t i v e l y h i g h c o n c e n t r a t i o n s i n the xylem sap, p a r t i c u l a r l y i n t h e e a r l y p a r t o f t h e s e a s o n (_2). There i s , as y e t , no d i r e c t e v i d e n c e t h a t f l o w e r i n i t i a t i o n i n a p p l e c a n be p r o m o t e d by a p p l i e d c y t o k i n i n s , t h o u g h i t i s o f i n t e r e s t t o n o t e t h a t i n P e r i l l a Beever and Woolhouse found increases i n c y t o k i n i n produced i n the r o o t s at the time of f l o r a l i n d u c t i o n . R a t h e r s t r o n g e r e v i d e n c e o f c y t o k i n i n i n v o l v e m e n t came f r o m M u l l i n s (4) who showed t h a t , i n t h e a b s e n c e o f r o o t s , i n f l o r e s c e n c e d e v e l o p m e n t i n g r a p e v i n e s c a n be s t i m u l a t e d b y t h e a p p l i c a t i o n o f β - b e n z y l a m i n o p u r i n e (BAP) and 6 - ( b e n z y l a m i n o ) - 9 - ( 2 t e t r a h y d r o p y r a n y l ) - 9 H - p u r i n e ( P B A ) , a n d f r o m Skene (j?) who f o u n d t h a t chlor m equat , w h i c h promotes f l o w e r i n i t i a t i o n i n the v i n e , a l s o i n c r e a s e s t h e c o n c e n t r a t i o n o f endogenous c y t o k i n i n i n t h e bleeding sap. M o n s e l i s e a n d H a l e v y ( 6 ) h a v e shown t h a t b e n z o t h i a z o l e - 2 - o x y a c e t a t e , a compound w i t h c y t o k i n i n - l i k e p r o p e r t i e s , t h o u g h d i f f e r e n t i n c h e m i c a l s t r u c t u r e , w i l l promote f l o w e r i n g i n Citrus. However, the p r e s e n t e v i d e n c e f o r the i n v o l v e m e n t o f c y t o k i n i n s i n f l o w e r i n i t i a t i o n i n pome f r u i t s , l i k e t h a t f o r t h e e x i s t e n c e of a s p e c i f i c f l o w e r i n g hormone, remains circumstantial and f u r t h e r e x p e r i m e n t a l e v i d e n c e i s n e e d e d .

Plimmer et al.; Pesticide Chemistry in the 20th Century ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

16.

LUCKWiLL

Flowering

and Fruit

295

Development


Seeds i n h i b i t f l o w e r i n d u c t i o n . The d a t a o f H u e t (j) i l l u s t r a t e the e f f e c t of leaves i n promoting f l o w e r i n i t i a t i o n i n the s e e d l e s s W i l l i a m s ( B a r t l e t t ) p e a r . Although normally seedless i n warm c l i m a t e s t h i s p e a r w i l l p r o d u c e s e e d s i f c r o s s - p o l l i n a t e d , a n d t h e same e x p e r i m e n t i l l u s t r a t e s t h e d r a m a t i c e f f e c t w h i c h these seeds have i n i n h i b i t i n g f l o w e r i n i t i a t i o n . T h i s e f f e c t of s e e d s was f i r s t n o t e d b y Tumanov a n d G a r e e v ( δ ) , a n d c o n f i r m e d b y t h e w o r k o f Chan a n d C a i n (_9). L u c k w i l l (1 Oj s u g g e s t e d t h a t t h e e f f e c t was due t o e n d o g e n o u s g i b b e r e l l i n s w h i c h a r e p r e s e n t i n v e r y h i g h c o n c e n t r a t i o n s i n seeds at c e r t a i n s t a g e s of development. E v i d e n c e t h a t g i b b e r e l l i n s a r e t h e o p e r a t i v e hormones i n v o l v e d i s p a r t l y i n d i r e c t and p a r t l y d i r e c t . Indirect evidence comes f r o m t h e o b s e r v a t i o n t h a t y o u n g f r u i t l e t s o n l y become i n h i b i t o r y t o f l o r a l i n i t i a t i o n a t 5 t o 6 weeks a f t e r f u l l b l o o m , w h i c h i s a l s o t h e t i m e t h e y s t a r t t o p r o d u c e l a r g e amounts o f GA4 a n d G A 7 (1θ): (11 ) , a n o b s e r v a t i o n , i n c i d e n t a l l y , w h i c h e x p l a i n s why f r u i t t h i n n i n g n e e d s t o be done w i t h i n t h i s t i m e l i m i t i f r e t u r n b l o o m f o r t h e f o l l o w i n g y e a r i s t o be i n c r e a s e d (Table i ) . The d i r e c t e v i d e n c e i s t h e f a c t t h a t i n a p p l e a n d many o t h e r s p e c i e s ( s t r a w b e r r y , p l u m , c h e r r y , p e a r , a l m o n d , a p r i c o t , orange, Fuchsia) sprays of g i b b e r e l l i c a c i d a p p l i e d s h o r t l y a f t e r bloom w i l l reduce or c o m p l e t e l y i n h i b i t f l o w e r i n g the f o l l o w i n g y e a r . T h i s i n h i b i t i n g e f f e c t o f GA o n f l o w e r TABLE Apple cv. different

I

Emneth E a r l y . E f f e c t o f f r u i t r e m o v a l a t t i m e s on f l o w e r i n i t i a t i o n i n b o u r s e b u d s

N o . o f weeks a f t e r f u l l b l o o m when t r e e s were d e - f r u i t e d

No. o f f r u i t buds f o r m e d a s fo o f t h o s e the p r e v i o u s y e a r

0 2

123

4 6 8

150 59 11

10

8 6

Not d e - f r u i t e d

146

G i b b e r e l l i n content of seeds μg GA3/IOOO s e e d s

< 1 .0 < 1 .0 3.2 19.2 27.1

—

i n i t i a t i o n appears i n d i r e c t c o n t r a s t to i t s r o l e i n l o n g day r o s e t t e type p l a n t s ( e . g . cabbage, r a d i s h , l e t t u c e , e t c . ) in w h i c h GA w i l l p r o m o t e f l o w e r i n g u n d e r n o n - i n d u c t i v e c o n d i t i o n s . The g e n e r a l s i t u a t i o n seems t o be t h a t g i b b e r e l l i n p r o m o t e s i n d u c t i o n i n t h o s e s p e c i e s w h i c h f l o w e r on l o n g s h o o t s , b u t i n h i b i t s i t i n s p e c i e s w h i c h f l o w e r on s h o r t s h o o t s , s u g g e s t i n g


296

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CHEMISTRY

I N T H E 20TH

CENTURY

t h a t t h e a c t i o n o f t h e hormone i s n o t on i n d u c t i o n p e r s e , b u t r a t h e r on the v e g e t a t i v e phase w h i c h p r e c e d e s i t ( 1 2 ) : ( T ) . I f g i b b e r e l l i n s produced i n seeds a r e the main cause of f l o w e r i n h i b i t i o n and hence o f b i e n n i a l b e a r i n g i n f r u i t t r e e s , we m i g h t e x p e c t t o f i n d d i f f e r e n c e s i n g i b b e r e l l i n p r o d u c t i o n b e t w e e n s t r o n g l y b i e n n i a l a n d more r e g u l a r c r o p p i n g v a r i e t i e s . In f a c t , although v a r i e t i e s d i f f e r i n their g i b b e r e l l i n prod u c t i o n , no c o r r e l a t i o n w i t h b i e n n i a l c r o p p i n g t e n d e n c i e s exists (10). A n a l t e r n a t i v e a n d more l i k e l y e x p l a n a t i o n i s s u g g e s t e d b y t h e w o r k o f G . V . Hoad a t L o n g A s h t o n (13) > w h i c h shows t h a t i n t h e s t r o n g l y b i e n n i a l L a x t o n s S u p e r b a much l a r g e r q u a n t i t y o f g i b b e r e l l i n c a n be c o l l e c t e d i n a n a g a r b l o c k p l a c e d o n t h e c u t b a s e o f t h e p e d i c e l t h a n i n t h e l e s s b i e n n i a l C o x ' s Orange P i p p i n , suggesting that g i b b e r e l l i n transport i s a key f a c t o r i n biennial cropping.


!

Chemical c o n t r o l of b i e n n i a l f l o w e r i n g . On t h e b a s i s o f t h e s e h y p o t h e s e s we c a n s u g g e s t s e v e n p o s s i b l e ways i n w h i c h g r o w t h r e g u l a t o r s m i g h t be u s e d t o c o n t r o l b i e n n i a l f l o w e r i n g and c r o p p i n g . . F o u r o f t h e s e a r e t r e a t m e n t s w h i c h c o u l d be a p p l i e d i n the ' o n ( f r u i t i n g ) year to i n c r e a s e flower i n d u c t i o n , and t h r e e a r e d e s i g n e d t o d e c r e a s e f l o w e r i n d u c t i o n and w o u l d t h e r e f o r e be a p p l i e d i n t h e ' o f f ( n o n - f r u i t i n g ) y e a r . To i n c r e a s e f l o w e r i n g we m i g h t : 1

1 . B l o c k GA s y n t h e s i s . T h e r e a r e a number o f a n t i g i b b e r e l ï i n c ô m p ô u n d s ~ " w h i c h p r o b a b l y f u n c t i o n i n t h i s w a y . On a p p l e s t h e most e f f e c t i v e i s s u c c i n i c a c i d - 2 , 2 - d i m e t h y l h y d r a z i d e ( d a m i n o z i d e , SADH, ' A l a r ' ) . T h i s compound i s w i d e l y u s e d f o r i n d u c i n g e a r l y c r o p p i n g , a n e x t r e m e example o f w h i c h i s t h e 'meadow o r c h a r d ' , a n e x p e r i m e n t a l s y s t e m o f a p p l e p r o d u c t i o n i n which t r e e s , planted 1 2 x 1 8 inches apart are sprayed w i t h daminozide to induce f l o w e r i n i t i a t i o n i n t h e i r f i r s t year of g r o w t h (H) · The e f f e c t o f d a m i n o z i d e o n f l o w e r i n d u c t i o n ( T a b l e iYJ c a n be e n h a n c e d b y m i x i n g i t w i t h 2 - c h l o r o e t h y l TABLE

II

A d d i t i v e e f f e c t s o f daminozide and ethephon on the i n d u c t i o n o f f l o w e r s o n o n e - y e a r - o l d t r e e s o f a p p l e c v . C o x ' s Orange P i p p i n . Mean number o f b l o s s o m c l u s t e r s / t r e e a s a r e s u l t o f a s i n g l e s p r a y a p p l i e d t h e p r e v i o u s summer Ethephon (ppm)

0

D a m i n o z i d e (ppm) 625 1250

2500

0

3

8

10

12

625 1250 2500

9 10 16

11 12 19

13

17 22 21

15 23


16.

LucKwiLL

Flowering

and

Fruit

297

Development

phosphonic a c i d (ethephon). While i t i s most e f f e c t i v e on nonf r u i t i n g t r e e s , daminozide w i l l a l s o i n c r e a s e flower i n d u c t i o n on f r u i t i n g t r e e s , "but where the crop i s very heavy, as i t o f t e n i s i n 'on years of s t r o n g l y b i e n n i a l v a r i e t i e s , i t s e f f e c t i s q u i t e small. I t i s t h e r e f o r e not very u s e f u l f o r the c o n t r o l of biennialism. 1

2. Block GA_transport. A number of growth r e g u l a t o r s are known which w i l l block~the t r a n s p o r t of g i b b e r e l l i n from seed to bourse, and compounds such as 2,3,5-tri-iodobenzoic a c i d are e f f e c t i v e i n p e r m i t t i n g flower i n d u c t i o n to take place even i n the presence of a heavy crop. But the flowers so induced set p o o r l y and no increase i n crop i s obtained, probably because the food reserves of the tree have been depleted (Table I I I ) . So again, t h i s i s not a p r a c t i c a l method.


(TIBA)

TABLE I I I E f f e c t of TIBA 150 ppm a p p l i e d as a spray to heavy cropping t r e e s of apple cv. George Cave i n 1969 Control

TIBA

Blossom c l u s t e r s per t r e e i n 1970

253

611

Crop per t r e e

27.6

(kg)

35.4

Significance

S i g n i f i c a n t at < Not

1.0$

significant

3. Induce seedless f r u i t . As we have seen, seedless f r u i t s do not i n h i b i t flower i n i t i a t i o n . In many v a r i e t i e s of apple and pear seedless f r u i t s can be induced by a p p l y i n g growth r e g u l a t o r s under c o n d i t i o n s where n a t u r a l p o l l i n a t i o n has f a i l e d or been prevented. But the most e f f e c t i v e growth r e g u l a t o r i s GA, p a r t i c u l a r l y when mixed with the r i g h t p r o p o r t i o n of an auxin, such as 2-naphthoxyacetic a c i d (2-NOA) and a c y t o k i n i n . Hence, the treatment which induces parthenocarpy i s i t s e l f i n h i b i t o r y to flower i n d u c t i o n . 4· Thin f r u i t l e t s . This i s the most p r a c t i c a l and widely used 'on' year""treatment to even out cropping from year to year. Naphthalene compounds, p a r t i c u l a r l y 1-naphthylacetic a c i d (NAZI) and i t s amide (NAm) and the i n s e c t i c i d e c a r b a r y l (1-naphthyl methyl^carbamate) have been widely used f o r many years to induce the a b s c i s s i o n of f r u i t l e t s : but timing i s c r i t i c a l and e f f e c t s can vary widely from season to season depending, amongst other f a c t o r s , on r a t e of uptake and metabolism. Recently there has been much i n t e r e s t i n the p o s s i b i l i t y of u s i n g ethephon f o r f r u i t t h i n n i n g but, here again, a t t e n t i o n to time of a p p l i c a t i o n i s r e q u i r e d to a v o i d complete d e - f r u i t i n g of the t r e e as f r u i t l e t s abscind much more r e a d i l y i n June, when n a t u r a l auxin production i s low, than i n May or J u l y (Table IV").


PESTICIDE C H E M I S T R Y IN T H E 2 0 T H C E N T U R Y

298

TABLE I V The v a r y i n g s e n s i t i v i t y o f a p p l e c v . C o x s Orange P i p p i n t o ethephon a p p l i e d a t d i f f e r e n t times as a f r u i t t h i n n i n g a g e n t . io f r u i t d r o p d u r i n g t h e 11 d a y s f o l l o w i n g s p r a y i n g 1


Ethephon cone,

May June July August September

(ppm)

0

200

1000

2.5 11.5 2.5 2.7 5.0

2.8

32.0 15.0 10.0

41.5 92.5 15.0 46.0 49.5

' O f f y e a r t r e a t m e n t s w h i c h have decrease flower i n i t i a t i o n i n c l u d e : -

12.8

been t r i e d

i n order

to

5. Reduce l e a f _ a r e a . A p p l i c a t i o n o f 1$ NaDNOC e a r l y i n t h e s e a s o n t o s c o r c h t h e y o u n g f o l i a g e i s a p o s s i b l e way o f r e d u c i n g flower induction; h o w e v e r , a n u n a c c e p t a b l e amount o f l e a f damage must be i n f l i c t e d t o g e t a w o r t h - w h i l e r e d u c t i o n i n b l o o m the f o l l o w i n g y e a r , and t h i s l e a f a r e a i s needed to b u i l d up the f o o d r e s e r v e s of the t r e e . 6. A p p l y G A . A n a t t r a c t i v e p o s s i b i l i t y i s t o s p r a y t h e t r e e w i t h GA i n t h e ' o f f year to prevent e x c e s s i v e f l o w e r i n i t i a t i o n , p a r t i c u l a r l y as t h i s does not a f f e c t the photosynthetic e f f i c i e n c y of the f o l i a g e . U n f o r t u n a t e l y , even at h i g h c o n centrations, GA h a s p r o v e d i n e f f e c t i v e on c o m p l e t e l y ' o f f year o r d e b l o s s o m e d t r e e s , a l t h o u g h i t w i l l i n h i b i t f l o w e r i n g when a p p l i e d to f r u i t i n g t r e e s . The e x p l a n a t i o n o f t h i s p a r a d o x i s n o t c l e a r b u t i t may be t h a t t h e g i b b e r e l l i n h a s t o combine w i t h some s e c o n d f a c t o r f r o m t h e f r u i t i t s e l f b e f o r e i t c a n become i n h i b i t o r y t o f l o w e r i n d u c t i o n (15)· 1

1

7* A p p l y o t h e r _ f l o w e r _ i n h i b i t o r s . Besides g i b b e r e l l i n , a number o f o t h e r compounds a r e ~ k n o w n w h i c h w i l l r e d u c e f r u i t b u d formation i n apple. T h e y i n c l u d e m e t a - t ο l y l p h t h a l a m i c a c i d and x a n t h i n e (1 θ ) and t h e h e r b i c i d e s b r o m o u r a c i l a n d t h i o u r a c i l u s e d i n low (50 ppm) c o n c e n t r a t i o n (16), b u t none o f t h e s e h a v e y e t found commercial a p p l i c a t i o n . To sum up - i t w o u l d seem t h a t r e d u c t i o n o f t h e o n y e a r ' crop by b l o s s o m o r f r u i t t h i n n i n g w i t h growth r e g u l a t o r s remains t h e most p r a c t i c a l way o f c o n t r o l l i n g b i e n n i a l b e a r i n g i n a p p l e s : b u t we s t i l l n e e d more r e l i a b l e a n d c o n s i s t e n t f r u i t t h i n n i n g agents. !


16.

LucKwiLL

Flowering

and Fruit

Development

299

F r u i t Development


1

'Direct a c t i o n hypothesis. In the w i l d , the f r u i t i s simply the packaging f o r the a l l - i m p o r t a n t seeds on which the f u t u r e of the species depends, so i t i s not s u r p r i s i n g i n c u l t i v a t e d f r u i t s to f i n d that seed and f r u i t development are c l o s e l y l i n k e d . Although amongst c u l t i v a t e d f r u i t s there are notable exceptions, the general r u l e i s - no seed development, no f r u i t . Moreover, the number and d i s p o s i t i o n of seeds i n the f r u i t determine i t s s i z e and shape, i t s l i a b i l i t y to drop before i t i s f u l l y grown and o f t e n i t s biochemistry and storage prop e r t i e s . These f a c t s have long been known. L a t e r i t was d i s covered that by a p p l y i n g growth r e g u l a t o r s of the auxin or g i b b e r e l l i n type, seedless f r u i t s of many species could be induced to develop without the usual p r e l i m i n a r i e s of p o l l i n a t i o n and f e r t i l i z a t i o n . The next discovery was that developing (though not mature) seeds were themselves r i c h sources of hormones such as c y t o k i n i n s , auxins and g i b b e r e l l i n s . These hormones are produced i n the seed, not at a steady r a t e , but i n strong f l u s h e s i n w e l l marked succession corresponding with the development of successive t i s s u e s w i t h i n the seed - f i r s t the n u c e l l u s , then the f r e e nuclear endosperm, the c e l l u l a r endosperm and f i n a l l y the embryo i t s e l f . At t h i s point i t seemed reasonable to propose the hypothesis that the f r u i t t i s s u e s grew i n d i r e c t response to hormonal s t i m u l i emanating from the seeds. In p a r t i c u l a r , i t seemed l o g i c a l to assume that c y t o k i n i n s , i n which the f r e e - n u c l e a r endosperm i s r i c h , were a s s o c i a t e d with the e a r l y phases of f r u i t growth i n which c e l l d i v i s i o n i s dominant, w h i l s t g i b b e r e l l i n s , which appear l a t e r , were r e s p o n s i b l e f o r s t i m u l a t i n g c e l l enlargement. Unfortunately, t h i s simple hypothesis was not s u b s t a n t i a t e d by more d e t a i l e d i n v e s t i g a t i o n s which, with few exceptions, showed no c l o s e c o r r e l a t i o n between the peaks of hormone production i n the seed and the v a r i o u s phases of f r u i t growth. In the apple, f o r i n s t a n c e , there i s no apparent c o r r e l a t i o n between the percentage i n c r e a s e i n volume of the f r u i t each week and the c o n c e n t r a t i o n of g i b b e r e l l i n i n the seeds ( F i g . 2 ) . 'Competing s i n k s ' hypothesis. Although a few adherents of the ' d i r e c t a c t i o n hypothesis' ( i n c l u d i n g most text books'.) s t i l l f i g h t a rearguard a c t i o n , most workers i n t h i s f i e l d have now t r a n s f e r r e d t h e i r a l l e g i a n c e to the hypothesis of 'competing s i n k s ' . This supposes that the f a c t o r normally l i m i t i n g the growth of an ovary i n t o a f r u i t i s not the minute q u a n t i t i e s of hormone r e q u i r e d f o r c e l l d i v i s i o n and c e l l expansion, but r a t h e r the carbohydrates and amino a c i d s needed f o r b u i l d i n g new t i s s u e s which are r e q u i r e d i n l a r g e q u a n t i t i e s . These have to be a t t r a c t e d from the general pool against the competing demands of the v e g e t a t i v e growing p o i n t s . Although the mechanism i s obscure there i s strong evidence that metabolites and mineral elements



300


c Figure 1. Hormonal factors influencing flower initiation in bourse bud of apple. G = gibberel lins, C = cytokinins, F = florigin ?

9

Γ

0

5

10

Weeks a f t e r

15

20

full-bloom

Figure 2. Gibberellin production in seeds of apple com pared with rate of increase in fruit volume (cv. Cox s Orange Pippin)



16.

LucKwiLL

Flowering

and

Fruit

Development

301

move p r e f e r e n t i a l l y t o w a r d s i t e s o f h i g h hormone c o n c e n t r a t i o n . Of t h e t h r e e m a j o r g r o u p s o f g r o w t h p r o m o t e r s a u x i n s a r e o f prime importance i n s t i m u l a t i n g t h i s h o r m o n e - d i r e c t e d t r a n s p o r t , but combination w i t h g i b b e r e l l i n s or c y t o k i n i n s , or b o t h , r e s u l t s i n strong s y n e r g i s t i c a c t i o n (17)· On t h i s h y p o t h e s i s t h e h i g h c o n c e n t r a t i o n s o f hormones f o u n d i n t h e s e e d s a r e n e c e s s a r y i n order to create a s t r o n g p h y s i o l o g i c a l s i n k capable of competing w i t h the stem and r o o t a p i c e s . The m a i n e x p e r i m e n t a l e v i d e n c e f o r t h i s h y p o t h e s i s comes f r o m e x p e r i m e n t s i n w h i c h t h e c o m p e t i t i o n between v e g e t a t i v e and f r u i t growth i s p a r t i a l l y r e l i e v e d by r e m o v i n g a l l the shoot t i p s q u i t e e a r l y i n t h e season. I f the s u p p r e s s i o n of shoot growth i s v e r y severe i t i s p o s s i b l e to induce p a r t h e n o c a r p i c or s e e d l e s s development of t h e f r u i t b y t h i s method ( 1 8 ) . In other s i t u a t i o n s f r u i t set c a n be g r e a t l y e n h a n c e d t h r o u g h a n i n c r e a s e d r e t e n t i o n o f f r u i t l e t s w h i c h o t h e r w i s e would have dropped o f f because t h e i r s e e d c o n t e n t was t o o low t o e n a b l e them t o c o m p e t e . The d a t a o f Q u i n l a n a n d P r e s t o n (j_9) c o n f i r m t h e e a r l i e r f i n d i n g s o f A b b o t t a n d s u g g e s t t h a t we h a v e h e r e a p o t e n t i a l l y v a l u a b l e t e c h n i q u e f o r i m p r o v i n g the c r o p p i n g o f a p p l e s at t h e expense o f shoot g r o w t h , much o f w h i c h i s n o t r e q u i r e d a n d w i l l , i n a n y e v e n t , be removed i n w i n t e r p r u n i n g . But manual removal o f shoot t i p s i s h a r d l y p r a c t i c a l on a n o r c h a r d s c a l e ; what we n e e d i s a g r o w t h r e g u l a t o r which w i l l a r r e s t shoot growth without c a u s i n g u n d e s i r a b l e s i d e e f f e c t s on the f r u i t . Daminozide has p r o v e d q u i t e e f f e c t i v e on y o u n g t r e e s , when a p p l i e d s h o r t l y a f t e r p e t a l f a l l a n d i s now u s e d as a r o u t i n e s p r a y i n t h e 'meadow o r c h a r d to increase f r u i t set (Table V ) . Dramatic i n c r e a s e s i n y i e l d are 1

TABLE V P r o m o t i o n of f r u i t - s e t i n a p p l e by s u p p r e s s i n g shoot growth w i t h d a m i n o z i d e 2500 ppm a p p l i e d a t p e t a l - f a l l s t a g e . Mean o f 100 t r e e s

Variety L o r d Lambourne Egremont R u s s e t

F r u i t s p e r 100 Control

10 17

blossom c l u s t e r s Daminozide

23 58

o b t a i n e d on t h e s e s m a l l t r e e s , b u t t h e method i s n o t so e f f e c t i v e on t h e more c o n v e n t i o n a l t y p e o f t r e e . Other growth r e g u l a t o r s w h i c h have been t e s t e d i n c l u d e ( 2 - c h l o r o e t h y l ) trimethylammonium c h l o r i d e (CCC, Chlormequat, C y c o c e l ) , A n c y m i d o l , m a l e i c h y d r a z i d e , m o r p h a c t i n s and f a t t y a c i d e s t e r s , b u t a l l produce u n d e s i r a b l e s i d e e f f e c t s on l e a f o r f r u i t g r o w t h o r s k i n f i n i s h : n e v e r t h e l e s s t h e p r i n c i p l e i s e s t a b l i s h e d as a s o u n d one - a l l


302



we n e e d i s

the

right

growth

regulator'.

I n d u c t i o n of parthenocarpy. A n a l t e r n a t i v e method o f i m p r o v i n g f r u i t s e t , or of c o m p l e t e l y c i r c u m v e n t i n g the need f o r p o l l i n a t i o n , i s b y hormone s p r a y i n g t o i n d u c e p a r t h e n o c a r p y , a method w h i c h h a s f o u n d c o m m e r c i a l a p p l i c a t i o n i n t h e p r o d u c t i o n o f s e e d l e s s g r a p e s , i n f i g s and t o m a t o e s , and a l s o i n p e a r s , where GA s p r a y s h a v e b e e n u s e d t o s a v e t h e c r o p a f t e r t h e f l o w e r s o r f r u i t l e t s h a v e b e e n damaged b y s p r i n g f r o s t s ( 2 0 ) . In species w h i c h r e s p o n d t o hormone s p r a y s i t i s o f t e n f o u n d t h a t s y n t h e t i c a u x i n s , g i b b e r e l l i n s and sometimes c y t o k i n i n s , a r e e q u a l l y e f f e c t i v e i n s t i m u l a t i n g f r u i t growth, and t h a t the t h r e e d i f f e r e n t t y p e s o f h o r m o n e s , when a p p l i e d i n m i x t u r e s , show s y n e r g i s t i c a c t i v i t y , an o b s e r v a t i o n which suggests t h a t the mechanism o f a c t i o n i s s i m i l a r t o t h a t s u g g e s t e d f o r the endogenous hormones, v i z . c r e a t i n g m o b i l i z a t i o n c e n t r e s f o r m e t a b o l i t e s r a t h e r than d i r e c t s t i m u l a t i o n of t i s s u e growth. The a p p l e h a s p r o v e d one o f t h e most d i f f i c u l t s u b j e c t s f o r t h e c h e m i c a l i n d u c t i o n o f p a r t h e n o c a r p y , and f o r t h i s r e a s o n the work o f Schwabe a n d h i s c o - w o r k e r s a t Wye C o l l e g e i n E n g l a n d i s o f g r e a t i n t e r e s t and p o t e n t i a l v a l u e to the f r u i t i n d u s t r y . They h a v e d e v e l o p e d a t r i p l e hormone f r u i t - s e t t i n g s p r a y c o n t a i n i n g g i b b e r e l l i c a c i d (600 p p m ) , t h e s y n t h e t i c a u x i n 2-naphthoxyacetic a c i d (40 ppm) a n d t h e c y t o k i n i n b e n z y l a d e n i n e (300 ppm) - more r e c e n t l y r e p l a c e d by d i p h e n y l u r e a (21). T r i a l s o n C o x ' s Orange P i p p i n o v e r e i g h t y e a r s have g i v e n c o n s i s t e n t i n c r e a s e s i n y i e l d on b o t h p o l l i n a t e d a n d u n p o l l i n a t e d f l o w e r s . On sweet c h e r r y (cvs E a r l y R i v e r s and M e r t o n G l o r y ) v e r y s p e c t a c u l a r yield i n c r e a s e s h a v e b e e n a c h i e v e d a n d t h e same m i x t u r e h a s g i v e n p r o m i s i n g r e s u l t s on E u r o p e a n plum ( c v . V i c t o r i a ) . Apart from t h e p o s s i b l e s i d e e f f e c t s o f t h i s s p r a y on f l o w e r p r o d u c t i o n f o r the f o l l o w i n g y e a r , the h i g h c o s t of g i b b e r e l l i c a c i d would p r o b a b l y make t h e t r e a t m e n t u n e c o n o m i c a t t h e p r e s e n t t i m e . C o n t r o l of f r u i t r i p e n i n g and a b s c i s s i o n . The r i p e n i n g o f f r u i t s s u c h as t h e a p p l e w h i c h show a r e s p i r a t i o n c l i m a c t e r i c h a s l o n g b e e n known t o be a s s o c i a t e d w i t h e t h y l e n e , and t h e a d v e n t o f compounds s u c h as e t h e p h o n , w h i c h r e l e a s e e t h y l e n e w i t h i n the t i s s u e s o f the p l a n t , has g i v e n us an u n p r e c e d e n t e d degree o f c o n t r o l over the r i p e n i n g p r o c e s s . It enables f r u i t growers to h a r v e s t h i g h q u a l i t y apples e a r l i e r i n the season t h a n w o u l d o t h e r w i s e be p o s s i b l e a n d t o s p r e a d t h e i r l a b o u r requirements f o r h a r v e s t over a l o n g e r p e r i o d than would o t h e r w i s e be p o s s i b l e . Ethephon a l o n e w i l l i n d u c e a b s c i s s i o n and t o c o u n t e r a c t t h i s i t n e e d s t o be a p p l i e d i n c o m b i n a t i o n w i t h a n a u x i n , s u c h as 2 , 4 , 5 - T P , or w i t h daminozide. A combination of 750 ppm a . i . e t h e p h o n a n d 15 ppm 2 , 4 , 5 - T P a p p l i e d a b o u t 10 d a y s b e f o r e t h e d e s i r e d h a r v e s t d a t e h a s p r o v e d h i g h l y e f f e c t i v e on e a r l y v a r i e t i e s s u c h as W o r c e s t e r P e a r m a i n ( T a b l e V I ) a n d E a r l y


16.

LUCKwiLL

Flowering

and

Fruit

303

Development

M a c i n t o s h , w h e r e a s m a i n c r o p v a r i e t i e s t e n d t o r e a c t more s l o w l y . The r a t e o f r e a c t i o n i s a f u n c t i o n , n o t o n l y o f v a r i e t y , b u t a l s o of temperature and degree of water s t r e s s a n d , i n p r a c t i c e , TABLE V I Effect the

of ethephon i n combination w i t h daminozide or 2 , 4 , 5 - T P

q u a l i t y of Worcester

Pearmain apples h a r v e s t e d

Control


% of f r u i t s •4 o r more Relative

1-6

amount

content

colour

(on

resistance

fo p r e - h a r v e s t

0

68

60

170

270

220

of

a

scale)

Pressure

Ethephon + 2,4,5-TP

with

surface

anthocyanin Starch

Ethephon + daminozide

on

on A u g . 2 5 t h

drop

(lb)

1.4

1.9

4.4

18.7

18.7

17.0

8.3

1.6

8.6

g r o w e r s a r e recommended t o f o l l o w t h e p r o g r e s s o f r i p e n i n g b y a simple starch/iodine t e s t (22). O t h e r compounds, s u c h as b e n z y l - i s o t h i o c y a n a t e (23)> a c t a s a n t i e t h y l e n e .agents - p r o b a b l y by b l o c k i n g n a t u r a l b i o s y n t h e s i s a n d t h e s e may f i n d a p p l i c a t i o n s f o r d e l a y i n g r i p e n i n g o f f r u i t s and perhaps p r o l o n g i n g t h e i r s t o r a g e l i f e . Conclusions G r o w t h r e g u l a t o r s c l e a r l y have many u s e s a n d p o t e n t i a l u s e s i n f r u i t growing. A l t h o u g h I h a v e c o n c e n t r a t e d on t h e a p p l e , o n w h i c h most w o r k h a s b e e n d o n e , a s i m i l a r s t o r y c o u l d h a v e b e e n t o l d f o r a l m o s t any o t h e r c u l t i v a t e d f r u i t i n w h i c h growth r e g u l a t o r s c a n m o d i f y c r o p p i n g b e h a v i o u r t h r o u g h e f f e c t s on f l o w e r i n d u c t i o n and f r u i t s e t . I h a v e s t r e s s e d how l o c a l c o n c e n t r a t i o n s o f e n d o g e n o u s h o r m o n e s , s u c h as o c c u r i n s h o o t t i p s and young s e e d s , r e g u l a t e the d i s t r i b u t i o n of p h o t o s y n t h a t e s by c r e a t i n g p h y s i o l o g i c a l ' s i n k s ' , the r e l a t i v e s t r e n g t h s of which determine the p r o p o r t i o n of the t r e e ' s r e s o u r c e s which i t d e v o t e s t o f r u i t p r o d u c t i o n as o p p o s e d t o v e g e t a t i v e g r o w t h , much o f w h i c h i s u n w a n t e d a n d i s d e s t i n e d t o be p r u n e d away t h e following winter. Some p r o g r e s s t o w a r d t h e c o n t r o l o f a s s i m i l a t e p a r t i t i o n i n g b y means o f g r o w t h r e g u l a t o r s h a s b e e n made, a n d one p r a c t i c a l outcome i s t h e n o v e l s y s t e m o f a p p l e p r o d u c t i o n known as t h e


304


'meadow orchard'. I t i s probably i n this f i e l d of assimilate partitioning that the greatest potential for the future use of growth regulators l i e s . Literature Cited 1. 2. 3.


4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

53,

22. 23.

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