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Hormonal control of shoot elongation in tulips
Scientia Horticulturae, 15 (1981) 363--372 Elsevier Scientific Publishing Company, Amsterdam --Printed in The Netherlands
HORMONAL
CONTROL
OF SHOOT ELONGATION
363
IN TULIPS
M. SANIEWSKI Institute of Pornology and Floriculture, Skierniewice (Poland) W.J. DE MUNK Bulb Research Centre, Lisse (The Netherlands) (Accepted for publication 10 February 1981)
ABSTRACT Saniewski, M. and De Munk, W.J., 1981. Hormonal control of shoot elongation in tulips. Scientia Hortic., 15: 363--372. Shoot elongation of both cooled and uncooled 'Apeldoorn' and 'Oxford' tulips, as regulated by the leaves and the flower-bud, was studied. Leaves and/or flower-buds were excised and IAA, BA, or GA 3 in lanolin paste, was applied at various sites. Excision of the leaves and flower-buds of cooled tulips inhibited the elongation of the stem internodes. Administration of auxin after leaf and flower excision restored the elongation, mainly basipetally from the site of application, The findings may indicate that both the leaves and the floral organs provide auxin-like substances which control the elongation growth of the stem. Stems of uncooled tulips also elongated after IAA administration to the stem, but the response was slower and weaker. One part of the effect of cooling might be the stimulation of an auxin-releasing activity in the leaves and the flowers, another part an effect on the auxin-response system.
INTRODUCTION Tulip b u l b s w i t h m a i n s h o o t s c o n t a i n i n g a t e r m i n a l f l o w e r require lowt e m p e r a t u r e t r e a t m e n t s f o r floral-stalk e l o n g a t i o n (Rees, 1 9 7 2 ) . This suggests a k i n d o f d o r m a n c y t h a t can be released b y e x p o s u r e t o l o w t e m p e r a t u r e s ( K a m e r b e e k et al., 1 9 7 0 ) . T h e a c t u a l e l o n g a t i o n a f t e r c o o l i n g c a n p r o c e e d q u i c k l y , a n d it t a k e s a b o u t 2--3 w e e k s at 2 0 ° C t o r e a c h anthesis. H a n k s a n d Rees ( 1 9 7 7 ) suggested t h a t t w o m e c h a n i s m s c o n t r o l t h e a c t u a l e l o n g a t i o n g r o w t h o f t h e tulip s t e m : an a u x i n - m e d i a t e d s y s t e m , d e p e n d i n g o n a u x i n m a i n l y p r o d u c e d in t h e g y n o e cium, w h i c h regulates t h e e l o n g a t i o n o f t h e u p p e r i n t e r n o d e s , and a gibberell i n - m e d i a t e d s y s t e m , w h i c h regulates especially t h e e l o n g a t i o n o f t h e l o w e r i n t e r n o d e s . A u n g a n d D e H e r t o g h ( 1 9 6 7 , 1 9 6 8 ) f o u n d t h a t d u r i n g t h e cooling o f t h e b u l b s t h e a m o u n t o f free gibberellins increases. A d m i n i s t r a t i o n o f gibberellins, h o w e v e r , o n l y partially equalled t h e e f f e c t o f c o l d (Van Bragt a n d Zilstra, 1 9 7 1 ; R u d n i c k i et al., 1 9 7 6 ) . 0304-4238/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
364 Op den Kelder et al. (1971) studied the influence of GA, kinetin (K), naphthyl-acetic acid (NAA), and IAA, applied in lanolin paste, on the growth of the last internode of the tulip cultivars ' T o m m y ' and 'Princess Beatrix'. They showed t h a t the action of exogenously applied NAA or IAA replaced t h a t of the tulip flower, whereas GA and K were ineffective. They could not obtain information about the control of the elongation of lower internodes by auxins after flower-bud decapitation b e c a u s e t h e y used plants with leaves. Furthermore, the lower internodes of their plants had almost reached their final length before the treatments were started (about 24 cm at bud stage). The purpose of the present study was to investigate further the hormonal control of stem extension in tulips, i.e. the influence of floral organs both alone and in concert with the leaves. Use was made of plants grown from cooled and uncooled tulip bulbs. MATERIAL AND METHODS The experiments were performed with 'Apeldoorn' and 'Oxford' from commercial stocks. After lifting, the bulbs (size 12 cm) were stored at 20°C until flower formation was completed (Stage G). When this stage was reached in the middle of August, the bulbs were stored at 5°C for periods of 12 weeks (cooled bulbs) or kept at 20°C until use at the end of December (uncooled bulbs). After the cooling-period, the bulbs were planted in boxes and cultivated at 17 + I ° C under fluorescent lamps (TL 44, 40 Watt m -2 ). Uncooled bulbs were also planted under similar conditions early in January. Some of the bulbs of 'Oxford' were given a specific t r e a t m e n t for retarded flowering, i.e. storage in planted condition at temperatures below 0°C ("ice tulips", obtained from the W. Blom nurseries, Hillegom, The Netherlands). This material had been harvested in 1976, planted in boxes, and delivered in frozen condition in the a u t u m n of 1977. The bulbs produced normal flowers after cultivation in a glasshouse at 17°C for 10--14 days in the period from October until December. The experiments involved the excision of leaves and/or floral organs at their base and the application of lanolin paste containing the plant growth substances, indol-3-ylacetic acid (IAA) or 6-benzyl-adenin (BA), each at a concentration of 0.2%, or gibberellic acid (GA3) at 2%, alone or in combinations. The paste was applied at various sites: on the cut surface of the stem, the leaf-scars, the flower-bud, and below the flower-bud. Treatments were performed during various stages of development, the first in the stage when the flower-bud has just emerged from the bulb neck, the last when the floral organs could be removed w i t h o u t too much damage to the leaves. The length o f the internodes was measured when the stems had reached their final length. The results of the experiments were statistically evaluated using analysis of variance and the Duncan t-test at 5%-level of significance.
365 RESULTS
C o o l e d tulips E x c i s i o n o f leaves a n d / o r f l o w e r - b u d . - - E x c i s i o n o f t h e basal l e a f r e d u c e d t h e e l o n g a t i o n o f all i n t e r n o d e s t o s o m e e x t e n t (Table I). E x c i s i o n o f all leaves a l m o s t c o m p l e t e l y i n h i b i t e d t h e e l o n g a t i o n o f t h e e n t i r e s t e m . R e m o v al o f t h e leaves plus t h e f l o w e r - b u d (Tables I a n d II) r e s u l t e d in t h e s a m e degree o f g r o w t h i n h i b i t i o n as was o b t a i n e d b y t h e e x c i s i o n o f all leaves. This is u n d e r s t a n d a b l e , b e c a u s e a f t e r r e m o v a l o f all leaves t h e d e v e l o p m e n t o f t h e f l o w e r - b u d ceases a n d t h e b u d usually shrivels. E x c i s i o n o f t h e f l o w e r - b u d a l o n e ( T a b l e II) e x e r t e d its i n f l u e n c e m a i n l y o n t h e e l o n g a t i o n g r o w t h o f t h e t o p i n t e r n o d e , w h i c h h a r d l y i n c r e a s e d in l e n g t h a f t e r d e c a p i t a t i o n a n d h a d , if so, a small e f f e c t o n l e a f - b e a r i n g intern o d e s 1, 2 a n d 3. TABLE I Effect of the excision of various plant parts on the elongation growth of stern internodes of 'Apeldoorn' tulips, bulb size: 12 cm, pre-cooled at 5°C before planting. Values are means of 18 plants. Means in the same column followed by the same letter do not differ at 5% level of significance. Treatment
Length of internodes (cm)
,3 7
Initial length Final length Intact plants Basal leaf excised All leaves excised All leaves and flower-bud excised
1st (basal)
2nd
3rd
1.2
0.5
0.5
8.2 a 7.3 a 1.1 1.0
8.6 a 7.4 a 0.6 0.6
11.4 9.8 2.2 2.8
a b c c
4th 0.5 20.2 a 17.8 a 1.0 0.8
Total stem length (cm) 2.7 48.4 42.3 4.8 5.2
a a b b
E x c i s i o n o f organs and a d m i n i s t r a t i o n o f I A A . -- A p p l i c a t i o n o f I A A in lanolin p a s t e t o t h e c u t s u r f a c e o f t h e s t e m o f d e c a p i t a t e d p l a n t s ( f l o w e r - b u d excised) restored the elongation growth of the top internode almost comp l e t e l y ( T a b l e II); i n t e r n o d e s 1, 2 a n d 3, still b e a r i n g leaves, e l o n g a t e d like the LP-control. Administration of IAA on the stem top of decapitated and d e f o l i a t e d p l a n t s r e s t o r e d t h e e l o n g a t i o n o f a l m o s t all i n t e r n o d e s ; o n l y t h e t h i r d i n t e r n o d e r e m a i n e d significantly s h o r t e r t h a n in t h e i n t a c t p l a n t s (Table II). S o m e e l o n g a t i o n o c c u r r e d a f t e r t h e a p p l i c a t i o n o f plain lanolin p a s t e , b u t it was negligible c o m p a r e d w i t h t h e e f f e c t o f I A A - c o n t a i n i n g lanolin. It proved unnecessary to apply the IAA to the cut surface of the stem, since a p p l i c a t i o n a r o u n d t h e t o p o f t h e s t e m j u s t b e l o w t h e f l o w e r - b u d o f
366 TABLE II Influence of application of 0.2% IAA in lanolin paste (IAA-LP) to the stem top on the elongation growth of the stem internode of 'Oxford' tulips after excision of the flowerbud or the flower-bud and the leaves. Planted bulbs (size 12 cm) were frozen for several months until cultivation at 17°C ("ice-tulips"). Measurements were made 7 days after treatment, when extension growth was completed. Values are means of 18 plants. Means in the same column followed by the same letter do not differ at 5% level of significance. Treatment
Flower-bud and leaves intact Final length Flower-bud excised, leaves intact Initial length Final length after application of: Plain LP (control) 0.2% IAA-LP Flower-bud and leaves excised Initial length Final length after application of: Plain LP (control) 0.2% IAA-LP
Length of internodes (cm) 1st (basal)
2nd
3rd
4th
Length of entire stem (cm)
10.6 a
6.1 a
9.0 a
18,4 a
44.1 a
3.6
2.0
3.0
17.6 c
5.2 a 5.4 a
6.9 b 6.9 b
4.2 17.1 a
27.9 b 40.9 a
2,0
2.0
2.5
12.5 d
3.1 5.1 a
2.5 6.5 b
3.1 18.2 a
15.8 c 40.4 a
9.0 b 11.6 a 11.5 a 6.0 7.1 10.6 a
d e f o l i a t e d plants was as effective as a p p l i c a t i o n t o t h e c u t surface o f t h e stem (Table III, T r e a t m e n t 4). E l o n g a t i o n was n o t e x p e c t e d t o o c c u r in these def o l i a t e d plants, because in ~he absence o f leaves t h e f l o w e r - b u d blasts, w h i c h has t h e same e f f e c t o n e l o n g a t i o n as d e c a p i t a t i o n (Table III, T r e a t m e n t 3). A d m i n i s t r a t i o n o f I A A b e l o w t h e f l o w e r did n o t p r e v e n t blasting o f t h e flower-bud. A p p l i c a t i o n o f I A A t o t h e t o p - l e a f scar o f d e f o l i a t e d a n d d e c a p i t a t e d plants r e s t o r e d e l o n g a t i o n o f t h e l o w e r i n t e r n o d e s (1, 2 a n d 3) b u t n o t o f t h e t o p i n t e r n o d e (Table III, T r e a t m e n t 7). T h e s t i m u l a t o r y e f f e c t o n t h e third, a n d t o s o m e degree also o n t h e basal, i n t e r n o d e was even higher t h a n a f t e r a p p l i c a t i o n o f I A A t o t h e t o p o f t h e stem. A p p l i c a t i o n o f I A A t o t h e scar o f t h e basal leaf (Table III, T r e a t m e n t 9) r e s t o r e d e l o n g a t i o n o f t h e basal i n t e r n o d e . T h e final length o f this i n t e r n o d e was t h e same as in i n t a c t plants a n d greater t h a n a f t e r a p p l i c a t i o n t o t h e scar o f t h e t o p leaf or s t e m t o p (Fig. 1). G r o w t h o f t h e s e c o n d i n t e r n o d e was prom o t e d t o s o m e degree; t h e effects on t h e t h i r d a n d t o p i n t e r n o d e were negligible. A p p l i c a t i o n o f I A A t o the c u t surface o f t h e s t e m a f t e r excision o f t h e leaves a n d i n t e r n o d e s 2, 3 a n d 4 also resulted in e l o n g a t i o n o f t h e basal intern o d e , b u t o n l y a b o u t half t h a t o b t a i n e d b y a p p l i c a t i o n t o t h e scar o f t h e basal leaf (Table III, T r e a t m e n t s 12 a n d 9, respectively).
367
Fig. 1. Effect of removal of leaves and flower-bud, and of IAA-application at various sites, on stem elongation in 5° C-pre-cooled 'Apeldoorn' tulips treated on 27 November. Initial lengths of internodes are given in Table III. Photographed on 5 December. From left to right: control, plain lanolin on stem top; IAA on the scar of the top leaf; IAA on the stem top; IAA on the scar of the basal leaf; GA~ on the scar of the basal leaf. It m a y be c o n c l u d e d t h a t t h e i n h i b i t i o n o f s t e m e l o n g a t i o n b y excision o f t h e leaves a n d f l o w e r - b u d can be n e u t r a l i z e d b y a d m i n i s t r a t i o n o f I A A in lanolin paste. This t r e a t m e n t is m a i n l y effective basipetally f r o m the site o f a p p l i c a t i o n , a n d has t h e greatest e f f e c t o n t h e i n t e r n o d e b e l o w t h a t site. T h e results p r o v i d e e v i d e n c e t h a t t h e leaves a n d t h e f l o w e r are sources o f auxinlike substances.
Adminstration o f GA3. ~-Because it was e x p e c t e d t h a t G A w o u l d also p r o m o t e s t e m e l o n g a t i o n ( H a n k s a n d Rees, 1 9 7 7 ) , we applied lanolin containing 2% GA3 (ICI, Berelex) t o t h e scar o f t h e basal leaf a f t e r r e m o v a l o f all leaves (Table III, T r e a t m e n t 10). A small p r o m o t i v e e f f e c t was observed, b o t h basipetally a n d a c r o p e t a l l y f r o m t h e site o f a p p l i c a t i o n . T h e e l o n g a t i o n o f t h e basal i n t e r n o d e was m u c h smaller (43%) t h a n t h a t p r o m o t e d b y I A A .
368 •T h e w e a k s t i m u l a t i o n o f e l o n g a t i o n g r o w t h i n t h e a c r o p e t a l d i r e c t i o n ( i n t e r n o d e s 2, 3 a n d 4 ) w a s t h e s a m e o r l i t t l e s t r o n g e r a f t e r t h e c o m p a r a b l e a p p l i cation of IAA. It has to be mentioned that the remnants of the leafbases e l o n g a t e d t o a b o u t 3 - - 8 c m i n b o t h t r e a t m e n t s (9 a n d 1 0 ) , a n d m i g h t h a v e i n f l u e n c e d t h e e l o n g a t i o n g r o w t h . O n t h i s basis, i t c a n n o t b e c o n c l u d e d t h a t t h e e l o n g a t i o n g r o w t h o f t h e b a s a l i n t e r n o d e is u n d e r t h e c o n t r o l o f G A 3 , n o r t h a t i t is n o t . TABLE III Influence on stem elongation of the application of 0.2% IAA in lanolin paste (IAA-LP) at various sites on 'Apeldoorn' tulip plants after the excision of the leaves, the leaves and the flower-bud, or the leaves and the upper internodes. Bulbs (size 12 cm) were pre~cooled at 5° C before planting. The plants were treated after emergence of the flower from the bulb neck. Values are means of 18 plants. Means in the same column followed by the same letter do not differ at 5% level of significance Treatment
Length of internodes (cm)
Length of entire stem (cm)
1st (basal)
2nd
3rd
4th
1.5 11.4c
0.5 8.2
0.5 8.6
0.5 20.2d
3.0 48.4
2.7 8.9 d
2.1 6.9
0.8 7.4
0.7 20.9 d
6.3 44.1
All leaves excised ; flower bud excised 5. Plain LP at stem top 2.7 6. 0.2% IAA-LP at stem top 8.5 d 7. 0.2% IAA-LP at scar of the top leaf 9.5 d 8. Plain LP at the scar of the basal leaf 2.8 a 9. 0.2% IAA-LP at the scar of the basal leaf 11.4 c 10. 2% GA3-LP at the scar of the basal leaf 6.5 b
2.2 6.5 6.3 1.0 a 4.3 b 4.4 b
0.9 6.3 9.1 0.6 a 1.0 b 2.0 c
0.8 21.2 1.2 0.8 1.0 1.3
6.6 42.5 26.1 5.2 17.7 14.2
Plant excised through the basal node 11. Plain LP on cut surface of stem 12. 0.2% IAA-LP on cut surface of stem
0.8 3.1
Intact plants 1. Initial length 2. Final length All leaves excised;flower-bud intact 3. Plain LP below the flower-bud 4. 0.2% IAA-LP below the flower-bud
3.1 5.5
d a a ab b
n
i
Uncooled tulips A d m i n i s t r a t i o n o f I A A to the s t e m . -- As c o u l d be e x p e c t e d , e l o n g a t i o n of the stem of uncooled tulips was very slow; 6 weeks after planting, the flowerb u d s h a d j u s t e m e r g e d f r o m the bulbs. E x c i s i o n of the leaves a n d the flowerb u d h a d h a r d l y a n y e f f e c t , a n d t h e i n t e r n o d e s e l o n g a t e d as i n u n t r e a t e d plants (Table IV). Application of 0.2% IAA to the cut surface of the stem
369 TABLE IV Stem elongation (length in cm) as influenced by the administration of IAA to the stem top of uncooled, defoliated, and decapitated 'Apeldoorn' tulip plants, or of IAA, GA, BA and their combinations to flower-buds of uncooled defoliated plants. Bulbs (size 12 cm) were stored at 20 ° C and planted in pots at 17 ° C at the end of December. Values are means of 18 plants unless otherwise stated. Means in the same column followed by the same letter do not differ at 5% level of significance. Treatment
Untreated intact plants Uncooled Cooled Leaves and flower-bud excised Plain lanolin on stem top 0.2% IAA-LP on stem top Leaves excised, flower intact Plain lanolin on flower-bud 0.2% I AA on flower-bud 2% GA 3 on flower-bud 0.2% BA on flower-bud 0.2% IAA + 0.2% BA on flower-bud 0.2% IAA + 2% GA 3 on flower-bud 2% GA3 + 0.2% BA on flower-bud 0.2% IAA + 2% GA 3 + 0.2% BA on flower-bud
Plants with blasted flower-buds
Flowering plants
1st internode
1st internode
4th internode
Entire stem
9.4
27.9
52.6
1.5
4th internode
Entire stem
0.9
3.0
N
1.8 4.7
0.6 10.3
3.4 21.1
1.7 1.6 1.6 1.9
0.5 0.9 0.9 1.4
3.1 3.6 3.7 5.2
3.2 a
14.6 a 23.8 a
5
1.7
1.5
5.5
3.5 a
15.8 a 26.1 a
7
1.7
1.1
4.0
1.6
1.4
4.0
3.7 a
14.5 a 23.0 a
2
1.6
1.2
3.8
2.5 a
11.3 b 19.7 b
4
after decapitation and excision of the leaves, stimulated the elongation g r o w t h o f all i n t e r n o d e s ( F i g . 2 A ) . A f t e r 6 w e e k s t h e f i n a l l e n g t h o f t h e s t e m w a s 2 1 . 1 c m , w h i c h is a b o u t h a l f t h a t i n c o o l e d t u l i p s , w h e r e t h e e l o n g a t i o n p e r i o d l a s t e d a b o u t 4 w e e k s ( s e e T a b l e I). It may be concluded that application of IAA to the plant stem of uncooled tulips stimulates the elongation growth of the internodes. Compared w i t h c o o l e d t u l i p s , t h e r e s p o n s e is m u c h w e a k e r a n d s l o w e r .
Administration o f I A A , GA3, B A and their combinations to the flowerbud. -- A d m i n i s t r a t i o n o f I A A , G A 3 , o r b o t h t o t h e f l o w e r - b u d a f t e r e x c i sion of the leaves did not noticeably stimulate the elongation growth of the
370
A
B
Fig. 2. Effect of IAA (A) and of BA (B) on stem elongation of uncooled 'Apeldoorn' tulips in January 1978. A, from left to right: untreated tulip plant; leaves and flower excised, plain lanolin applied to the cut surface of the stem; leaves and flower excised and 0.2% IAA on the cut surface of the stem. B, left, defoliated tulip plant, flower-bud blasted;right, 0.2% BA in lanolin applied to the flower-bud resulted in flowering after which the stem started to elongate. s t e m i n t e r n o d e s (Table IV). A d m i n i s t r a t i o n o f BA, B A + I A A , BA + G A 3 , o r BA + I A A + GA3 did n o t result in i m m e d i a t e s t e m e l o n g a t i o n , b u t in s o m e plants t h e s t e m s t a r t e d t o e l o n g a t e a f t e r s o m e time. T h e final stem length was c o m p a r a b l e t o t h a t f o u n d a f t e r a d m i n i s t r a t i o n o f I A A t o the stem t o p , b u t t h e basal i n t e r n o d e r e m a i n e d s h o r t e r and t h e u p p e r i n t e r n o d e e l o n g a t e d m o r e t h a n a f t e r a p p l i c a t i o n o f I A A t o t h e s t e m t o p . This p h e n o m e n o n was o n l y o b s e r v e d in plants given BA alone o r in c o m b i n a t i o n s and w h o s e flowerb u d s r e a c h e d t h e flowering-stage (Fig. 2B). T h e o n s e t o f stem e l o n g a t i o n coi n c i d e d w i t h t h e start o f p e r i a n t h p i g m e n t a t i o n . It s h o u l d be m e n t i o n e d t h a t
371 the pistil of such plants showed a tremendous development; its final length was a b o u t 50 mm, which is quite unusual for 'Apeldoorn' tulips. DISCUSSION Our results show clearly that the elongation growth of the stem is under the control of the leaves and the flower, probably because these organs produce and release auxin-like substances. These findings are in agreement with those made in earlier investigations. (E.J. Fortanier, 1970, personal communication; Op den Kelder et al., 1971; Hanks and Rees, 1977). Hanks and Rees (1977) found that soft-drenches with ancymidol reduced tulip-stem elongation. The effects were mainly on the first internodes and were reversed b y GA. Such observations, and the fact that administration of GA to bulbs when the cold requirement is n o t fully satisfied stimulates the elongation growth, led the authors to the suggestion that a gibberellin-mediated system regulates the growth of lower internodes. Our results show that elongation of all internodes is p r o m o t e d b y auxins, probably produced in the leaves and the flower-bud. It can be an indication that GA stimulates the auxin-release or response system. Excision of the leaves resulted, in most cases, in complete inhibition of all internodes, even though the flower-bud was still present. This is understandable, because the growth of the flower usually ceases and the floral organs desiccate after the removal of the leaves. The effect of leaf excision on flower-bud development can be overcome b y application of cytokinins to the flower-bud, which might be an indication that the leaves provide cytokinins to the flower. Cytokinins strengthen the sink of the flower-bud, and sustain the flow of substrate from the bulb-scales to the flower bud (De Munk and Gijzenberg, 1977). The elongation of the stem and leaves of tulips is due almost entirely to elongation of cells produced during earlier developmental stages (Gilford and Rees, 1973). This process is greatly enhanced b y low-temperature treatments either before or after planting and rooting of the bulbs. It has been reported (Van Bragt and Zijlstra, 1971; l~udnicki et al., 1976) that part of the coolingeffect could be replaced b y gibberellins. In our experiments b o t h cooled and uncooled tulips reacted to the administration of auxin, although uncooled tulips elongated much more slowly and to a lower degree than cooled tulips. Thus, part of the cooling-effect can also be replaced b y auxin administration. We cannot conclude whether these effects of IAA and GA concern the same process. The crux of the question is the nature of the cooling-effect. It seems likely that the levels of b o t h cytokinins and gibberellins in the shoot increase during cooling, thus promoting the sink activity of the shoot organs, which in turn produce their own auxins. This is in agreement with the analyses made b y Rakhimbayev et al. (1978),who studied the biological activity of endogenous auxins, inhibitors, gibberellins and cytokinins in bulbs of Tulipa alberti Regel, stored at 22 or 4°C for 8 weeks. The hormonal situation
372
changed in the tissue during exposure of the bulbs to cold: the bound forms of gibberellins were found in the free state and the activity of cytokinins i m creased, whereas the activity of auxins did n o t change appreciably. The abscisic-acid-like growth inhibitor did n o t disappear on cold treatment, but its level dropped. On the other hand, it must be kept in mind that phytohormones also require a response mechanism. It might be that the receptor sites of the p h y t o h o r m o n e s are affected by the cold-treatment, which could explain why the application if IAA to the flowers of uncooled tulip plants had no effect.
REFERENCES Aung, L.H. and De Hertogh, A.A., 1967. The occurrence of gibberellin-like substances in tulip bulbs (Tulipa sp.). Plant Cell Physiol., 8: 201--205. Aung, L.H. and De Hertogh, A.A., 1968. Gibberellin-like substances in non-cold and cold treated tulip bulbs (Tulipa sp.). In: F. Wightman and G. Setterfield (Editors), Biochemistry and Physiology of Plant Growth Substances. Runge Press, Ottawa, pp. 943--956. De Munk, W.J. and Gijzenberg, J., 1977. Flower-bud blasting in tulip plants mediated by the hormonal status of the plant. Scientia Hortic., 7 : 2 5 5 - - 2 6 8 . Gilford, J.McD. and Rees, A.R., 1973. Growth of the tulip shoot. Scientia Hortic., 1: 143--156. Hanks, G.R. and Rees, A.R., 1977. Stem elongation in tulip and narcissus: the influence of floral organs and growth regulators. New Phytol., 78: 579--591. Kamerbeek, G.A., Beijersbergen, J.C.M. and Schenk, P.K., 1970. Dormancy in bulbs and corms. Proc. 18th. Int. Hortic. Congr., Tel Aviv, 1970, Vol. 5: 233--240. Op den Kelder, P., Benschop, M. and De Hertogh, A.A., 1971. Factors affecting floral stalk elongation of flowering tulips. J. Am. Soc. Hortic. Sci., 96: 603--605. Rakhimbayev, I.R., Syrtanova, G.A. and Solomina, V.F., 1978. The effect of cold treatment on the level of biological activity of endogenous growth regulators of tulip bulbs. Plant Physiol. (Russ.), 2 5 : 2 4 9 - - 2 5 3 (in Russian). Rees, A.R., 1972. The growth of bulbs. Academic Press, London, 311 pp. Rudnieki, R.M., Nowak, J. and Saniewski, M., 1976. The effect of gibberellic acid on sprouting and flowering of some tulip cultivars. Scientia Hortic., 4: 387--397. Van Bragt, J. and Zijlstra, F.A., 1971. Effects of gibberellins on flowering of tulip cv. 'Apeldoorn'. Z. Pflanzenphysiol., 64: 139--144.
HORMONAL
CONTROL
OF SHOOT ELONGATION
363
IN TULIPS
M. SANIEWSKI Institute of Pornology and Floriculture, Skierniewice (Poland) W.J. DE MUNK Bulb Research Centre, Lisse (The Netherlands) (Accepted for publication 10 February 1981)
ABSTRACT Saniewski, M. and De Munk, W.J., 1981. Hormonal control of shoot elongation in tulips. Scientia Hortic., 15: 363--372. Shoot elongation of both cooled and uncooled 'Apeldoorn' and 'Oxford' tulips, as regulated by the leaves and the flower-bud, was studied. Leaves and/or flower-buds were excised and IAA, BA, or GA 3 in lanolin paste, was applied at various sites. Excision of the leaves and flower-buds of cooled tulips inhibited the elongation of the stem internodes. Administration of auxin after leaf and flower excision restored the elongation, mainly basipetally from the site of application, The findings may indicate that both the leaves and the floral organs provide auxin-like substances which control the elongation growth of the stem. Stems of uncooled tulips also elongated after IAA administration to the stem, but the response was slower and weaker. One part of the effect of cooling might be the stimulation of an auxin-releasing activity in the leaves and the flowers, another part an effect on the auxin-response system.
INTRODUCTION Tulip b u l b s w i t h m a i n s h o o t s c o n t a i n i n g a t e r m i n a l f l o w e r require lowt e m p e r a t u r e t r e a t m e n t s f o r floral-stalk e l o n g a t i o n (Rees, 1 9 7 2 ) . This suggests a k i n d o f d o r m a n c y t h a t can be released b y e x p o s u r e t o l o w t e m p e r a t u r e s ( K a m e r b e e k et al., 1 9 7 0 ) . T h e a c t u a l e l o n g a t i o n a f t e r c o o l i n g c a n p r o c e e d q u i c k l y , a n d it t a k e s a b o u t 2--3 w e e k s at 2 0 ° C t o r e a c h anthesis. H a n k s a n d Rees ( 1 9 7 7 ) suggested t h a t t w o m e c h a n i s m s c o n t r o l t h e a c t u a l e l o n g a t i o n g r o w t h o f t h e tulip s t e m : an a u x i n - m e d i a t e d s y s t e m , d e p e n d i n g o n a u x i n m a i n l y p r o d u c e d in t h e g y n o e cium, w h i c h regulates t h e e l o n g a t i o n o f t h e u p p e r i n t e r n o d e s , and a gibberell i n - m e d i a t e d s y s t e m , w h i c h regulates especially t h e e l o n g a t i o n o f t h e l o w e r i n t e r n o d e s . A u n g a n d D e H e r t o g h ( 1 9 6 7 , 1 9 6 8 ) f o u n d t h a t d u r i n g t h e cooling o f t h e b u l b s t h e a m o u n t o f free gibberellins increases. A d m i n i s t r a t i o n o f gibberellins, h o w e v e r , o n l y partially equalled t h e e f f e c t o f c o l d (Van Bragt a n d Zilstra, 1 9 7 1 ; R u d n i c k i et al., 1 9 7 6 ) . 0304-4238/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
364 Op den Kelder et al. (1971) studied the influence of GA, kinetin (K), naphthyl-acetic acid (NAA), and IAA, applied in lanolin paste, on the growth of the last internode of the tulip cultivars ' T o m m y ' and 'Princess Beatrix'. They showed t h a t the action of exogenously applied NAA or IAA replaced t h a t of the tulip flower, whereas GA and K were ineffective. They could not obtain information about the control of the elongation of lower internodes by auxins after flower-bud decapitation b e c a u s e t h e y used plants with leaves. Furthermore, the lower internodes of their plants had almost reached their final length before the treatments were started (about 24 cm at bud stage). The purpose of the present study was to investigate further the hormonal control of stem extension in tulips, i.e. the influence of floral organs both alone and in concert with the leaves. Use was made of plants grown from cooled and uncooled tulip bulbs. MATERIAL AND METHODS The experiments were performed with 'Apeldoorn' and 'Oxford' from commercial stocks. After lifting, the bulbs (size 12 cm) were stored at 20°C until flower formation was completed (Stage G). When this stage was reached in the middle of August, the bulbs were stored at 5°C for periods of 12 weeks (cooled bulbs) or kept at 20°C until use at the end of December (uncooled bulbs). After the cooling-period, the bulbs were planted in boxes and cultivated at 17 + I ° C under fluorescent lamps (TL 44, 40 Watt m -2 ). Uncooled bulbs were also planted under similar conditions early in January. Some of the bulbs of 'Oxford' were given a specific t r e a t m e n t for retarded flowering, i.e. storage in planted condition at temperatures below 0°C ("ice tulips", obtained from the W. Blom nurseries, Hillegom, The Netherlands). This material had been harvested in 1976, planted in boxes, and delivered in frozen condition in the a u t u m n of 1977. The bulbs produced normal flowers after cultivation in a glasshouse at 17°C for 10--14 days in the period from October until December. The experiments involved the excision of leaves and/or floral organs at their base and the application of lanolin paste containing the plant growth substances, indol-3-ylacetic acid (IAA) or 6-benzyl-adenin (BA), each at a concentration of 0.2%, or gibberellic acid (GA3) at 2%, alone or in combinations. The paste was applied at various sites: on the cut surface of the stem, the leaf-scars, the flower-bud, and below the flower-bud. Treatments were performed during various stages of development, the first in the stage when the flower-bud has just emerged from the bulb neck, the last when the floral organs could be removed w i t h o u t too much damage to the leaves. The length o f the internodes was measured when the stems had reached their final length. The results of the experiments were statistically evaluated using analysis of variance and the Duncan t-test at 5%-level of significance.
365 RESULTS
C o o l e d tulips E x c i s i o n o f leaves a n d / o r f l o w e r - b u d . - - E x c i s i o n o f t h e basal l e a f r e d u c e d t h e e l o n g a t i o n o f all i n t e r n o d e s t o s o m e e x t e n t (Table I). E x c i s i o n o f all leaves a l m o s t c o m p l e t e l y i n h i b i t e d t h e e l o n g a t i o n o f t h e e n t i r e s t e m . R e m o v al o f t h e leaves plus t h e f l o w e r - b u d (Tables I a n d II) r e s u l t e d in t h e s a m e degree o f g r o w t h i n h i b i t i o n as was o b t a i n e d b y t h e e x c i s i o n o f all leaves. This is u n d e r s t a n d a b l e , b e c a u s e a f t e r r e m o v a l o f all leaves t h e d e v e l o p m e n t o f t h e f l o w e r - b u d ceases a n d t h e b u d usually shrivels. E x c i s i o n o f t h e f l o w e r - b u d a l o n e ( T a b l e II) e x e r t e d its i n f l u e n c e m a i n l y o n t h e e l o n g a t i o n g r o w t h o f t h e t o p i n t e r n o d e , w h i c h h a r d l y i n c r e a s e d in l e n g t h a f t e r d e c a p i t a t i o n a n d h a d , if so, a small e f f e c t o n l e a f - b e a r i n g intern o d e s 1, 2 a n d 3. TABLE I Effect of the excision of various plant parts on the elongation growth of stern internodes of 'Apeldoorn' tulips, bulb size: 12 cm, pre-cooled at 5°C before planting. Values are means of 18 plants. Means in the same column followed by the same letter do not differ at 5% level of significance. Treatment
Length of internodes (cm)
,3 7
Initial length Final length Intact plants Basal leaf excised All leaves excised All leaves and flower-bud excised
1st (basal)
2nd
3rd
1.2
0.5
0.5
8.2 a 7.3 a 1.1 1.0
8.6 a 7.4 a 0.6 0.6
11.4 9.8 2.2 2.8
a b c c
4th 0.5 20.2 a 17.8 a 1.0 0.8
Total stem length (cm) 2.7 48.4 42.3 4.8 5.2
a a b b
E x c i s i o n o f organs and a d m i n i s t r a t i o n o f I A A . -- A p p l i c a t i o n o f I A A in lanolin p a s t e t o t h e c u t s u r f a c e o f t h e s t e m o f d e c a p i t a t e d p l a n t s ( f l o w e r - b u d excised) restored the elongation growth of the top internode almost comp l e t e l y ( T a b l e II); i n t e r n o d e s 1, 2 a n d 3, still b e a r i n g leaves, e l o n g a t e d like the LP-control. Administration of IAA on the stem top of decapitated and d e f o l i a t e d p l a n t s r e s t o r e d t h e e l o n g a t i o n o f a l m o s t all i n t e r n o d e s ; o n l y t h e t h i r d i n t e r n o d e r e m a i n e d significantly s h o r t e r t h a n in t h e i n t a c t p l a n t s (Table II). S o m e e l o n g a t i o n o c c u r r e d a f t e r t h e a p p l i c a t i o n o f plain lanolin p a s t e , b u t it was negligible c o m p a r e d w i t h t h e e f f e c t o f I A A - c o n t a i n i n g lanolin. It proved unnecessary to apply the IAA to the cut surface of the stem, since a p p l i c a t i o n a r o u n d t h e t o p o f t h e s t e m j u s t b e l o w t h e f l o w e r - b u d o f
366 TABLE II Influence of application of 0.2% IAA in lanolin paste (IAA-LP) to the stem top on the elongation growth of the stem internode of 'Oxford' tulips after excision of the flowerbud or the flower-bud and the leaves. Planted bulbs (size 12 cm) were frozen for several months until cultivation at 17°C ("ice-tulips"). Measurements were made 7 days after treatment, when extension growth was completed. Values are means of 18 plants. Means in the same column followed by the same letter do not differ at 5% level of significance. Treatment
Flower-bud and leaves intact Final length Flower-bud excised, leaves intact Initial length Final length after application of: Plain LP (control) 0.2% IAA-LP Flower-bud and leaves excised Initial length Final length after application of: Plain LP (control) 0.2% IAA-LP
Length of internodes (cm) 1st (basal)
2nd
3rd
4th
Length of entire stem (cm)
10.6 a
6.1 a
9.0 a
18,4 a
44.1 a
3.6
2.0
3.0
17.6 c
5.2 a 5.4 a
6.9 b 6.9 b
4.2 17.1 a
27.9 b 40.9 a
2,0
2.0
2.5
12.5 d
3.1 5.1 a
2.5 6.5 b
3.1 18.2 a
15.8 c 40.4 a
9.0 b 11.6 a 11.5 a 6.0 7.1 10.6 a
d e f o l i a t e d plants was as effective as a p p l i c a t i o n t o t h e c u t surface o f t h e stem (Table III, T r e a t m e n t 4). E l o n g a t i o n was n o t e x p e c t e d t o o c c u r in these def o l i a t e d plants, because in ~he absence o f leaves t h e f l o w e r - b u d blasts, w h i c h has t h e same e f f e c t o n e l o n g a t i o n as d e c a p i t a t i o n (Table III, T r e a t m e n t 3). A d m i n i s t r a t i o n o f I A A b e l o w t h e f l o w e r did n o t p r e v e n t blasting o f t h e flower-bud. A p p l i c a t i o n o f I A A t o t h e t o p - l e a f scar o f d e f o l i a t e d a n d d e c a p i t a t e d plants r e s t o r e d e l o n g a t i o n o f t h e l o w e r i n t e r n o d e s (1, 2 a n d 3) b u t n o t o f t h e t o p i n t e r n o d e (Table III, T r e a t m e n t 7). T h e s t i m u l a t o r y e f f e c t o n t h e third, a n d t o s o m e degree also o n t h e basal, i n t e r n o d e was even higher t h a n a f t e r a p p l i c a t i o n o f I A A t o t h e t o p o f t h e stem. A p p l i c a t i o n o f I A A t o t h e scar o f t h e basal leaf (Table III, T r e a t m e n t 9) r e s t o r e d e l o n g a t i o n o f t h e basal i n t e r n o d e . T h e final length o f this i n t e r n o d e was t h e same as in i n t a c t plants a n d greater t h a n a f t e r a p p l i c a t i o n t o t h e scar o f t h e t o p leaf or s t e m t o p (Fig. 1). G r o w t h o f t h e s e c o n d i n t e r n o d e was prom o t e d t o s o m e degree; t h e effects on t h e t h i r d a n d t o p i n t e r n o d e were negligible. A p p l i c a t i o n o f I A A t o the c u t surface o f t h e s t e m a f t e r excision o f t h e leaves a n d i n t e r n o d e s 2, 3 a n d 4 also resulted in e l o n g a t i o n o f t h e basal intern o d e , b u t o n l y a b o u t half t h a t o b t a i n e d b y a p p l i c a t i o n t o t h e scar o f t h e basal leaf (Table III, T r e a t m e n t s 12 a n d 9, respectively).
367
Fig. 1. Effect of removal of leaves and flower-bud, and of IAA-application at various sites, on stem elongation in 5° C-pre-cooled 'Apeldoorn' tulips treated on 27 November. Initial lengths of internodes are given in Table III. Photographed on 5 December. From left to right: control, plain lanolin on stem top; IAA on the scar of the top leaf; IAA on the stem top; IAA on the scar of the basal leaf; GA~ on the scar of the basal leaf. It m a y be c o n c l u d e d t h a t t h e i n h i b i t i o n o f s t e m e l o n g a t i o n b y excision o f t h e leaves a n d f l o w e r - b u d can be n e u t r a l i z e d b y a d m i n i s t r a t i o n o f I A A in lanolin paste. This t r e a t m e n t is m a i n l y effective basipetally f r o m the site o f a p p l i c a t i o n , a n d has t h e greatest e f f e c t o n t h e i n t e r n o d e b e l o w t h a t site. T h e results p r o v i d e e v i d e n c e t h a t t h e leaves a n d t h e f l o w e r are sources o f auxinlike substances.
Adminstration o f GA3. ~-Because it was e x p e c t e d t h a t G A w o u l d also p r o m o t e s t e m e l o n g a t i o n ( H a n k s a n d Rees, 1 9 7 7 ) , we applied lanolin containing 2% GA3 (ICI, Berelex) t o t h e scar o f t h e basal leaf a f t e r r e m o v a l o f all leaves (Table III, T r e a t m e n t 10). A small p r o m o t i v e e f f e c t was observed, b o t h basipetally a n d a c r o p e t a l l y f r o m t h e site o f a p p l i c a t i o n . T h e e l o n g a t i o n o f t h e basal i n t e r n o d e was m u c h smaller (43%) t h a n t h a t p r o m o t e d b y I A A .
368 •T h e w e a k s t i m u l a t i o n o f e l o n g a t i o n g r o w t h i n t h e a c r o p e t a l d i r e c t i o n ( i n t e r n o d e s 2, 3 a n d 4 ) w a s t h e s a m e o r l i t t l e s t r o n g e r a f t e r t h e c o m p a r a b l e a p p l i cation of IAA. It has to be mentioned that the remnants of the leafbases e l o n g a t e d t o a b o u t 3 - - 8 c m i n b o t h t r e a t m e n t s (9 a n d 1 0 ) , a n d m i g h t h a v e i n f l u e n c e d t h e e l o n g a t i o n g r o w t h . O n t h i s basis, i t c a n n o t b e c o n c l u d e d t h a t t h e e l o n g a t i o n g r o w t h o f t h e b a s a l i n t e r n o d e is u n d e r t h e c o n t r o l o f G A 3 , n o r t h a t i t is n o t . TABLE III Influence on stem elongation of the application of 0.2% IAA in lanolin paste (IAA-LP) at various sites on 'Apeldoorn' tulip plants after the excision of the leaves, the leaves and the flower-bud, or the leaves and the upper internodes. Bulbs (size 12 cm) were pre~cooled at 5° C before planting. The plants were treated after emergence of the flower from the bulb neck. Values are means of 18 plants. Means in the same column followed by the same letter do not differ at 5% level of significance Treatment
Length of internodes (cm)
Length of entire stem (cm)
1st (basal)
2nd
3rd
4th
1.5 11.4c
0.5 8.2
0.5 8.6
0.5 20.2d
3.0 48.4
2.7 8.9 d
2.1 6.9
0.8 7.4
0.7 20.9 d
6.3 44.1
All leaves excised ; flower bud excised 5. Plain LP at stem top 2.7 6. 0.2% IAA-LP at stem top 8.5 d 7. 0.2% IAA-LP at scar of the top leaf 9.5 d 8. Plain LP at the scar of the basal leaf 2.8 a 9. 0.2% IAA-LP at the scar of the basal leaf 11.4 c 10. 2% GA3-LP at the scar of the basal leaf 6.5 b
2.2 6.5 6.3 1.0 a 4.3 b 4.4 b
0.9 6.3 9.1 0.6 a 1.0 b 2.0 c
0.8 21.2 1.2 0.8 1.0 1.3
6.6 42.5 26.1 5.2 17.7 14.2
Plant excised through the basal node 11. Plain LP on cut surface of stem 12. 0.2% IAA-LP on cut surface of stem
0.8 3.1
Intact plants 1. Initial length 2. Final length All leaves excised;flower-bud intact 3. Plain LP below the flower-bud 4. 0.2% IAA-LP below the flower-bud
3.1 5.5
d a a ab b
n
i
Uncooled tulips A d m i n i s t r a t i o n o f I A A to the s t e m . -- As c o u l d be e x p e c t e d , e l o n g a t i o n of the stem of uncooled tulips was very slow; 6 weeks after planting, the flowerb u d s h a d j u s t e m e r g e d f r o m the bulbs. E x c i s i o n of the leaves a n d the flowerb u d h a d h a r d l y a n y e f f e c t , a n d t h e i n t e r n o d e s e l o n g a t e d as i n u n t r e a t e d plants (Table IV). Application of 0.2% IAA to the cut surface of the stem
369 TABLE IV Stem elongation (length in cm) as influenced by the administration of IAA to the stem top of uncooled, defoliated, and decapitated 'Apeldoorn' tulip plants, or of IAA, GA, BA and their combinations to flower-buds of uncooled defoliated plants. Bulbs (size 12 cm) were stored at 20 ° C and planted in pots at 17 ° C at the end of December. Values are means of 18 plants unless otherwise stated. Means in the same column followed by the same letter do not differ at 5% level of significance. Treatment
Untreated intact plants Uncooled Cooled Leaves and flower-bud excised Plain lanolin on stem top 0.2% IAA-LP on stem top Leaves excised, flower intact Plain lanolin on flower-bud 0.2% I AA on flower-bud 2% GA 3 on flower-bud 0.2% BA on flower-bud 0.2% IAA + 0.2% BA on flower-bud 0.2% IAA + 2% GA 3 on flower-bud 2% GA3 + 0.2% BA on flower-bud 0.2% IAA + 2% GA 3 + 0.2% BA on flower-bud
Plants with blasted flower-buds
Flowering plants
1st internode
1st internode
4th internode
Entire stem
9.4
27.9
52.6
1.5
4th internode
Entire stem
0.9
3.0
N
1.8 4.7
0.6 10.3
3.4 21.1
1.7 1.6 1.6 1.9
0.5 0.9 0.9 1.4
3.1 3.6 3.7 5.2
3.2 a
14.6 a 23.8 a
5
1.7
1.5
5.5
3.5 a
15.8 a 26.1 a
7
1.7
1.1
4.0
1.6
1.4
4.0
3.7 a
14.5 a 23.0 a
2
1.6
1.2
3.8
2.5 a
11.3 b 19.7 b
4
after decapitation and excision of the leaves, stimulated the elongation g r o w t h o f all i n t e r n o d e s ( F i g . 2 A ) . A f t e r 6 w e e k s t h e f i n a l l e n g t h o f t h e s t e m w a s 2 1 . 1 c m , w h i c h is a b o u t h a l f t h a t i n c o o l e d t u l i p s , w h e r e t h e e l o n g a t i o n p e r i o d l a s t e d a b o u t 4 w e e k s ( s e e T a b l e I). It may be concluded that application of IAA to the plant stem of uncooled tulips stimulates the elongation growth of the internodes. Compared w i t h c o o l e d t u l i p s , t h e r e s p o n s e is m u c h w e a k e r a n d s l o w e r .
Administration o f I A A , GA3, B A and their combinations to the flowerbud. -- A d m i n i s t r a t i o n o f I A A , G A 3 , o r b o t h t o t h e f l o w e r - b u d a f t e r e x c i sion of the leaves did not noticeably stimulate the elongation growth of the
370
A
B
Fig. 2. Effect of IAA (A) and of BA (B) on stem elongation of uncooled 'Apeldoorn' tulips in January 1978. A, from left to right: untreated tulip plant; leaves and flower excised, plain lanolin applied to the cut surface of the stem; leaves and flower excised and 0.2% IAA on the cut surface of the stem. B, left, defoliated tulip plant, flower-bud blasted;right, 0.2% BA in lanolin applied to the flower-bud resulted in flowering after which the stem started to elongate. s t e m i n t e r n o d e s (Table IV). A d m i n i s t r a t i o n o f BA, B A + I A A , BA + G A 3 , o r BA + I A A + GA3 did n o t result in i m m e d i a t e s t e m e l o n g a t i o n , b u t in s o m e plants t h e s t e m s t a r t e d t o e l o n g a t e a f t e r s o m e time. T h e final stem length was c o m p a r a b l e t o t h a t f o u n d a f t e r a d m i n i s t r a t i o n o f I A A t o the stem t o p , b u t t h e basal i n t e r n o d e r e m a i n e d s h o r t e r and t h e u p p e r i n t e r n o d e e l o n g a t e d m o r e t h a n a f t e r a p p l i c a t i o n o f I A A t o t h e s t e m t o p . This p h e n o m e n o n was o n l y o b s e r v e d in plants given BA alone o r in c o m b i n a t i o n s and w h o s e flowerb u d s r e a c h e d t h e flowering-stage (Fig. 2B). T h e o n s e t o f stem e l o n g a t i o n coi n c i d e d w i t h t h e start o f p e r i a n t h p i g m e n t a t i o n . It s h o u l d be m e n t i o n e d t h a t
371 the pistil of such plants showed a tremendous development; its final length was a b o u t 50 mm, which is quite unusual for 'Apeldoorn' tulips. DISCUSSION Our results show clearly that the elongation growth of the stem is under the control of the leaves and the flower, probably because these organs produce and release auxin-like substances. These findings are in agreement with those made in earlier investigations. (E.J. Fortanier, 1970, personal communication; Op den Kelder et al., 1971; Hanks and Rees, 1977). Hanks and Rees (1977) found that soft-drenches with ancymidol reduced tulip-stem elongation. The effects were mainly on the first internodes and were reversed b y GA. Such observations, and the fact that administration of GA to bulbs when the cold requirement is n o t fully satisfied stimulates the elongation growth, led the authors to the suggestion that a gibberellin-mediated system regulates the growth of lower internodes. Our results show that elongation of all internodes is p r o m o t e d b y auxins, probably produced in the leaves and the flower-bud. It can be an indication that GA stimulates the auxin-release or response system. Excision of the leaves resulted, in most cases, in complete inhibition of all internodes, even though the flower-bud was still present. This is understandable, because the growth of the flower usually ceases and the floral organs desiccate after the removal of the leaves. The effect of leaf excision on flower-bud development can be overcome b y application of cytokinins to the flower-bud, which might be an indication that the leaves provide cytokinins to the flower. Cytokinins strengthen the sink of the flower-bud, and sustain the flow of substrate from the bulb-scales to the flower bud (De Munk and Gijzenberg, 1977). The elongation of the stem and leaves of tulips is due almost entirely to elongation of cells produced during earlier developmental stages (Gilford and Rees, 1973). This process is greatly enhanced b y low-temperature treatments either before or after planting and rooting of the bulbs. It has been reported (Van Bragt and Zijlstra, 1971; l~udnicki et al., 1976) that part of the coolingeffect could be replaced b y gibberellins. In our experiments b o t h cooled and uncooled tulips reacted to the administration of auxin, although uncooled tulips elongated much more slowly and to a lower degree than cooled tulips. Thus, part of the cooling-effect can also be replaced b y auxin administration. We cannot conclude whether these effects of IAA and GA concern the same process. The crux of the question is the nature of the cooling-effect. It seems likely that the levels of b o t h cytokinins and gibberellins in the shoot increase during cooling, thus promoting the sink activity of the shoot organs, which in turn produce their own auxins. This is in agreement with the analyses made b y Rakhimbayev et al. (1978),who studied the biological activity of endogenous auxins, inhibitors, gibberellins and cytokinins in bulbs of Tulipa alberti Regel, stored at 22 or 4°C for 8 weeks. The hormonal situation
372
changed in the tissue during exposure of the bulbs to cold: the bound forms of gibberellins were found in the free state and the activity of cytokinins i m creased, whereas the activity of auxins did n o t change appreciably. The abscisic-acid-like growth inhibitor did n o t disappear on cold treatment, but its level dropped. On the other hand, it must be kept in mind that phytohormones also require a response mechanism. It might be that the receptor sites of the p h y t o h o r m o n e s are affected by the cold-treatment, which could explain why the application if IAA to the flowers of uncooled tulip plants had no effect.
REFERENCES Aung, L.H. and De Hertogh, A.A., 1967. The occurrence of gibberellin-like substances in tulip bulbs (Tulipa sp.). Plant Cell Physiol., 8: 201--205. Aung, L.H. and De Hertogh, A.A., 1968. Gibberellin-like substances in non-cold and cold treated tulip bulbs (Tulipa sp.). In: F. Wightman and G. Setterfield (Editors), Biochemistry and Physiology of Plant Growth Substances. Runge Press, Ottawa, pp. 943--956. De Munk, W.J. and Gijzenberg, J., 1977. Flower-bud blasting in tulip plants mediated by the hormonal status of the plant. Scientia Hortic., 7 : 2 5 5 - - 2 6 8 . Gilford, J.McD. and Rees, A.R., 1973. Growth of the tulip shoot. Scientia Hortic., 1: 143--156. Hanks, G.R. and Rees, A.R., 1977. Stem elongation in tulip and narcissus: the influence of floral organs and growth regulators. New Phytol., 78: 579--591. Kamerbeek, G.A., Beijersbergen, J.C.M. and Schenk, P.K., 1970. Dormancy in bulbs and corms. Proc. 18th. Int. Hortic. Congr., Tel Aviv, 1970, Vol. 5: 233--240. Op den Kelder, P., Benschop, M. and De Hertogh, A.A., 1971. Factors affecting floral stalk elongation of flowering tulips. J. Am. Soc. Hortic. Sci., 96: 603--605. Rakhimbayev, I.R., Syrtanova, G.A. and Solomina, V.F., 1978. The effect of cold treatment on the level of biological activity of endogenous growth regulators of tulip bulbs. Plant Physiol. (Russ.), 2 5 : 2 4 9 - - 2 5 3 (in Russian). Rees, A.R., 1972. The growth of bulbs. Academic Press, London, 311 pp. Rudnieki, R.M., Nowak, J. and Saniewski, M., 1976. The effect of gibberellic acid on sprouting and flowering of some tulip cultivars. Scientia Hortic., 4: 387--397. Van Bragt, J. and Zijlstra, F.A., 1971. Effects of gibberellins on flowering of tulip cv. 'Apeldoorn'. Z. Pflanzenphysiol., 64: 139--144.