Ripe pollen carbohydrate changes in Trachycarpus fortunei: the effect of relative humidity

Pollen of the palm Trachycarpus fortunei was kept at 25°C and relative humidities (RH) of 20, 55 and 98%. Changes in viability, water content and carbohydrates were measured over 2–17 days. Water content remained almost constant at 20 and 50% RH and increased dramatically at 98%. Pollen viability and germination rate remained almost constant over 14 days at 20% RH and decreased to about 2% after 7–9 days at 55% and to even less at 98% RH. Although the three experimental conditions were constant, qualitative and quantitative variations in pollen carbohydrates were recorded, even after pollen had lost its viability. The quantities of mono-, di- and polysaccharides varied with the period of pollen storage at the various RH. The greatest changes in glucose, fructose and sucrose content were recorded at 55 and 98% RH. At these relative humidities, maximum glucose and fructose content and minimum sucrose content occurred at maximum water content. Starch was not present in mature pollen but appeared and peaked after 7–9 days of pollen storage at 55 and 98%. Appearance of starch coincided with an increase in pectin content. PAS-positive cytoplasmic polysaccharides showed an increasing trend at 20% RH. A relation was found between pollen viability, water content and monosaccharide content. Pollen viability and germination capacity remained high at 20% RH for 14 days. At this relative humidity, pollen water, glucose and fructose contents remained almost constant, while sucrose reached its maximum value. The fluctuations of more complex carbohydrates (starch, pectins and PAS-positive cytoplasmic polysaccharides) were less easy to interpret. Changes observed under experimental conditions could simulate processes occurring in nature during pollen presentation and dispersal.     

甘草叶片渗透调节物质及蔗糖代谢相关酶对干旱胁迫的响应特性

该研究以甘草幼苗为试材,采用盆栽自然干旱方法,设计对照(CK)、轻度(LS)、中度(MS)、重度(SS)干旱胁迫处理,测定分析甘草叶片的渗透调节物质及蔗糖代谢相关酶[蔗糖磷酸合成酶(SPS)、蔗糖合成酶合成方向(Ss+)、蔗糖合成酶分解方向(Ss-)、中性转化酶(NI)、酸性转化酶(AI)、淀粉磷酸化酶(SP)]活性的变化,以探讨甘草的渗透调节特性以及糖分调节的酶学机制,揭示甘草对干旱胁迫的响应机理。结果显示:(1)随着干旱胁迫程度的加剧,甘草叶片可溶性糖、可溶性蛋白和脯氨酸含量呈逐渐增加的趋势,束缚水/自由水的比值呈先增加后降低的趋势。(2)随着干旱胁迫程度的加剧,甘草叶片蔗糖、葡萄糖、果糖含量均呈先升高后降低的趋势,但不同胁迫强度出现峰值的时间不同;其中在CK和LS干旱胁迫时蔗糖含量淀粉含量葡萄糖含量果糖含量,在MS和SS干旱胁迫时淀粉含量葡萄糖含量蔗糖含量果糖含量,表明随着干旱程度的增加,甘草叶片中蔗糖转化成了淀粉。(3)随着干旱胁迫程度的加重,甘草叶片的SPS活性呈先升高后降低的趋势,Ss活性和Inv(蔗糖转化酶)活性呈逐渐升高的趋势,SP活性呈逐渐降低的趋势;各胁迫处理的Ss+活性与CK差异不显著,而Ss-活性与CK差异显著,并且Ss-活性在各胁迫处理下远大于Ss+活性,表明甘草叶片Ss-活性在蔗糖代谢过程中占主导作用。(4)相关分析结果显示,在LS中,NI与蔗糖呈负相关关系,Ss-与淀粉呈显著正相关关系、与蔗糖呈负相关关系;在MS中,蔗糖和葡萄糖与SPS、Ss+、Ss-、NI和AI均呈正相关关系,与SP呈负相关关系;在SS中,SP和NI与蔗糖呈正相关关系,而与淀粉呈负相关关系;表明在轻度干旱处理下,Ss参加了蔗糖的分解,继而合成淀粉;在中度和重度干旱条件下,SP主要催化淀粉的分解来增加蔗糖含量以此平衡蔗糖代谢。     

Storage,mobilization and interrelations of starch,sugars, protein and fat in the ray storage tissue of poplar trees

The seasonal pattern in starch, various sugars, protein, and fat, and their interrelationship, has been followed in 3-year-old branch wood of poplar trees (Populus x canadensis Moench robusta) under natural site conditions. The deposition of starch, protein and fat proceeds at different times. Starch accumulates from May until October, fat mainly during the summer months, and protein when the leaves are yellowing in September and October. The maximum concentrations in the branch wood were 15–18 g starch, 6–9 g protein, 4–8 g fat, 10–15 g sucrose, and up to 30 g total sugars per milligram dry weight (DW). During starch deposition periods no increased sucrose level is found in the tissue. The maximum daily starch deposition rate was 0.2–0.4 g starch/day/mg DW of wood. During starch hydrolysis in late autumn and winter, a dramatic increase in sucrose and its galactosides is measured (up to 15–27 g/mg DW in total). In early spring, before budbreak, the concentrations of these sugars diminishes sharply. In contrast to this clear-cut starch-to-sugar conversion in autumn no significant starch-to-fat conversion is detected. An elevated content of free glycerol, however, is found in winter. In spring, starch and storage protein are mobilized completely, or almost completely, in poplar twig wood. A noteworthy pool of maltose is found transiently during autumn (up to 8 g/mg DW) and again in spring. The results demonstrate that the individual storage materials, e.g. starch, protein, and fat, are accumulated fairly independently in the wood storage parenchyma. Tissue sugar levels, in contrast, appear to be closely related to the seasonal variations in starch content, on the one hand, and to the acclimation and deacclimation of the cells, on the other. The interrelations of the storage materials and sugars are discussed.     

Seasonal Dynamic of Nonstructural Saccharides in a Rhizomatous Grass Calamagrostis Epigeios

Seasonal dynamic of total nonstructural saccharides (TNS) and individual saccharides (starch, sucrose, glucose, fructose, fructans) was followed in rhizomes and stem bases of Calamagrostis epigeios (L.) Roth at two types of meadows communities in the South Moravia (Czech Republic): cnidion and molinion alliances, which differ in their water regime. The TNS were formed mainly by fructans and starch, while glucose, sucrose and fructose were low. The amount of TNS in rhizomes and stem bases of plants from wet cnidion site was higher than in plants from drier molinion site. The seasonal trends of all saccharides were similar in the both sites. During growing season (June to October) the main storage sugar was fructan (18 – 21 % of dry biomass). At the beginning of September the content of fructan decreased to 10 – 12 % and simultaneously the content of sucrose increased from 1 to 3 %. This may increase frost resistance. The content of TNS in the stem bases was lower than in the rhizomes. During winter time the stem bases contained 2 to 2.5 % of sucrose. Plant height and aboveground biomass were also higher in molinion site.     

Influence of cell turgor on sucrose partitioning in potato tuber storage tissues

Sucrose uptake and partitioning in potato (Solanum tuberosum L.) tuber discs were examined under a range of mannitol and ethylene-glycol concentrations. Mannitol caused the same changes in turgor over a wide range of incubation periods (90 min-6 h), indicating that it did not penetrate the tissue. In comparison, ethylene glycol reduced turgor losses but did not eliminate them, even after 6 h. Between 100 mM and 300 mM mannitol, turgor fell by 350 kPa, compared with 35 kPa in ethylene glycol. Uptake experiments in mannitol alone showed that total sucrose uptake was strongly correlated with both osmotic potential and with turgor potential. In subsequent experiments sucrose uptake and partitioning were examined after 3 h equilibration in 100 mM and 300 mM concentrations of mannitol and ethylene glycol. Total sucrose uptake and the conversion of sucrose to starch were enhanced greatly only at 300 mM mannitol, indicating an effect of turgor, rather than osmotic potential on sucrose partitioning. The inhibitors p-chloromercuribenzenesulfonic acid and carbonylcyanide m-chlorophenylhydrazone (CCCP) both reduced sucrose uptake, but in quite different ways. p-Chloromercuribenzenesulfonic acid reduced total sucrose uptake but did not affect the partitioning of sucrose to starch. By contrast, CCCP inhibited total uptake and virtually eliminated the conversion of sucrose to starch. Despite this, sucrose uptake in the presence of CCCP continued to increase as the mannitol concentration increased, indicating an increase in passive transport at higher mannitol concentrations. Increased sucrose uptake above 400 mM mannitol was shown to be the result of uptake into the free space. The data show that starch synthesis is optimised at low but positive turgors and the relation between sucrose partitioning and the changing diurnal water relations of the tuber are discussed.Abbreviations CCCP carbonylcyanide m-chlorophenylhydrazone - PCMBS p-chloromercuribenzenesulfonic acid     

Carbohydrates and carbohydrate enzymes in developing cotton ovules

Patterns of carbohydrates and carbohydrate enzymes were investigated in developing cotton ovules to establish which of these might be related to sink strength in developing bolls. Enzymatic analysis of extracted tissue indicated that beginning 1 week following anthesis, immature cotton seeds (Gossypium hirsutum L. cv. Coker 100A glandless) accumulated starch in the tissues which surround the embryo. Starting at 15 days post anthesis (DPA), this starch was depleted and starch simultaneously appeared in the embryo. Sucrose entering the tissues surrounding the embryo was rapidly degraded, apparently by sucrose synthase; the free hexose content of these tissues reached a peak at about 20 DPA. During the first few weeks of development these tissues contained substantial amounts of hexose but little sucrose; the reverse was true for cotton embryos. Embryo sucrose content rose sharply from the end of the first week until about 20 DPA; it then remained roughly constant during seed maturation. Galactinol synthase (EC 2.4.1.x) appeared in the embryos approximately 25 days after flowering. Subsequently, starch disappeared and the galactosides raffinose and stachyose appeared in the embryo. Except near maturity, sucrose synthase (EC 2.4.1.13) activity in the embryos predominated over that of both sucrose phosphate synthase (EC 2.4.1.14) and acid invertase (EC 3.2.1.26). Activities of the latter enzymes increased during the final stages of embryo maturation. The ratio of sucrose synthase to sucrose phosphate synthase was found to be high in young cotton embryos but the ratio reversed about 45 DPA, when developing ovules cease being assimilate sinks. Insoluble acid invertase was present in developing cotton embryos, but at very low activities; soluble acid invertase was present at significant activities only in nearly mature embryos. From these data it appears that sucrose synthase plays an important role in young cotton ovule carbohydrate partitioning and that sucrose phosphate synthase and the galactoside synthesizing enzymes assume the dominant roles in carbohydrate partitioning in nearly mature cotton seeds. Starch was found to be an important carbohydrate intermediate during the middle stages of cotton ovule development and raffinose and stachyose were found to be important carbohydrate pools in mature cotton seeds.     

The role of the hexose transporter in the chloroplasts of Arabidopsis thaliana L.

The metabolism of wild-type Arabidopsis thaliana L. and its mutant TC265 were compared in order to reveal the role of the chloroplast glucose transporter. Plants were grown in a 12-h photoperiod. From 20 to 40 days after germination, starch per gram fresh weight of shoot in the mutant was four times that in the wild type. The extent of this difference did not alter during this period. Stereological analysis showed that the chloroplasts in the mutant were larger than those in the wild type; the thylakoids appeared to be distorted by the high starch content. [U-¹⁴C]Glucose and [U-¹⁴C]glycerol were supplied, separately, to excised leaves in the dark. [U-¹⁴C]Glucose was a good precursor of sucrose in the wild type and mutant; [U-¹⁴C]glycerol was a poor precursor of sucrose in both. The distribution of ¹⁴C in the wild type was used to calculate that the net flux was from hexose monophosphates to triose phosphates, not vice versa. During the first 4 h of the night the sugar content (75% sucrose, 20% glucose) of the leaves of the mutant dropped sharply, and at all times during the night it was less than that of the wild-type leaves. This drop in sugar coincided with a decrease in the rate of respiration. The growth rate of the mutant was less than that of the wild type. Addition of sucrose restored the rate of respiration at night and increased the rate of growth. It is argued that a major function of the glucose transporter in Arabidopsis chloroplasts is export of the products of starch breakdown that are destined for sucrose synthesis at night.We thank Professor C.R. Somerville for his generous gift of seed of the Arabidopsis mutant TC265. We are also grateful to Mr B. Chapman for assistance with the preparation of the sections for electron microscopy. R.N.T. thanks the Science and Engineering Research Council for a studentship.     

Metabolism of free sugars in relation to starch synthesis in the developing Sorghum vulgare grain

Accumulation of starch at expense of its free-sugar precursors was studied in the developing grains of the ‘SL-44’variety of Sorghum vulgare Pers. The content of starch gradually increased with the maturation of the grain and this increase was relatively fast until 18 days after anthesis. The daily rate of starch accumulation was at a maximum 15 days after anthesis. The content of total free sugars, reducing sugars, non-reducing sugars other than sucrose, total and non-sucrosyl fructose, and glucose also increased, reaching maximum values at 18 days after anthesis. Sucrose content gradually increased with a concomitant decrease in the activity of invertase, and sucrose was the major non-reducing sugar in the matured grains. Detached heads incubated in labelled sugars indicated that, compared to sucrose and fructose. ¹⁴C was more efficiently incorporated from glucose into grain starch, which was maximally synthesized at the mid-milky stage of grain development. Exogenous supply of NAD⁺ plus ATP stimulated the in vivo incorporation of ¹⁴C from sucrose to starch. The decline in the rate of starch accumulation did not synchronise with that of protein synthesis.     

Osmotic regulation of starch synthesis in potato tubers?

Using potato (Solanum tuberosum L.) tuber discs incubated in a range of mannitol concentrations it has been demonstrated that both sucrose uptake and the conversion of sucrose to starch are sensitive to the osmotic environment of the storage cells. Starch synthesis was optimised at 300 mM but declined sharply at both lower and higher osmotic concentrations. The decline in starch synthesis on either side of optimum was not proportional to the change in mannitol concentration, indicating different inhibitory mechanisms under low and high osmotica. The fraction of the total sucrose converted to starch i.e. the partitioning between sucrose and starch, was also influenced by osmotic environment. The amount of soluble material taken up by the storage cells, but not converted to starch, was maintained under mannitol concentrations (300–400 mM) which inhibited starch synthesis, indicating that sucrose uptake continued during declining starch synthesis. At mannitol concentrations above 400 mM, sucrose uptake was greatly enhanced but no significant change in starch synthesis occurred.     

The effect of sulphite on chlorophyll fluorescence and sucrose metabolism in poplar leaves

Sulphite at concentrations from 0.5 to 5.0 mM was supplied to illuminated, detached poplar (Populus deltoides Bartr. ex Marsh) leaves via the transpiration stream. Chlorophyll a fluorescence parameters, the contents of fructose-2,6-bisphosphate (Fru2,6BP) and starch, and extractable specific activity of sucrose-phosphate synthase (SPS), sucrose synthase (SuSy), acid invertase (AI), neutral invertase (NI), ATP-dependent fructose-6-phosphate 1-phosphotransferase (PFK) and pyrophosphate-dependent fructose-6-phosphate 1-phosphotransferase (PFP) were measured. Chlorophyll fluorescence parameters appeared to be unaffected by sulphite. Application of ≥ 1.0 mM sulphite led to an increase in the content of Fru2,6BP and starch. There was also a decline in the activity of SPS, NI and PFK. On the other hand, the influence of sulphite on the activity of AI and PFP was negligible. Specific activity of SuSy was inhibited by 1.0 and 2.5 mM but activated by 5.0 mM of sulphite. On the basis of the results obtained in the present study, we postulate that sulphite at concentrations ≥ 1.0 mM inhibits primarily sucrose synthesis, favours starch accumulation and has an indirect effect on the sucrolytic activities in poplar leaves.     

Role of calonyctin on free sugars in relation to starch accumulation in developing sweet potatoes

Calonyctin, a natural plant growth regulator extracted from the leaves of Calonyction aculeatum (L.) House, can promote crop growth and increase crop yield. The specific reasons for this response are unknown. This study was conducted to determine the effect of calonyctin treatment on the free sugars of sweet potato [Ipomoea batatas (L.) Lam.] as related to starch accumulation. The sweet potatoes were grown in the field in 1992, treated by foliar spray with Calonyctin concentrations of 0 (control) and 0.1 activity unit (CTSP) at 20 days after planting (DAP) at the rate of 190 liters of diluted solution/ha., and sampled periodically to determine free sugars. The response of sweet potato to calonyctin was first detected at 40 days after treatment (on 60 DAP). Data indicated that calonyctin treatment significantly increased starch synthesis in storage roots, decreased the fluctuation tendency of total sugar level during the growth period, and kept the sugar level relatively constant with a gradual rise regardless of variations in weather. The level of the reducing sugars in CTSP leaves was higher at 60 and 160 DAP and lower at 100, 120, and 140 DAP. During rainy days (100 DAP), the reducing sugars in CTSP storage roots remained at a lower level when those in controls reached high levels. The sucrose content in CTSP leaves was 40–138% greater than that in controls except at 80 and 120 DAP, and the ratio of sucrose to total nonreducing sugars remained at 100% in CTSP leaves even on rainy and cool days and above 96% in CTSP storage roots except on cool days (140 and 160 DAP), suggesting that calonyctin treatment promoted the synthesis and transfer of sucrose and supplied abundant sugar precursors for starch accumulation in storage roots.Abbreviations DAP days after planting - CTSP calonyctin-treated sweet potato with 0.1 activity unit     

Molecular regulation of sink–source transition in rice leaf sheaths during the heading period

The upper leaf sheath of rice (Oryza sativa L.) serves as a temporary starch sink before heading, subsequently becoming a carbon source tissue to the growing panicle at the post-heading stage. The time of sink–source transition in upper leaf sheaths is highly correlated to the panicle exsertion. Here, we found that the expression profiles of starch synthesis genes such as ADP-glucose pyrophosphorylase large subunit 2, granule-bound starch synthase II, soluble starch synthase I, starch branching enzyme (SBE) I, SBEIII, and SBEIV were highly correlated with starch content changes during the heading period in the second leaf sheath below the flag leaf. In addition, the α-amylase2A and β-amylase were considered as major genes that were in charge of starch degradation at the post-heading period. Of the five sucrose transporter (OsSUT) genes, OsSUT1 and OsSUT4 appeared to play an important role in sucrose loading into the phloem of source leaf sheaths. Moreover, the microarray-based data implied that the dominant processes associated with functional leaf sheath transition from sink to source were carbohydrate metabolism and the translocation of the carbon and nitrogen sources and inorganic phosphate.     

Magnesium deficiency in sugar beets alters sugar partitioning and phloem loading in young mature leaves

Magnesium deficiency has been reported to affect plant growth and biomass partitioning between root and shoot. The present work aims to identify how Mg deficiency alters carbon partitioning in sugar beet (Beta vulgaris L.) plants. Fresh biomass, Mg and sugar contents were followed in diverse organs over 20 days under Mg-sufficient and Mg-deficient conditions. At the end of the treatment, the aerial biomass, but not the root biomass, of Mg-deficient plants was lower compared to control plants. A clear inverse relationship between Mg and sugar contents in leaves was found. Mg deficiency promoted a marked increase in sucrose and starch accumulation in the uppermost expanded leaves, which also had the lowest content of Mg among all the leaves of the rosette. The oldest leaves maintained a higher Mg content. [¹⁴C]Sucrose labelling showed that sucrose export from the uppermost expanded leaves was inhibited. In contrast, sucrose export from the oldest leaves, which are close to, and export mainly to, the roots, was not restricted. In response to Mg deficiency, the BvSUT1 gene encoding a companion cell sucrose/H⁺ symporter was induced in the uppermost expanded leaves, but without further enhancement of sucrose loading into the phloem. The observed increase in BvSUT1 gene expression supports the idea that sucrose loading into the phloem is defective, resulting in its accumulation in the leaf.     

A convenient method for the synthesis of methyl 2-acetamido-2-deoxy-3-O-(2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl)-α-d-glucopyranoside

In the seeds of Tropaeolum and Tamarindus, which have been found to have the highest content of amyloids, these polysaccharides are associated with sucrose and O-d-galactosylsucroses of the raffinose-stachyose series. Balsamine seeds have a low content of amyloid and do not contain the aforementioned galactosides, which are otherwise widely distributed in plant seeds. They contain sucrose and a mono-O-D-galactosylsucrose that has been crystallized and identified as planteose. Planteose has previously been isolated from some Sympetalous plants. As far as we know, this is the first report of its occurrence in an Archichlamydeaeous plant seed.     

Intercellular compartmentation of sucrose synthesis in leaves of Zea mays L.

The incorporation of ¹⁴C into sucrose and hexose phosphates during steady-state photosynthesis was examined in intact leaves of Zea mays L. plants. The compartmentation of sucrose synthesis between the bundle sheath and mesophyll cells was determined by the rapid fractionation of the mesophyll and comparison of the labelled sucrose in this compartment with that in a complete leaf after homogenisation. From these experiments it was concluded that the majority of sucrose synthesis occurred in the mesophyll cell type (almost 100% when the time-course of sucrose synthesis was extrapolated to the time of ¹⁴C-pulsing). The distribution of enzymes involved in sucrose synthesis between the two cell types indicated that sucrose-phosphate synthetase was predominantly located in the mesophyll, as was cytosolic (neutral) fructose-1,6-bisphosphatase activity. Stromal (alkaline) fructose-1,6-bisphosphatase activity was found almost exclusively in the bundle-sheath cells. No starch was found in the mesophyll tissue. These data indicate that in Zea mays starch and sucrose synthesis are spatially, separated with sucrose synthesis occurring in the mesophyll compartment and starch synthesis in the bundle sheath.     

Effects of Chilling Temperature on the Activity of Enzymes of Sucrose Synthesis and the Accumulation of Saccharides in Leaves of Three Sugarcane Cultivars Differing in Cold Sensitivity

The effects of short-term exposure to chilling temperature (10 °C) on sucrose synthesis in leaves of the cold-tolerant sugarcane cultivars Saccharum sinense R. cv. Yomitanzan and Saccharum sp. cv. NiF4, and the cold-sensitive cultivar S. officinarum L. cv. Badila were studied. Plants were grown at day/night temperatures of 30/25 °C, and then shifted to a constant day/night temperature of 10 °C. After 52-h exposure to the chilling temperature, sucrose content in the leaves of NiF4 and Yomitanzan showed a 2.5- to 3.5-fold increase relative to that of the control plants that had been left on day/night temperatures of 30/25 °C. No such increase was observed in Badila leaves. Similarly, starch content in the leaves of NiF4 and Yomitanzan was maintained high, but starch was depleted in Badila leaves after the 52-h exposure. During the chilling temperature, sucrose phosphate synthase (SPS; E.C.2.4.1.14) activity was relatively stable in the leaves of NiF4 and Yomitanzan, whereas in Badila leaves SPS activity significantly decreased. There was no significant change in cytosolic fructose-1,6-bisphosphatase activity for the three cultivars at the chilling temperature. This supports the hypothesis that: (1) on exposure to chilling temperature, sucrose content in sugarcane leaves is determined by the photosynthetic rate in the leaves, and is not related to SPS activity; (2) SPS activity in sugarcane leaves at chilling temperature is to be determined by sugar concentration in the leaves.     

Changes in Mulberry Leaf Metabolism in Response to Water Stress

A series of experiments were conducted to characterize the water stress-induced changes in the activities of RuBP carboxylase (RuBPCO) and sucrose phosphate synthase (SPS), photosystem 2 activity, and contents of chlorophylls, carotenoids, starch, sucrose, amino acids, free proline, proteins and nucleic acids in mulberry (Morus alba L. cv. K-2) leaves. Water stress progressively reduced the activities of RuBPCO and SPS in the leaf extracts, the chlorophyll content, and PS2 activity in isolated chloroplasts. Plants exposed to drought showed lower content of starch and sucrose but higher total sugar content than control plants. While the soluble protein content decreased under water stress, the amino acid content increased. Proline accumulation (2.5-fold) was noticed in stressed leaves. A reduction in the contents of DNA and RNA was observed. Reduced nitrogen content was associated with the reduction in nitrate reductase activity. SDS-PAGE protein profile showed few additional proteins (78 and 92 kDa) in the water stressed plants compared to control plants.     

Changes in carbohydrate metabolism and assimilate export in starch-excess mutants of Arabidopsis

The aim of this work was to investigate the effects on carbohydrate metabolism of a reduction in the capacity to degrade leaf starch in Arabidopsis. The major roles of leaf starch are to provide carbon for sucrose synthesis, respiration and, in developing leaves, for biosynthesis and growth. Wild-type plants were compared with plants of a starch-excess mutant line (sex4) deficient in a chloroplastic isoform of endoamylase. This mutant has a reduced capacity for starch degradation, leading to an imbalance between starch synthesis and degradation and the gradual accretion of starch as the leaves age. During the night the conversion of starch into sucrose in the mutant is impaired; the leaves of the mutant contained less sucrose than those of the wild type and there was less movement of ¹⁴C-label from starch to sucrose in radio-labelling experiments. Furthermore, the rate of assimilate export to the roots during the night was reduced in the mutant compared with the wild type. During the day however, photosynthetic partitioning was altered in the mutant, with less photosynthate partitioned into starch and more into sugars. Although the sucrose content of the leaves of the mutant was similar to the wild type during the day, the rate of export of sucrose to the roots was increased more than two-fold. The changes in carbohydrate metabolism in the mutant leaves during the day compensate partly for its reduced capacity to synthesize sucrose from starch during the night.     

Effect of sink isolation on sugar uptake and starch synthesis by potato-tuber storage parenchyma

Import into potato (Solarium 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 ADP-glucose pyrophosphorylase. Activities of UDP-glucose pyrophosphorylase, starch phosphorylase, starch synthase nd both PPi- and ATP-dependent phosphofructokinases remained unchanged. Acid-invertase activity increased fivefold.