Planta (1990)182:113-117

P l a n t a 9 Springer-Verlag 1990

Effect of sink isolation on sugar uptake and starch synthesis by potato-tuber storage parenchyma K.J. Oparka, H.V. Davies, K.M. Wright, R. Viola, and D.A.M. Prior Department of Cellular and EnvironmentalPhysiology,Scottish Crop Research Institute, Invergowrie,Dundee DD2 5DA, UK Received 4 January; accepted 4 April 1990

Abstract. Import into potato (Solanum tuberosum L. cv. Record) tubers was terminated by removing the sink at its connection with the stolon. The ability of discs of storage tissue from the excised tubers to take up exogenous sugars and convert them to starch was compared with that of discs from untreated tubers from the same plant population. In rapidly-growing control tubers, glucose and fructose were taken up to a greater extent than sucrose, 77% of the glucose being converted to starch within 3 h (compared with 64% and 27% for fructose and sucrose, respectively). These values fell as the tubers aged but the ranking (glucose > fructose > sucrose) was maintained, emphasising a severe rate-limiting step following the import of sucrose into the growing tuber. Sink isolation had little effect on the ability of the storage cells to take up exogenous sucrose across the plasmalemma for up to 7 d after sink isolation. However, the ability of the same cells to convert the sucrose to starch was severely inhibited within 24 h, as was the sensitivity of starch synthesis to turgor. In the case of glucose, sink isolation inhibited both the uptake and the conversion to starch, the latter being inhibited to a greater degree. A detailed metabolic study of tubers 7 d after excision showed that, with sucrose as substrate, 94% of the radioactivity in the soluble sugar pool was recovered in sucrose following sink isolation (92% in control tubers). However, with glucose as substrate, 80% of the radioactivity was recovered as sucrose following tuber excision (28% in control tubers), providing evidence that sucrose synthesis acts as a major alternative carbon sink when starch synthesis is inhibited. In the same tubers, sucrose-synthase activity decreased by 70% following sink isolation, compared with a 45% reduction in ADPglucose pyrophosphorylase. Activities of UDP-glucose pyrophosphorylase, starch phosphorylase, starch synthase and both PPi- and ATP-dependent phosphofructokinases remained unchanged. Acid-invertase activity increased fivefold.

Key words: Enzyme activity (potato tuber) - Sink isolation (sugar uptake) - Solanum (tuber) - Starch synthesis - Sugar transport

Introduction Carbon import into sink tissues is maintained by a variety of mechanisms depending on the nature of the sink (Thorne 1986). It appears that the rate of import is affected mainly by the predominant allocation process. In 'utilisation' sinks, e.g. meristematic tissues, most of the imported assimilate is used for growth, only a small amount being stored temporarily, whereas in 'storage' sinks, substantial amounts of imported carbon are stored for long periods in insoluble forms, commonly as starch (Ho 1988). A popular view in the literature is that in sinks which store predominantly starch, starch synthesis may be the controlling step for assimilate import into the sink cell, the rate of starch synthesis regulating the rate of import (see reviews by Thorne 1986; Ho 1988). The potato tuber represents a massive storage sink in which starch represents about 70% of the dry matter (Oparka 1986). In previous publications we have demonstrated that both the uptake of exogenous sucrose and its subsequent conversion to starch are acutely sensitive to the water relations of the storage-parenchyma cells (Oparka and Wright 1988a, b). Starch synthesis is optimised at low, but positive cell turgors (80 kPa), considerably lower than the estimated turgor of fresh tissue (320 kPa). One possible explanation for enhanced starch synthesis in the presence of external osmotica is that cytosolic solute levels are raised at low turgot via the action of the plasmalemma H+-ATPase, resulting in increased substrate availability for starch synthesis (Oparka and Wright 1988 b). If this hypothesis is correct then the plasmalemma emerges as a potential regulator of starch synthesis via the control of sucrose fluxes into the storage cell. We were interested in determining the effect of depleting cytosolic substrate levels on both the uptake and the conversion of sugars to starch. We therefore terminated carbon import into the tubers by sink isolation and subsequently examined the ability of discs derived from the excised tubers to take up and convert exogenous sugars relative to discs from untreated tubers from the

114 s a m e p l a n t p o p u l a t i o n . In this p a p e r we c o m p a r e the u p t a k e a n d c o n v e r s i o n o f sucrose with t h a t o f glucose a n d d e m o n s t r a t e m a j o r differences in the m e t a b o l i s m o f these sugars f o l l o w i n g sink i s o l a t i o n .

Material and methods

Plant material. Potato (Solanum tuberosum L. cv. Record) plants were grown from seed tubers (40~5 mm diameter) in square (20.20 cm 2) pots containing compost. When the plants were 10 cm high (16 June 1988) individual stems were replanted into separate pots. There was no supplementary lighting. Glasshouse temperatures were maintained at 20~ C during the day and 15~ C at night. Sink isolation. On five occasions (between 8 15 weeks following planting), import was terminated by cutting the stolons at the base of the tubers. The excised (treated) tubers were then left undisturbed in the compost. Control plants were left intact. In experiments using sucrose alone, import was terminated after 8 weeks. The plants were then harvested at 24 h, 3 d, 7 d and 14 d after treatment. In subsequent experiments, in which sucrose uptake was compared with that of glucose, the plants were harvested at 7 d only following tuber excision. The experiments were conducted between 8 and 15 weeks after planting at dates specified in the figure legends. At each harvest, the daughter tubers growing on three plants per treatment were removed and discs cut from the perimedulla of each tuber. The discs from each treatment were selected for use at random. Sucrose-uptake experiments. In experiments with sucrose alone, the procedures for washing discs, equilibration in different mannitol concentrations and uptake of [14C]sucrose were exactly as described previously (Oparka and Wright 1988 a, b). Following washing in 300 mM mannitol, 25 mM 2-(N-morpholino)ethanesulfonic acid (Mes)-KOH pH 6.5, the discs were preincubated with the appropriate osmoticum for 30 min before transfer to a second solution containing 5 mM or 50mM sucrose and [U-14C]sucrose (9.25 kBq. ml-1, specific activity 20 GBq. mmol-1; Amersham International, Amersham, Bucks., UK). After 3 h incubation the discs were washed for 3 • 4 min and the radiolabel in the soluble and starch fractions determined as described previously (Oparka and Wright 1988a, b). This procedure does not take account of effiux of radiolabel during the incubation period and the data were therefore expressed as net total sugar accumulation to reflect this loss. Comparative sugar-uptake experiments. Radiolabelled sucrose (specific activity 20 GBq. mmol- 1), glucose (11.47 GBq- mmol- 1) and fructose (8.10 GBq-mmol-1) were supplied separately to discs at 10 kBq.m1-1 in either 5 mM or 50 mM unlabelled carrier sugar plus 25 mM Mes-KOH, pH 6.5. Discs from control and treated plants were incubated simultaneously under identical conditions for 3 h. All incubations were conducted in the presence of 300 mM mannitol (also determined as the osmotic optimum for the conversion of glucose and fructose to starch; data not shown). Extraction of soluble ~4C and starch was as described above. Analysis of radioactive sugars. After treating extracts with ion-exchange resin to remove organic acids and amino acids (Davies 1987), individual sugars from day-7 samples were separated by high-performance liquid chromatography on a 15-cm NH 2 column. The solvent was acetonitrile:water (85:15, v/v). Determination of enzyme activities. Enzymes were extracted from the tuber discs at 7 d following tuber excision and also from control tubers. Extraction was performed at 4~ in 0.1 M Tris-acetate, pH 8.5, containing EDTA (20 mM), diethyldithiocarbamic acid (10 mM), cysteine-HC1 (15 mM) and 6% polyethylene glycol (MW 3 350). Aliquots were removed for the measurement of total starch-

K.J. Oparka et al. : Sink isolation in potato-tuber storage synthase activity (Hawker et al. 1979) and the remainder centrifuged at 3000 .g prior to desalting on Sephadex G-25 (Sigma, Poole, Dorset, UK) columns. Both ADP-glucose (ADPGlc) and UDPGlc pyrophosphorylases were determined by the method of Sowokinos (1981), starch synthase and invertase by the protocols of Davies et al. (1989), hexokinase, PPi and ATP-dependent phosphofructokinases by the methods of Morrell and ap Rees (1986) and sucrose synthase by the method of Sung et al. (1989).

Results

Effect of sink isolation on turgor-sensitive sucrose partitioning. P l a n t s t r e a t e d b e t w e e n 8 a n d 15 weeks after p l a n t i n g s h o w e d a similar r e s p o n s e w i t h respect to the t o t a l u p t a k e o f sugar, a l t h o u g h b y 15 weeks levels o f s t a r c h synthesis were reduced. T h e effect o f sink isolation on n e t t o t a l [x4C]sucrose a c c u m u l a t i o n over a r a n g e o f e x t e r n a l m a n n i t o l c o n c e n t r a t i o n s is s h o w n in Fig. 1 ( a - d ) for p l a n t s t r e a t e d 8 weeks after p l a n t i n g . N o t e t h a t net a c c u m u l a t i o n into the tissue discs was m a i n t a i n e d at relatively c o n s t a n t levels for at least 3 d following the c e s s a t i o n o f i m p o r t , b u t declined after this, p r o b a b l y as a result o f i n c r e a s i n g t u b e r age ( W r i g h t a n d O p a r k a 1989). H o w e v e r , d e s p i t e the decline, the discs f r o m the t r e a t e d t u b e r s a c c u m u l a t e d similar a m o u n t s o f sucrose as those f r o m the c o n t r o l t u b e r s at each m a n nitol c o n c e n t r a t i o n . A t each s a m p l i n g d a t e there was a n increase in net a c c u m u l a t i o n w i t h increasing e x t e r n a l m a n n i t o l c o n c e n t r a t i o n (see also O p a r k a a n d W r i g h t 1988b). A p a r t f r o m the t o t a l a m o u n t o f s u g a r e n t e r i n g the tissue, there was no significant difference in the o b served r e s p o n s e with either 50 m M o r 5 m M sucrose ( d a t a n o t shown). T h e influence o f sink i s o l a t i o n o n the c o n v e r s i o n o f sucrose to starch is s h o w n in Fig. 2 ( a - d ) . In c o n t r a s t to net t o t a l sucrose a c c u m u l a t i o n , the c o n v e r s i o n o f sucrose to s t a r c h was greatly r e d u c e d in discs f r o m the t r e a t e d p l a n t s w i t h i n 24 h o f sink i s o l a t i o n (Fig. 2a). D e s p i t e the large r e d u c t i o n in s t a r c h synthesis after 24 h, the o s m o t i c o p t i m u m for s t a r c h synthesis at 300 m M m a n n i t o l (see O p a r k a a n d W r i g h t 1988a, b) was still evident, a l t h o u g h g r e a t l y reduced. By d a y 7, h o w e v e r , this o p t i m u m was b a r e l y e v i d e n t (Fig. 2a~d). T h e a m o u n t o f s t a r c h synthesised in the c o n t r o l p l a n t s also d e c r e a s e d with time, p r o b a b l y also as a c o n s e q u e n c e o f increasing t u b e r age (see W r i g h t a n d O p a r k a 1989).

Comparative uptake and conversion of sucrose, glucose and fructose. T h e a m o u n t s o f sugars t a k e n u p by the discs a n d c o n v e r t e d to s t a r c h a t 300 m M m a n n i t o l are s h o w n in Table 1 for r a p i d l y - g r o w i n g c o n t r o l t u b e r s 8 weeks after planting. N o t e t h a t c o n s i d e r a b l y m o r e glucose was t a k e n u p t h a n either fructose o r sucrose a n d t h a t u p to 7 7 % o f the glucose was c o n v e r t e d to s t a r c h w i t h i n 3 h ( c o m p a r e d with 6 4 % a n d 2 7 % for fructose a n d sucrose respectively). T h e a b o v e values were f o u n d to decline as the t u b e r s a g e d ( d a t a n o t shown). N o n e t h e less, the r a n k i n g (glucose > fructose > sucrose) with respect to s t a r c h synthesis was m a i n t a i n e d .

Effect of sink isolation on sucrose and glucose metabolism. Because o f the e x t r e m e differences o b s e r v e d b e t w e e n glu-

K.J. Oparka et al. : Sink isolation in potato-tuber storage -

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Fig. 1 a-tl. Net sucrose accumulation (soluble + starch) into potatotuber discs in response to increasing mannitol concentrations. Discs were isolated from excised tubers ( m - - m ) , or control (zx--zx) tubers at 24 h (a), 3 d (b), 7 d (e) and 14 d (d) after sink isolation

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Fig. 2a-d. The amount of sucrose converted to starch in isolated potato-tuber discs. Time course and treatments as in Fig. I

cose and sucrose, the uptake and conversion of these sugars was examined at 7 d following sink isolation. Table 2 shows the effect of sink isolation on sugar uptake and starch synthesis in tubers which were excised from the plants at 15 weeks following planting. Note that the net accumulation of sucrose was reduced by only 11%, compared with 37% for glucose. However, there was approximately a 70% reduction in the conversion of both sugars to starch. In the case of glucose, despite the inhibition of starch synthesis, the amount of sugar entering the soluble fraction was unaltered.

Table 1. Net accumulation (nmol. (g F W ) - t. h - 1 ) and partitioning

to starch of 5 m M sucrose, glucose and fructose by potato-tuber discs derived from control tubers. The experiment was conducted 8 weeks after planting. Data are presented as mean + SE (n = 3) Sugar

Net accumulation (soluble + starch)

Starch

% Starch

Sucrose Glucose Fructose

164.8-1-10.4 554.5+_ 0.7 190.9+10.2

45.4+ 4.2 430.5-t- 2.1 122.9-1-11.2

27.5 77.6 64.1

116

K.J. Oparka et al. : Sink isolation in potato-tuber storage

Table 2. Effect of sink isolation on the net total accumulation and partitioning to starch of 5 mM sucrose and glucose. The experiment was initiated 15 weeks after planting. Discs from control and treated potato tubers were compared at 7 d after treatment. Data are presented as mean + SE; n=5 tubers Sugar

Accumulation (nmol. (g FW)- 1.h 1) as Soluble

Sucrose Control Excised Glucose Control Excised

Starch

88.0_+3.9 92.6_+3.1

% Starch

Total

23.1+ 5.9 6.5_+ 0.6

169.7+6.8 206.1-+20.7 171.7-+6.2 64.4_+ 5.4

111.2+ 9.2 99.1_+ 2.9

19.7 6.6

375.8-+21.7 236.1_+ 1.6

54.4 27.3

Table 3. Percentage distribution of 14C between glucose, fructose and sucrose after incubating discs from treated and control tubers for 3 h with either 5 mM [14C]glucoseor [14C]sucrose. The experiment was initiated at 15 weeks after planting and the tubers harvested at 7 d after sink isolation. Data are presented as mean _+SE; n = 5 tubers

14C p r e c u r s o r

Distribution of 14C (%) Glucose

Fructose

Sucrose

Sucrose Control Excised

3.5_+ 1.9 4.2 _+1.3

4.6_+0.3 2.1 _+0.3

91.9_+ 10.7 93.7 -I-12.2

Glucose Control Excised

63.6+4.8 15.0_+2.2

8.4_+ 1.9 2.0+1.1

28.0_+ 4.7 80.1 _+ 8.0

Table 4. Effect of sink isolation on enzyme activities in potato tubers. The experiment was initiated 15 weeks after planting and activities in discs derived from treated tubers were determined at 7 d after excision (mean -+ SE; n = 5 tubers)

Enzyme

Sucrose synthase ADPGlc-pyrophosphorylase UDPGlc-pyrophosphorylase Invertase (acid) PPi-phosphofructokinase ATP-phosphofructokinase Starchsynthase Hexokinase Glucose substrate Fructose substrate

Activity (nmol. (g FW)- 1. rain- 1) Control

Excised

52.3 _+ 7.9 137.4 _+ 10.2 1563.7 _+120.7 0.95_+ 0.4 663.0 + 27.1 96.0 -+ 8.3 222.0 -+ 1 8 . 7

15.9_+ 3.2 75.6_+ 9.9 1480.7+ 100.9 4.4_+ 0.7 718.9_+37.1 99.5-+ 12.6 189.3_+15.7

17.6 _+ 3.2 38.7 _+ 4.1

16.7+ 2.9 32.9_+ 4.9

Analysis of radioactive sugars. In both control and treated tubers more than 90% o f the label recovered in the soluble sugar fraction following incubation with [14C]sucrose was recovered in sucrose (Table 3). However, tuber excision increased the incorporation of [14C]glucose into sucrose almost threefold.

Enzyme activities. As shown in Table 4, sucrose-synthase activity in tubers 7 d after treatment was 70% lower than in controls, A D P G l c pyrophosphorylase 45% lower and invertase fivefold higher. Activities o f UDPGlc-pyrophosphorylase, starch phosphorylase, starch synthase, hexokinase (glucose substrate) and PPiand ATP-dependent phosphofructokinases were not significantly affected. Discussion

Removal of potato tubers from the parent plant caused a rapid loss in the starch-synthesising capacity of the storage cells. This inhibition of starch synthesis appeared to have little effect on the ability of the storage cells to take up sucrose from the apoplast. The net accumulation of sucrose into the tissue was relatively unaffected by sink isolation at either 5 m M or 50 m M external sugar concentrations. In the case o f 5 m M sucrose, this concentration is clearly on the saturable (carrier-mediated) phase of the sugar-uptake curve (Oparka and Wright 1988a, b). It is therefore unlikely that sugar uptake at this concentration was maintained by diffusion alone. Sucrose is the major translocated sugar into potato tubers (Oparka and Prior 1987). If starch synthesis alone was driving import of sucrose into the tuber then one might expect that its inhibition would feed back on sucrose uptake from the apoplast. However, this did not occur, at least in the short term (24 h), and it would appear that the events influencing sucrose uptake at the plasmalemma may be regulated independently from those influencing starch synthesis. In the case of glucose, sink isolation had a considerable influence on the partitioning of carbon within the storage cell. After 7 d, uptake occurred at a reduced level relative to controls. As Table 3 shows, sucrose biosynthesis formed an important sink for carbon not utilised in starch production. It is likely that much of the sucrose synthesised in the cytosol is stored in the vacuole. Hydrolysis of sucrose in the cytosol of excised tubers is likely to be limited by the low activity of sucrose synthase, coupled with its low affinity for substrate (sucrose K m= 130 m M ; Pressey 1969). This low affinity would also explain why in attached, growing tubers only approx. 25% of sucrose is incorporated into starch (see also Oparka and Wright 1988a, b), compared with up to 80% of glucose, i.e. sucrose synthase represents a rate-limiting step for starch synthesis in vivo. Similar differences in glucose and sucrose incorporation into starch have been noted for developing seeds of Vicia faba (data not shown), adding weight to the argument of Sung et al. (1989) that in many storage sinks sucrose-synthase activity may be an important determinant of sink strength. As yet we have no explanation for the intermediate value (65% conversion to starch in rapidly-growing tubers) obtained for fructose (see Table 1) although the consistent differences between sugars merit further investigation. In the case of sucrose, uptake is unaffected by sink isolation, despite the reduction in starch synthesis, presumably by continued compartmentation into the va-

K.J. Oparka et al. : Sink isolation in potato-tuber storage cuole. Studies of carbon import into potato tubers in vivo have shown that the rate of import into the tuber is positively correlated with its sucrose content (Oparka 1985; Engels and Marschner 1986). The reverse relationship might be expected of a sink in which starch synthesis alone was maintaining a sucrose concentration gradient between source and sink. Thus, available data support the concept that a large proportion of the imported sucrose enters the vacuole. Similar c o m p a r t m e n t a t i o n of sucrose taken up f r o m the apoplast of isolated discs would account for the observed unaffected uptake of sucrose in the absence of starch synthesis. Competition between sucrose synthase in the cytosol and an efficient sucrose-transport mechanism on the tonoplast might together limit the availability o f sucrose for starch biosynthesis. In previous studies we have demonstrated that the incorporation of sucrose into starch increases as the sucrose-uptake rate is increased by lowering cell turgor (Oparka and Wright 1988 a, b). This p h e n o m e n o n might also be explained, in part, by the low affinity of sucrose synthase for its substrate, increased uptake across the p l a s m a l e m m a raising the concentration of sucrose in the cytosol. In the present study, the turgot-stimulated uptake of sucrose into excised-tuber discs would not be expected to induce increased partitioning into starch because of the adverse effect of sink isolation on sucrosesynthase activity (see Fig. 2). As noted previously (Richardson et al. 1990) tuber excision stimulated the development of acid-invertase activity. However, activity was extremely low c o m p a r e d with the other enzymes assayed. In the presence of the proteinaceous invertase inhibitor (Pressey 1966) activity in vivo m a y be insufficient to provide the hexoses required to maintain high rates of starch production in excised tubers. Low rates o f hexose effiux f r o m the vacuole could also contribute to the observed effect. The reduced rate of starch synthesis observed in excised tubers with glucose as a precursor clearly indicates that, in addition to sucrose synthase, other processes and-or reactions are affected adversely. The absence of any significant effect of excision on hexokinase activity suggests that the capacity for phosphorylation is unaffected. The reduction in A D P G l c - p y r o p h o s p h o r y l a s e activity m a y contribute to the observed events but it cannot be stated categorically that this is the primary reason for a reduction in starch synthesis. Restricted transport of intermediates into the amyloplast m a y also have contributed to the observed inhibition of starch synthesis. Recent studies (Viola et al. 1990) have shown that a sixcarbon c o m p o u n d enters the amyloplast to support starch synthesis in the potato tuber. The accumulation of phosphorylated hexoses would provide additional substrate for sucrose formation catalysed by sucrosephosphate synthase. Fine (metabolite) control of key reactions might also have been affected. In s u m m a r y , glucose is preferred to sucrose as a substrate

for starch synthesis in growing potato tubers, the supply of this sugar bypassing the enzyme sucrose synthase,

117 which has a low affinity for its substrate. Sink isolation severely inhibited starch synthesis when either sucrose or glucose were provided as substrates, sucrose uptake being unaffected for up to 7 d following sink isolation. In the case of glucose, net uptake was reduced by sink isolation and sucrose biosynthesis provided a m a j o r carbon sink following the inhibition o f starch synthesis. It now remains to be shown to what extent the above observations apply to other types o f storage sinks in which starch is the m a j o r reserve.

References Davies, H.V. (1987) Rapid determination of glucose, fructose and sucrose in potato tubers by capillary gas chromatography. Potato Res. 31,569-572 Davies, H.V., Jefferies, R.A., Scobie, L. (1989) Hexose accumulation in cold-stored tubers of potato (Solanum tuberosum L.). The effects of water stress. J. Plant Physiol. 134, 471-475 Engels, C.H., Marschner, H. (1986) Allocation of photosynthate to individual tubers of Solanum tuberosum L. J. Exp. Bot. 37, 1804-1812 Hawker, J.S., Marschner, H., Krauss, A. (1979) Starch synthesis in developing potato tubers. Physiol. Plant. 46, 25-30 Ho, L.C. (1988) Metabolism and compartmentation of imported sugars in sink organs in relation to sink strength. Annu. Rev. Plant Physiol. Plant Mol. Biol. 39, 355-378 Morrell, S. ap Rees, T. (1986) Sugar metabolism in developing tubers of Solanum tuberosum L. Phytochemistry 25, 1579-1585 Oparka, K.J. (1985) Changes in partitioning of current assimilate during tuber bulking in potato (Solanum tuberosum L.) Ann. Bot. 55, 705-713 Oparka, K.J. (1986) Phloem unloading in the potato tuber. Pathways and sites of ATPase. Protoplasma 131,201-210 Oparka, K.J., Prior, D.A.M. (1987) 14C sucrose efflux from the perimedulla of growing potato tubers. Plant Cell Environ. 10, 667-675 Oparka, K.J., Wright, K.M. (1988a) Osmotic regulation of starch synthesis in potato tubers? Planta 174, 123-126 Oparka, K.J., Wright, K.M. (1988b) Influence of cell turgor on sucrose partitioning in potato tuber storage tissues. Planta 175, 520-526 Pressey, R. (1966) Separation and properties of potato invertase and invertase inhibitor. Arch. Biochem. Biophys. 133, 667-674 Pressey, R. (1969) Potato sucrose synthetase - purification, properties and changes in activity associated with maturation. Plant Physiol. 44, 759-764 Richardson, D.L., Davies, H.V., Ross, H.A., Mackay, G.R. (1990) Invertase activity and its relation to hexose accumulation in potato tubers. J. Exp. Bot. 41, 95-99 Sowokinos, J.R. (1981) Pyrophosphorylases in Solanum tuberosum. 2. Catalytic properties and regulation of ADP-glucose and UDP-glucose pyrophosphorylase activities in potatoes. Plant Physiol. 69, 1459-1466 Sung, S-J. S., Xu, D.-P., Black, C.C. (1989) Identification of actively filling sucrose sinks. Plant Physiol. 89, 1117-1121 Thorne, J.H. (1986) Sieve tube unloading. In: Phloem transport, pp. 211-224, Cronshaw, J., Lucas, W.J., Giaquinta, R.T., eds. Liss, New York Viola, R., Davies, H.V., Chudeck, J.A. (1990) NMR investigations on starch and sucrose interconversion in growing storage organs. J. Exp. Bot. Abstracts of Papers and Posters on Topics Relevant to Plant Biology Presented at the 1990 Annual Meeting of the Society for Experimental Biology [P8.05] Wright, K.M., Oparka, K.J. (1989) Sucrose uptake and partitioning in discs derived from source versus sink potato tubers. Planta 177, 237-244