Planta (Berl.) 77, 224--232 (1967)

The Influence of Light, Gibberellic Acid and CCC on Sprout Growth and Mobilization of Tuber Reserves in the Potato (Solanum tuberosum L.) D. A. Mom~is University of Nottingham School of Agriculture, Sutton Bonington, England Received August 2, 1967

Summary. 'Warm white' fluorescent light inhibited the elongation of potato sprouts and reduced the rate at which reserve materials in the mother tuber were mobilized. The evidence indicated that the influence of light on mobilization was independent of light effects on the magnitude of the 'sink' for translocates in the sprout. However, dark-grown sprouts exerted a greater directive influence than illuminated sprouts on the translocation of the mobilized materials. The rate of elongation and increase in dry weight of sprouts was promoted by exogenous GA independently of the effects of light on these processes, and the absence of any effects of CCC on sprout length and dry weight in these experiments was interpreted to indicate that light did not influence these parameters by controlling the level of endogenous gibberellins. Neither GA nor CCC modified the effects of light on the dry matter content of the sprouts, suggesting that growth in darkness and growth induced by GA were qualitatively different. The results are compared with the effects of light and GA on growth and stem elongation in other species. Introduction The r a t e of s p r o u t g r o w t h in t h e p o t a t o (Solanum tuberosum L.) during storage is closely c o r r e l a t e d w i t h t h e initial weight of t h e t u b e r piece to which t h e s p r o u t is a t t a c h e d (HEADFO:aD, ]962; MORRIS, 1966). I n a previous s t u d y it was f o u n d t h a t s p r o u t g r o w t h c o n t i n u e d after t h e onset of complete interference b e t w e e n t h e s p r o u t s on m u l t i - s p r o u t t u b e r s s t o r e d in light, a n d t h a t t h e r a t e of g r o w t h was i n v e r s e l y r e l a t e d to t h e n u m b e r of c o m p e t i n g sprouts (Mo~ms, 1966). I t was concluded t h a t s p r o u t g r o w t h was l i m i t e d b y t h e r a t e a t which t h e reserve m a t e r i a l s in t h e t u b e r were mobilized.

Many studies have been made of the pathways of synthesis and degradation of reserve materials in the potato tuber (cf. BgRTON, 1961 ; EMrLSSON and LII~D~LOM, 1963) but the mechanism by which the reactions involved in these pathways are regulated and the part, if any, played by the sprouts in this regulation have received relatively little attention. EDEn~AN and SI~GH (1966) demonstrated a very rapid

Influence of Light and GA on Sprout Growth in the Potato

225

mobilization of carbohydrates in tubers bearing a single apical sprout which was allowed to root in moist sand in darkness at 25 o C. Degradation of starch also occurred in debudded control tubers, but at a greatly reduced rate, indicating t h a t while mobihzation was not entirely dependent on the presence of growing sprouts, it was greatly enhanced b y them. ED~LMA~ and SI~G~ (1966) raise the i m p o r t a n t question of whether the mobilization of the reserves is regulated t h r o u g h the removal of substrates b y the growing sprouts, or whether the sprouts exert a more direct control possibly b y secreting into the tuber hormones which m a y influence enzyme activity. Observations t h a t competition between sprouts on multi-sprout tubers during storage and emergence can result in large reductions in individual sprout weight (Mon~IS, 1966; 1967), and t h a t the total d r y weight of sprouts per tuber m a y be constant irrespective of the n u m b e r of growing sprouts (H~ADFG~D, 1961 ; Mon~IS, 1966), support the view t h a t the rate of mobilization is independent of the size of the ' s i n k ' for d r y m a t t e r in the sprout. On the other h a n d it is well k n o w n t h a t under certain conditions (as in darkness and after planting) the rate of transfer of d r y m a t t e r from tuber to sprouts is considerably greater t h a n it is during normal storage conditions in light. This suggests t h a t mobilization can be modified b y external factors acting via the sprouts themselves. I n order to gain more information about the factors controlling mobilization of tuber reserves the rate of sprout growth has been modified b y storing t h e m in darkness and b y the application of gibberelHc acid (GA) and (2-chloroethyl) t r i m e t h y l a m m o n i u m chloride (CCC), and the effects of these treatments on the transfer of d r y material from the tuber to the sprout has been investigated.

Materials and Methods In all the experiments described tubers of S. tuberosum cv. Arran Pilot were used. These were grown from certified Scottish seed tubers which were treated at the time of planting with the systemic aphicide 'Disyston' (Baywood Chemicals Ltd.) to reduce the risk of virus infection. All the experiments were carried out in constant temperature rooms controlled to • 1~ Where treatments involved exposure of the plants to light this was provided by fluorescent tubes (Philips 'Warm White') at a visible radiation intensity of approximately 1.0 cal cm2 hour -1. GA was applied as a solution of a commercially available sample (British Drug Houses Ltd.) and the solutions were made up by dissolving a known weight in a few drops of ethanol and making up to volume with glass distilled water. CCC (Koch-Light Laboratories Ltd.) was applied as an aqueous solution. The solutions were applied to tubers through polyethylene tubes (10.0 cm long • 0.5 cm internal diameter) inserted into holes cut 1.0 cm deep in the tuber with a sterile cork borer and approximately 1.0 cm from the base of the bud under study. The tubers were thoroughly washed and were surface sterilized with 0.1 per cent HgCl 2

226

D.A. MORRIS:

before use. Aseptic techniques were employed to insert the tubes and to apply the solutions. Dose-response curves to GA in light and darkness (Fig. 1) were determined using essentially the same techniques.

IP~IA 30

--,f /o

o

7L____.~.,,

I /

I /o

I I /gO ppm 100o

~A concen/ra/ion

Fig. 1. Dose-response curves for GA-treated sprouts grown in the presence (open circles) and absence (closed circles) of light at 20~C. Measurements made 120 hours from start of experiment Experiments and Results

1. Comparative e//ects o/ light and darkness on sprout growth Tubers in the weight range 95 g to 105 g were removed from storage at 4~ debudded to leave either 1 or 2 lateral buds per tuber (mean initial length 8.0:~ 0.18 mm), and stored at 15~ either in light or in continuous darkness. Each treatment was replicated 5 times. D r y weight m e a s u r e m e n t s m a d e 35 days later i n d i c a t e d t h a t b o t h i n light a n d i n darkness sprout growth was limited b y the s u p p l y of substrates from the tuber. U n d e r b o t h conditions of storage, the m e a n d r y weight of sprouts on single-sprout t u b e r s was a p p r o x i m a t e l y twice Table 1. Ef/ect of light and number of sprouts per ~uber on the growth of sprouts at

15 ~ C ( • S.E. of means). Duration of treatments 35 days Storage treatment

~o. of sprouts per tuber

Fresh weight (mg per sprout)

Dry weight (mg per sprout)

Light Light Dark Dark

1 2 1 2

1,588 • 148.6 935• 57.3 6,688 • 485.0 3,838 -4-409.9

358 • 35.4 2 0 3 i 11.8 781 4- 51.2 436 4- 41.0

Influence of Light and GA on Sprout Growth in the Potato

227

t h a t of s p r o u t s on t w o - s p r o u t t u b e r s (Table 1) i n d i c a t i n g c o m p l e t e interference for essential g r o w t h factors. H o w e v e r , t h e t o t a l weight of d r y m a t e r i a l t r a n s f e r r e d from t u b e r to s p r o u t in d a r k n e s s was more t h a n twice t h e weight t r a n s f e r r e d in light, i n d i c a t i n g t h a t light influenced t h e a v a i l a b i l i t y of s u b s t r a t e s in t h e tuber. The d a r k - g r o w n s p r o u t s c o n t a i n e d a c o n s i d e r a b l y g r e a t e r p r o p o r t i o n of w a t e r p e r u n i t d r y weight t h a n s p r o u t s e x p o s e d to light. The m e a n w a t e r c o n t e n t s were 3.38 :J= 0.191 g a n d 7.71 ~= 0.406 g p e r g dry m a t t e r

/

8V g/sp)ouf 6

o

2

T

0.2

[

I

I

Qz/

06"

08

Meandryw@hl

I

I0

9~sprout

Fig. 2. Relationship between water content and dry weight for sprouts grown in the presence (open circles) and absence (closed cire]es) of light. Regression coefficients (b) = 7.710 4- 0.4063 (dark) and 3.376 • 0.1910 (light)

in light- and dark-grown sprouts respectively. Within each light treatment there was a close correlation between the water content of the sprouts and their dry weight (Fig. 2). In view of the observation that exposure of sprouts to light reduced the availability of substrates in the tuber, a second experiment was carried out to investigate the effect of exposing sprouts to light on the growth of a sprout maintained in darkness on the same tuber. Twenty-four tubers weighing 57.2-t-1.92 g were debudded to leave two lateral buds of uniform size on each tuber (initial bud length 4.5 4-0.14 ram). Eight tubers were stored in continuous light, and eight in continuous darkness. One bud on each of the remaining tubers was shaded with a light-tight black paper tube sealed to the tuber with modelling clay, and these tubers were then stored in continuous light at the same intensity as the fullyilluminated controls. All the tubers were maintained at 15~ The length and fresh and dry weights of each sprout were measured 23 days later. E a c h s p r o u t on t h e t u b e r s which received t h e differential light t r e a t m e n t b e h a v e d quite i n d e p e n d e n t l y of t h e o t h e r s p r o u t on t h e same t u b e r . The i n d i v i d u a l s p r o u t s on these t u b e r s d i d n o t differ significantly from

228

D.A. MORRIS:

t h e s p r o u t s in t h e corresponding control t r e a t m e n t s in a n y of t h e characters m e a s u r e d (Table 2). As in t h e previous e x p e r i m e n t t h e r a t e of a c c u m u l a t i o n of d r y m a t t e r b y t h e sprouts grown in darkness was a p p r o x i m a t e l y twice t h a t of t h e s p r o u t s exposed to light. These results suggest t h a t a l t h o u g h t h e r a t e of m o b i l i z a t i o n of t h e reserves was c o n s i d e r a b l y g r e a t e r in t u b e r s b e a r i n g s p r o u t s grown in darkness, all t h e a d d i t i o n a l s u b s t r a t e s released were i n c o r p o r a t e d into t h e s h a d e d sprouts. Table 2. E//ect o] di//erential light treatment on the growth o/sprouts at 15~ C ( • S.E. o/means). Duration of experiment 23 days Storage treatnaent

Length (ram per sprout)

Fresh weight (nag per sprout)

Dry weight (nag per sprout)

Dark control Differential treatment: Dark Light Light control

48.7 ~= 3.43

946 • 108.3

116 4- 7.2

41.34-3.50 19.4~0.67 17.5 ~=0.47

1,040• 96.6 3394- 27.3 302 4- 15.0

136• 10.0 71=L 5.1 63 =E 3.0

2. E//ects o / G A , CCC and light on sprout growth and mobilization o] tuber reserves Tubers weighing 22 g to 26 g were debudded to leave a single lateral bud on each tuber (initial length 7.3 • 0.21 ram) and stored at 15~ in darkness or under constant artificial illumination. One ml of a saturating (200 ppm) solution of GA (cf. Fig. 1), 1.0 ml of 10-s M CCC, or 1.0 m] of sterile distilled water was supplied to each tuber initially and on days 2, 6 and 10 by the method outlined above. There were 10 tubers in each treatment. Sixteen days later the final length and the fresh and dry weights of the sprouts were determined. E x p o s u r e of t h e sprouts to white light significantly r e d u c e d t h e r a t e of s p r o u t elongation b o t h in t h e presence a n d absence of s a t u r a t i n g doses of GA, t h e r e l a t i v e r e d u c t i o n in l e n g t h due to light being a p p r o x i m a t e l y t h e same in t r e a t e d a n d u n t r e a t e d t u b e r s (Table 3). These o b s e r v a t i o n s Table 3. E/feet o/GA (200 ppm), CCC (10-a M) and light on the elongation o/potato sprouts at 15~ C. Mean initial sprout length 7.3 ~ 0.21 mm; duration o] experiment 16 days Treatment

Final length (mm per sprout)

Mean increase in length (nana)

t~eduction in length due to light (%)

Light Light Light Dark Dark Dark

22.3 ~ 0.86 44.9 • 3.63 21.0 • 0.46 39.4 ~=4.62 80.1 ~ 8.80 45.9 -J: 3.58

15.0 37.6 13.7 32.1 72.8 37.7

53.3 48.4 63.7 ----

0 ~- GA + CCC 0 + GA + CCC

Influence of Light and GA on Sprout Growth in the Potato

229

suggested that light and GA influenced stem elongation b y independent mechanisms. CCC did not significantly influence sprout length either in light or in darkness. Changes in dry weight followed a similar pattern to changes in sprout length (Table 4). The mean sprout dry weight was reduced by exposure to light, the percentage reduction being of the same order in both GA-treated and untreated tubers. I n GA-trcated tubers light reduced the mean dry weight of the sprouts b y 23.3 per cent compared with a reduction of 30.4 per cent in untreated tubers. GA treatment itself increased the rate of transfer of dry material from tubers to sprouts. Sprouts treated with CCC did not differ significantly from untreated sprouts, either in light or darkness. As in the previous experiments, the water content of the dark-grown sprouts was considerably greater than t h a t of the illuminated sprouts, but none of the chemical treatments significantly influenced water content (Table 4). Table 4. E//ect o/ GA, CCC and light on the accumulation o/ dry matter by potato sprouts and on their water content. Experimental conditions as in Table 3

Treatment

Fresh weight (rag per sprout)

Dry weight (rag per sprout)

Mean water content (g per g dry matter)

Light Light Light Dark Dark Dark

385 • 35.1 546 ~ 42.4 358 ~: 18.2 850 ~: 103.1 1,172 =~ 71.8 890 • 65.2

73 • 6.2 89 :~ 6.4 69 • 3.6 95 • 7.0 128 • 6.2 98 • 5.3

4.27 5.13 4.32 7.95 8.14 8.08

0 + GA @ CCC 0 + GA + CCC

Discussion Previous experiments (Mon~IS, 1966) have indicated t h a t the limit imposed on sprout growth during storage of seed tubers is probably the rate at which reserve materials in the mother tuber are mobilized. The proportional reduction in mean sprout dry weight when the number of sprouts per tuber was increased from one to two, both in light and darkness, indicated t h a t under both conditions of illumination sprout growth was limited by the availability of substrates from the tuber. This suggests t h a t light in some way reduced the rate at which the tuber reserves were mobilized, and furthermore, t h a t the effect of light on mobilization was not merely an indirect one operating by increasing the magnitude of the ' s i n k ' for substrates in the growing sprouts. However, the results presented in Table 2 indicate t h a t shaded sprouts exert a greater directive influence on the translocation of the mobilized materials than do sprouts exposed to light. I t therefore seems t h a t the effects of

230

D.A. M o ~ i s :

light on mobilization of the reserves and on sprout growth per se are largely independent. A number of experiments not reported here have clearly shown that the light influencing mobilization is perceived by the sprout itself and not by the tuber. This suggests that the inhibitory effect of light on mobilization is mediated by a hormonal messenger. Saturating doses of GA were shown to increase the length (Table 3) and dry weight (Table 4) of sprouts on tubers stored either in light or in darkness, the relative influence of GA being the same under both conditions of illumination. LOCKItAI~T(1956, 1959, 1961, 1964) has shown that in Pisum sativum and in Phaseolus vulgaris red-light inhibition of stem elongation may be reversed by exogenous GA, and has proposed that light may control internode elongation in these plants by influencing the level of endogenous gibberellins. M o ~ and APPvE~ (1962) were unable to confirm these observations during their studies of hypoeotyl elongation in Sinapsis alba. In this species GA did not remove the controlling influence of the phytochrome system on elongation, and the relative inhibitory influence of light was essentially the same in plants treated with saturating doses of GA as in untreated plants. No evidence was found to suggest that in Sinapsis alba light acted to reduce the effective level of endogenous gibberellins. The results presented here for Solanum tuberosum are in agreement with the findings of M o ~ and A P P c ~ (1962) and indicate absence of interaction between light and exogenous GA in their effects both on the rate of sprout elongation and on the rate of transfer of dry material from the tuber to the sprout. It was concluded that in the potato the action of light and exogenous GA on these processes were independent. Such a conclusion does not exclude the possibility that light may still control sprout growth by influencing the level of endogenous gibberellins if these differ from GA. CCC has been shown to block gibberellin biosynthesis in Fusarium monili/orme (KENDE et al., 1963; NI~N~MANN et al., 1964; HAVANA and LA~O, 1965), although recently it has been shown that strains of the fungus differ in their sensitivity to CCC and similar growth retardants (ME~Tz and H ~ s o ~ , 1967). There is some evidence that CCC may similarly inhibit biosynthesis of gibberellins in higher plants ( K 6 E z ~ , 1965; Z ~ V ~ T , 1966; REIn and CA~, 1967). If CCC inhibits gibberellin synthesis in potato sprouts, it is unlikely that the response to light exhibited by the sprouts studied in the present investigation was brought about by an effect of light on the level of endogenous gibberellins. Provided that its cells are living and fully turgid, the water content of ~n organ m~y be regarded as a crude estimate of its total cell volume (cf. McCoMB, 1966), and therefore the relationship between the dry weight of a plant organ and its water content is a measure of dry matter

Influence of Light and GA on Sprout Growth in the Potato

231

per unit cell volume. I t is clear t h a t in the present experiments the weight of d r y material accumulated per unit cell volume was considerably higher in light t h a n in darkness (cf. Fig. 2 and Tables 2 and 4). This suggests t h a t in light a greater proportion of secondary wall material was laid down in the cells of the sprouts. Neither GA nor CCC significantly influenced the relationship between the d r y weight and water content of the sprouts, suggesting t h a t etiolated growth and growth induced b y GA were qualitatively different. A similar failure of GA to influence the relationship between d r y weight and water content of pea internodes was observed b y McCoMB (1966), who concluded t h a t exogenous GA stimulated the synthesis of p r i m a r y cell wall material. I n w o o d y species WA~EING (1958) found t h a t GA stimulated division of the c a m b i u m and t h a t indoleacetic acid resulted in the vacuolation and lignifieation of the new cambial derivatives. I t is possible t h a t in the present experiments light stimulated the deposition of secondary cell wall material b y controlling the level of a hormone other t h a n gibberellin. The author is indebted to Professor F. L. MILT~OR~E and Dr. J. MOORBYfor their interest and advice during the course of this work which was financed by a Research Grant from the Agricultural Research Council. Technical assistance from Miss V. J. CAR~VT~E~S and Miss J. E. CE~LENGn~ is gratefully acknowledged. Part of this work was included in a thesis submitted for the Ph.D. degree of Nottingham University. References BURTON,W. G. : The physiology of the potato: problems and present status. Proc. 1st Triennial Conf. Europ. Assoc. Potato l%es. 1960, 79--117 (1961). EDELMAN,J., and S. P. SI~G~: Studies on the biochemical basis of physiological processes in the potato tuber. Changes in carbohydrates in the sprouting tuber. J. exp. Bot. 17, 696--702 (1966). EMILSSON,B., and It. LI~DBLO~: Physiological mechanisms concerned in sprout growth. In: The growth of the potato. Proc. 10th Easter School agrie. Sci. Univ. Nott. 45--62 (1963). HA~ADA,H., and A. LA~G: Effects of some (2-cMoroethyl) trimethylammonium chloride analogs and other growth retardants on gibberellin biosynthesis in Fusarium monili/orme. Plant Physiol. 40, 176--183 (1965). HEADFOI%D,D. W. R. : Sprout growth of the potato. Ph.D. thesis, Univ. of Nottingham (1961). - - Sprout development and subsequent plant growth. Eur. Potato J. 5, 14--22 (1962). KENDE, H., H. ~II~NE~I~I% and A. LANG: Inhibition of gibberellic acid biosynthesis in Fusarium moniliforme by Amo-1618 and CCC. Naturwissenschaften 50, 599--600 (1963). KSHLEtr D. : Die Wirkung yon schwachem Rotlicht und Chlorocholinchlorid auf den Gibberellingehalt normaler Erbsens~mlinge und die Ursaehe der unterschiedlichen Empfindliehkeit yon Zwerg- und iNormalerbsensi~mlingen gegen ihr eigenes Gibberellin. Planta (Berl.) 67, 44--54 (1965).

232

D.A. MORRIS: Influence of Light and GA on Sprout Growth in the Potato

LOCKI~ART,J. A. : Reversal of the light inhibition of pea stem growth by the gibberellins. Proe. nat. Acad. Sci. (Wash.) 42, 841--848 (1956). -Studies on the mechanism of stem growth inhibition by visible radiation. Plant Physiol. 34, 4 5 7 ~ 6 0 (1959). - - Interactions between gibberellin and various environmental factors on stem growth. Amer. J. Bot. 48, 516--525 (1961). - - Physiological studies on light sensitive stem growth. Planta (BEE.) 62, 97--115 (1964). McCoMB, A. J.: The stimulation by gibberellic acid of cell wall synthesis in the dwarf pea plant. Ann. Bot. N. S. 39, 155--163 (1966). MEI~TZ,D., and W. HENSON: The effect of the plant growth retardants Amo 1618 and CCC on gibberellin production in Fusarium monili/orme: Light stimulated biosynthesis of gibberellins. Physiol. Plant. (Kobenhavn) 20, 187--199 (1967). M o i l , H., and U. APPIzI~N:Die Stcuerung des tIypocotylwachstums yon Sinapsis alba L. durch Licht und Gibberellins~Lnre.Planta (Berl.) 59, 49--67 (1962). MORRIS, D. A. : Intcrsprout competition in the potato I. Effects of tuber size, sprout number and temperature on sprout growth during storage. Eur. Potato J. 9, 69--85 (1966). - - Intersprout competition in the potato II. Competition for nutrients during pre-emergence growth after planting. Eur. Potato J. (in press). ~INNEMANlq,H., J. A. D. ZEEYAART,H. K:ENDE, and A. L)_~G: The plant growth retardant CCC as inhibitor of gibberellin biosynthesis in Fusarium monili/orme. Planta (Berl.) 61, 229--235 (1964). REID, D./-Y[., and D. J. CAI~: Effects of a dwarfing compound, CCC, on the production and export of gibberellin-likesubstances by root systems. Pl~nta (Berl.) 73, 1--11 (1967). WAREINC,P . F . : Interaction between indole-acetic acid and gibberellic acid in cambial activity. Nature (Lond.) 181, 174--175 (1958). ZEEVAART,J. A. D.: Reduction of the gibberellin content of Pharbitis seeds by CCC: After-effects on the progeny. Plant Physiol. 41, 856--862 (1966). Dr. D . A . MORRIS Department of Botany University of Southampton, England