Spikelet Sterility and Flowering Response of Rice to Water Stress at Anthesis

Water deficits at the anthesis stage of rice (Oryza sativa L.)induce a high percentage of spikelet sterility and reduce grainyield. This study attempted to elucidate the direct effectsof water stress on panicle exsertion, spikelet opening, andspikelet desiccation leading to spikelet sterility. A well-wateredtreatment and two water stress levels were imposed in pot-grownplants of IRAT 13 (upland cultivar) and IR20 (lowland cultivar)at the time of flowering under greenhouse conditions A cultivar difference was observed in the flowering responseto water stress with a high sensitivity in IR20. The time courseof panicle exsertion showed an inhibitory effect due to thelow panicle water status. Low panicle water potentials significantlyreduced the number of opened spikelets. Spikelet opening wascompletely inhibited at panicle water potentials below –1·8MPa and –2·3 MPa in IR20 and IRAT 13, respectively.However, the peak spikelet opening time in a day was not influencedby the stress treatment. Spikelets in stressed panicles wereobserved to remain open for a longer period than in the well-wateredpanicles. The role of turgor in spikelet opening is also discussedin the study. At low panicle water potentials, severe desiccationof spikelets and anthers was noted. The deleterious effectsof water deficits on spikelet opening and spikelet water losscontributed to reduced spikelet fertility Oryza sativa L., rice, spikelet sterility, flowering, water stress, panicle water potentials, turgor potentials, desiccation     

Contribution of adenosine 5′-diphosphoglucose pyrophosphorylase to the control of starch synthesis is decreased by water stress in growing potato tubers

Water stress stimulates sucrose synthesis and inhibits starch and cell-wall synthesis in tissue slices of growing potato (Solanum tuberosum L. cv. Desirée) tubers. Based on the analysis of fluxes and metabolites, Geigenberger et al. (1997, Planta 201: 502–518) proposed that water deficits up to −0.72 MPa stimulate sucrose synthesis, leading to decreased starch synthesis as a result of the resulting decline of phosphorylated metabolite levels, whereas more-severe water deficits directly inhibit the use of ADP-glucose. Potato plants with decreased expression of adenosine 5′-diphosphoglucose pyrophosphorylase (AGPase) have been used to test the prediction that the contribution of AGPase to the control of starch synthesis should decrease in severely water-stressed tuber material. Freshly cut slices from wild-type and antisense tubers were incubated at a range of mannitol concentrations (20, 300 and 500 mM) and the metabolism of [¹⁴C]glucose was analysed. A 86–97% reduction of AGPase activity led to a major but non-stoichiometric inhibition of starch accumulation in intact growing tubers attached to the plant (40–85%), and an inhibition of starch synthesis in non-stressed tuber slices incubated in 20 mM mannitol (60–80%). The inhibition of starch synthesis was accompanied by a 2- to 8-fold increase in the levels of sugars in intact tubers and a 2- to 3-fold stimulation of sucrose synthesis in tuber slices, whereas respiration and cell-wall synthesis were not significantly affected. The strong impact of AGPase on carbon partitioning in non-stressed tubers and tuber slices was retained in slices subjected to moderate water deficit (300 mM mannitol, corresponding to −0.72 MPa). In discs incubated in 500 mM mannitol (corresponding to −1.2 MPa) this response was modified. A 80–97% reduction of AGPase resulted in only a 0–40% inhibition of starch synthesis. Further, the water stress-induced stimulation of sucrose synthesis was abolished in the transformants. The results provide direct evidence that the contribution of AGPase to the control of starch synthesis can be modified by environmental factors, leading to a lower degree of control during severe water deficits. There was also a dramatic decrease in the labelling of cell-wall components in wild-type tuber slices incubated with 300 or 500 mM mannitol. The water stress-induced inhibition of cell-wall synthesis occurred independently of AGPase expression and the accompanying changes in starch and sucrose metabolism, indicating a direct inhibition of cell-wall synthesis in response to water stress. Received: 24 February 1999 / Accepted: 28 May 1999     

Comparison of the effect of rapidly and gradually developing water-stress on carbohydrate metabolism in spinach leaves

Abstract. The effect of water-stress on photosynthetic carbon metabolism in spinach ( Spinacia oleracea L.) has been studied in experiments in which water-stress was induced rapidly by floating leaf discs on sorbitol solutions or wilting detached leaves, and in experiments in which water-stress was allowed to develop gradually in whole plants as the soil dried out. In both short- and long-term water stress, the rate of photosynthesis in saturating CO₂ did not decrease until leaf water potential decreased below -1.0 MPa. However, at smaller water deficits there was already an inhibition of starch synthesis, while sucrose synthesis remained constant or increased. This change in partitioning was accompanied by an increase in activation of sucrose-phosphate synthase (revealed as an increase in activity assayed in the presence of low hexose-phosphate and inorganic phosphate, while the activity assayed with saturating hexosephosphates remained unaltered). Water-stressed leaves had a two- to three-fold higher sucrose content at the end of the night, and contained less starch than non-stressed leaves. When leaves were held in the dark, sucrose was mobilized initially, while starch was not mobilized until the sucrose had decreased to a low level; in water-stressed leaves, starch mobilization commenced at a two-fold higher sucrose content. It is concluded that water-stressed leaves maintain higher sucrose and lower starch levels than non-stressed leaves. This response is found in rapid and long-term stress, and represents an inherent response to water deficits.     

The Effect of Low Temperature upon Starch, Sucrose and Fructan Synthesis in Leaves

Measurements on plants in a temperature gradient tunnel showthat diurnal accumulation of starch in illuminated leaves wasgreatly reduced at temperatures below 8 °C, whereas sucrosesynthesis was less affected under similar conditions. High chillingsensitivity for leaf starch accumulation was observed in a numberof chilling resistant temperate species. Enzymes of sucroseand fructan metabolism from mature leaves of Lolium temulentumwere less strongly inhibited at low temperatures than enzymesinvolved in starch synthesis. These results are discussed inrelation to carbon partitioning in species which grow and aremetabolically active at chilling temperatures. Lolium temulentum, starch, sucrose, fructan, temperature, enzyme activity, carbon partitioning     

Effect of modified carbon allocation on turgor, osmolality, sugar and potassium content, and membrane potential in the epidermis of transgenic potato (Solanum tuberosum L.) plants

The effects of modification in sugar concentrations on turgor pressure and membrane potential in epidermal leaf cells of transgenic potato (Solanum tuberosum cv. Desirée) plants were studied. Measurements of turgor pressure were performed by insertion of a micro pressure probe. Osmolality and sugar concentrations were determined by micro analysis of single cell extracts. Membrane potentials and cell diameters were calculated from repeated, computer-controlled scans with voltage-sensitive microelectrodes. Epidermal cells of sucrose transporter antisense plants showed a more than 100% elevation in osmolality and turgor pressure compared to the wild-type. As a consequence, cell diameters were enhanced in this transgenic line. However, membrane potentials were only slightly reduced in sucrose transporter antisense plants. In addition to sucrose transporter antisense lines, transgenic plants that were reduced in their capacity to accumulate starch due to antisense inhibition of the chloroplastic fructose-1,6-bisphosphatase (FBPase) were investigated. These antisense plants maintained membrane potential and turgor pressure comparable to the wild-type.     

Nitrate Accumulation and its Relation to Leaf Elongation in Spinach Leaves

The leaf elongation rate (LER) of spinach leaves during theday was twice that during the night when grown at a photon fluxdensity of 145 µmol m^–2 s^–1. All leaves showedthe same LER-pattern over 24 h. Due to low turgor, LER was lowin the afternoon and in the first hours of the night until wateruptake restored full turgor. Osmotic potential remained constantdue to increased nitrate uptake and starch degradation in thisperiod. LER increased to high rates in the second part of thenight and in the morning. The lower rate in the dark comparedto the light was not caused by the lower night temperatures,as increased photon flux density during growth resulted in equalrates in the light and the dark. Increased relative humiditydecreased LER and afternoon rates were most sensitive to waterstress. A ‘low light’ night period did not changeLER-pattern during the night or on the following day. We concludethat nitrate is not an obligatory osmoticum during the nightand can be exchanged for organic osmotica without decreasingLER. During the night the turgor is first restored by increasingwater uptake, nitrate uptake and starch degradation. This resultedin increased leaf fresh weight in this period. Thereafter, elongationincreased by simultaneous uptake of nitrate and water. Nitrateconcentration was, therefore, constant in the older leaves.In the younger leaves nitrate concentration increased to replacesoluble carbohydrates. The vacuoles of the old leaves were filledwith nitrate before those of the young leaves. Key words: Spinacia oleracea L., nitrate accumulation, osmotic potential, organic acids     

Sucrose metabolism and cell elongation in developing sunflower hypocotyls

The relationships between cell elongation and changes in specificactivities of enzymes of sucrose metabolism were investigatedin the growing region of hypocotyls of sunflower seedlings {Helianthusannuus L.) that were grown either in darkness or irradiatedwith continuous white light (WL). After transfer of dark–grownseedlings into WL an inhibition of cell elongation was observed.In etiolated stems, changes in enzymes of sucrose breakdown(acid invertase, sucrose synthase) were closely correlated withthe rate of cell elongation. Irradiation with WL induced a largedrop in acid invertase and a significant decrease in sucrosesynthase. The changes in concentration of sucrose were inverselycorrelated with the activities of the sucrose breakdown enzymes.A short–term experiment revealed that the effect of WLon growth was more rapid than the inhibitory effect on invertaseactivity. In dark–grown stems the activities of enzymesof sucrose biosynthesis (sucrose–phosphate synthase, ribulose1,5 bisphosphate carboxylase/oxygenase) were very low. AfterWL irradiation significant enhancements were measured. However,activities of enzymes of sucrose breakdown were still much largerthan those of sucrose biosynthesis, indicating that the green(de–etiolated) stem remains a sink for sucrose. We suggestthat the relative maintenance of cell osmotic pressure and turgorduring rapid cell elongation in darkness is due to enhancedhydrolysis of imported sucrose, which is cleaved by two enzymes(invertase, sucrose synthase). This process is regulated bylight and hence is under environmental control. Key words: Cell elongation, organ growth, Helianthus annuu, sucrose metabolism, source-sink relationship     

Relative water content and water potential of tissue 1

Relative water content (RWC) and water potential as measuredwith the pressure chamber were evaluated as indicators of waterstatus of tissue-cultured apple shoots and plantlets (shootswith roots). During the hydration required for RWC measurement,both water content and water potential exhibited the same hydrationkinetics, indicating that 10 h were required for full hydration.Once full hydration was reached, shoot mass remained relativelyconstant. Moisture release characteristics were also constructedand the associated shoot and plantlet water relations parameterswere estimated. Underin vitroconditions, both shoot and plantletwater potential were similar to the water potential of the culturemedium in which they were grown. The moisture release characteristicof shoots and plantlets was consistent with that expected fortypical plant tissues, and gave estimates of maximum modulusof elasticity (6.201.14 MPa), osmotic potential at saturation(–0.85 0.10 MPa), osmotic potential at zero turgor (–1.16 0.14 MPa) and RWC at zero turgor (78 2%) which were similarto values in the literature. Higher values of leaf conductanceand RWC were found in shoots and plantlets placed at 95% RH(21 C) compared to those at 90% RH. Plantlets had higher valuesof both conductance and RWC compared to shoots, suggesting thatinvitroroots are functional in water uptake. Relative water contentwas related to measures of physiological activity such as leafconductance, and it was also easier to measure than water potential.Relative water content is suggested as a sound index of waterstatus in tissue culture plants. Key words: Conductance, microculture, water status, water stress.     

Sucrose uptake by the phloem parenchyma of carrot storage root

The characteristics of sucrose uptake into the symplast of phloemtissue discs harvested from fresh, actively-growing carrot storageroots are described. Sucrose uptake exhibited a curvilinearresponse with increasing sucrose concentration. The inhibitorsp-chloromercuribenzenesulphonic acid (PCMBS) and carbonyl cyanidem-chlorophenylhydrazone (CCCP) decreased uptake and resultedin solely linear relationships between uptake and sucrose concentration.These results suggest that active carrier-mediated transportoccurs at the plasmalemma in addition to a diffusive mechanism.The former saturates at a lower concentration (approximately20 mM) than the latter which does not saturate below 100 mM.Though similar in their effect on the ethanol-soluble fraction,CCCP and PCMBS had different effects on the conversion of sucroseto ethanol-insoluble material. Varying the osmotic environment with different mannitol concentrationsdid not affect uptake between 0 and 400 mM mannitol, but didcause an increase at 600 mM mannitol: an effect which may havebeen an artefact of plasmolysis. Metabolic conversion to ethanol-insolubleforms remained unchanged from 0 to 250 mM mannitol and declinedabove this. Thus metabolism, but not uptake may be responsiveto changes in turgor. Key words: carrot, sucrose, uptake, transport, turgor     

Characteristics of the Photosynthetic Metabolism of Carbon in Deuterated Chlorella ellipsoidea C-27

The photosynthetic metabolism of carbon in fully deuteratedcells of Chlorella ellipsoidea C-27 (D-Chlorella), obtainedby culture in medium prepared with 100 mol% D₂O, was characterizedby examining the activities of several enzymes and the levelsof metabolic regulators in a comparison with those of ordinarycells (H-Chlorella). The cellular content of starch in D-Chlorellawas more than twice that in H-Chlorella, whereas those of sucroseand glucose were significantly lower in D-Chlorella. Deuterationof Chlorella caused marked alterations in the activities ofenzymes involved in starch metabolism. There was a significantdecrease in the activity of phosphorylase, a catabolic enzyme,and a significant increase in the activity of starch synthase,an anabolic enzyme. These alterations are probably responsiblefor the increase in the amount of starch in cells. By contrast,no marked changes were observed in the activities of enzymesand the levels of metabolic inhibitors that are involved inthe synthesis of sucrose. It seems likely, therefore, that thedecrease in the amount of sucrose in D-Chlorella was causedmainly by a deficiency in sources of carbon in the cytoplasm,as a consequence of an increase in levels of starch in chloroplasts. (Received May 13, 1992; Accepted December 1, 1992)     

Aspects of the Water Relations of Ileostylus micranthus (Hook. f.) Tieghem, a New Zealand Mistletoe

Leaf water potentials in the mistletoe, Ileostylus micranthusgrowing outdoors decreased rapidly during the early part ofthe day but remained relatively steady in the early afternoondespite increases in atmospheric vapour pressure deficit (vpd).Minimum water potentials of the mistletoe were relatively constant.They were held at values lower than those of hosts when thelatter maintained high water potentials but approached or evenexceeded those of hosts when they developed low water potentials.In contrast, cut shoots of Ileostylus usually maintained higherwater contents and leaf water potentials than those of its hostswhen both were desiccated separately in the laboratory. Pressure-volumeanalyses indicated that Ileostylus had lower water potentialat full turgor, a lower water potential but higher relativewater content at turgor loss, and a higher bulk modulus of elasticitythan the following four hosts: the native Kunzea ericoides andCoprosmapropinqua, and the introduced Ribes sanguineum and Teline monspessulana.Water potential at turgor loss (_tlp) was strongly correlatedwith the minimum field water potential of both mistletoes andhosts. When _tlpof mistletoe and host is similar (as on Kunzeaand Ribes) field water potentials are also similar, but when_tlpis lower in the mistletoe (as on Coprosma and Teline), thefield water potential of the mistletoe is lower than that ofits host. Consequently, I. micranthus is likely to be more frequenton hosts that maintain high field water potentials than on hoststhat develop low water potentials. Copyright 1999 Annals ofBotany Company Water relations, water potential, osmotic potential, pressure-volume, Ileostylus micranthus , mistletoe, New Zealand.     

Growth Rate and Water Relations of Citrus Leaf Flushes

Growth rates of seasonal leaf flushes of ‘Valencia’orange [Citrus sinensis (L.) Osbeck] were measured and waterrelations characteristics of young (new) and over-wintered (old)citrus leaves were compared. New flush leaves had lower specificleaf weights and lower midday leaf water potentials than comparablyexposed old leaves. Spring and summer flush new leaves had higherosmotic potentials than old leaves. These differences becamenon-significant as the new leaves matured. During summer conditions,water-stressed new leaves reached zero turgor and stomatal conductancealso began to decrease in them at higher leaf water potentialsthan in old leaves. Old leaves were capable of maintaining openstomata at lower leaf water potentials. Opened flowers and newflush leaves lost more water, on a dry weight basis, than flowerbuds, fruit or mature leaves. The results illustrate differencesin leaf water potential and stomatal conductance which can beattributed to the maintenance of leaf turgor by decreases inleaf osmotic potentials as leaves mature. These changes in citrusleaf water relations are especially important since water stressresulting from high water loss rates of new tissues could reduceflowering and fruit set. Citrus sinensis (L.) Osbeck, orange, Citrus paradisi Macf., grapefruit, growth rate, leaf water relations, osmotic potential, water potential, stomatal conductance     

Turgor-Regulated Sugar Release from the Source Leaves of Sugarbeet (Beta vulgaris L.)

Under mild water stress conditions, a potential site of regulationfor distribution of sucrose between osmotic adjustment and exportmay be at the mesophyll plasmalemma and/or tonoplast. This possibilitywas examined in attached leaves of sugarbeet (Beta vulgarisL.), labeled with ¹⁴CO₂. Leaf discs were exposed to solutionscontaining 400 or 50 mM mannitol to generate "low" or "high"cellular turgor, respectively and release of labeled soluteswas monitored. Response to changes in cell turgor was rapidand reversible. High turgor increased solute efflux rates todouble those at low turgor conditions. Approximately 30% and55% of the released label was in the sugar (sucrose and hexose)fractions at low and high turgor, respectively. Paramercuribenzenesulfonic acid (PCMBS) had no effect on efflux, but N-ethylmaleimide(NEM) and carbonylcyanide-m-chlorophenyl hydrazone (CCCP) enhancedefflux, especially at high turgor. Presence of unlabeled sucrosegreatly enhanced efflux in a turgor-dependent manner; suggestinga sucrose exchange system. While influx across the plasmalemmais both turgor sensitive and carrier-mediated, turgor-regulatedplasmalemma efflux did not appear to involve a carrier. Boththe tonoplast and plasmalemma appeared to be involved in turgor-inducedsugar efflux. Turgor-regulated efflux of solutes from vacuole-containingcells (mesophyll), may contribute to the establishment of ahomeostatic turgor pressure in these cells. (Received June 9, 1989; Accepted September 5, 1989)     

Potassium Loss from Stomatal Guard Cells at Low Water Potentials

The potassium content of guard cells and the resistance to viscousflow of air through the leaf were determined in sunflower (Helianthusannuus) subjected to low leaf water potentials under illuminatedconditions. In intact plants desiccated slowly by withholdingwater from the soil, large losses in guard cell K occurred asleaf water potentials decreased. Leaf viscous resistance increased,indicating stomatal closure. Similar results were obtained whendetached leaf segments were desiccated rapidly. Upon rehydrationof leaves, no stomatal opening was observed initially, despiteleaf water potentials at predesiccated levels. After severalhours, however, re-entry of K occurred and stomata became fullyopen. Turgid leaf segments floated on an ABA solution showedlosses of guard cell K and closure of stomata as rapidly andcompletely as those brought about by desiccation. It is concludedthat stomatal closure at low water potentials under illuminatedconditions is not controlled solely by water loss from the tissuebut involves the loss of osmoticum from the guard cells as well.This in turn decreases the turgor difference between the guardcells and the surrounding cells, and closing occurs.     

Osmotic Adjustment and Stomatal Response to Water Deficits in Maize

A pot experiment was carried out using five maize {Zea maysL.) cultivars under three soil moisture levels (MPa 0 to –0.05,–0.3 to –0.9 and –1.2 to –1.5) to investigatethe effects of water deficits on osmotic adjustment and stomatalconductance. The degree of leaf rolling and the sugar and nutrientconcentrations in leaf cell sap were measured. Leaf water potential and osmotic potential decreased and stomatalconductance decreased with increasing water deficits. Stomatalconductance correlated positively with leaf water potentialand osmotic potential. Degree of leaf rolling was lower in cultivarswhich maintained higher turgor. Osmotic adjustment of 0.08 to0.43 MPa was found under the lowest soil moisture level in fivecultivars used. Sugar and K were the major osmotic substancesin the maize plant. Sugar, K and Mg concentrations increasedunder water deficit, and correlated negatively with a decreasein osmotic potential. Key words: Zea mays L., leaf water relations, leaf rolling, osmotic adjustment, stomatal conductance, water deficit     

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     

Hexose Accumulation and Turgor-Sensitive Starch Synthesis in Discs Derived from Source versus Sink Potato Tubers

The net total uptake (sum of soluble and insoluble components)of the hexoses, D-glucose and D-fructose, into sink potato (Solanumtuberosum L.) storage parenchyma was biphasic with respect tosubstrate concentration. Analysis of radioactive products revealedthat the biphasic kinetics were composed of a linear, solublecomponent superimposed on saturating starch synthesis. In contrast,in source tuber tissue, there was negligible conversion of D-glucoseto starch and the shape of the kinetic was the result of a biphasicsoluble component. The uptake of D-fructose into source tissuewas linear with respect to substrate concentration. Uptake ofthe non-metabolizable glucose analogue, 3-oxymethyl-D-glucose(3-OMG), into both sink and source tissue, demonstrated biphasickinetics, indicating the presence of a carrier for glucose.The data demonstrate that in sink potato tubers, metabolismgreatly influences apparent uptake kinetics, the kinetics ofstarch synthesis masking the kinetics of hexose transport atthe plasmalemma. Uptake of L-glucose was linear with respectto substrate concentration, an observation consistent with thissugar not being recognized by a carrier. As in the case of sucrose, in sink tuber tissue the conversionof D-glucose and D-fructose to starch was sensitive to turgor,showing a marked optimum in external osmotica containing 300mol m^–3 mannitol. The mechanisms controlling turgor-sensitivestarch synthesis in the potato tuber would, therefore, appearto be common to all three sugars. Key words: Hexose (transport), partitioning, Solanum (source, sink tubers), starch synthesis     

Sucrose Metabolism in Bean Plants Under Water Deficit

The effects of water stress on sucrose metabolism were evaluatedin bean plants of Tacarigua variety grown for 25 d. Decreasingwater potential and relative water content were observed. Waterstress effects resulted in a decrease of sucrose phosphate synthase(SPS) in both total (substrate saturating conditions) and Pi-insensitive(substrate limiting conditions plus inorganic phosphate) activities.The SPS Pi-insensitive activity was lower than the total SPSactivity, but the decrease in activity induced by water deficitwas relatively lower in the Pi-insensitive; however the activationstate increased during the water deficit period. An increasein sucrose synthase activity increased the activities of bothneutral and acid invertases at moderate water stress (–0·8MPa) and decreased activities at severe water stress(–1·45 MPa). The activity values of neutral invertasewere lower than those for the acid invertase. The starch/sucroseratio decreased and the ratio of total glucose/total fructoseincreased. These results indicate a relevant physiological roleof SPS in bean plants under water stress. Key words: Acid invertase, sucrose phosphate synthase, sucrose synthase     

Regulation of sucrose and starch metabolism in potato tubers in response to short-term water deficit

To investigate the effect of water stress on carbon metabolism in growing potato tubers (Solanum tuberosum L.), freshly cut and washed discs were incubated in a range of mannitol concentrations corresponding to external water potential between 0 and −1.2 MPa. (i) Incorporation of [¹⁴C]glucose into starch was inhibited in water-stressed discs, and labeling of sucrose was increased. High glucose overrode the changes at low water stress (up to −0.5 MPa) but not at high water stress. (ii) Although [¹⁴C]sucrose uptake increased in water-stressed discs, less of the absorbed [¹⁴C]sucrose was metabolised. (iii) Analysis of the sucrose content of the discs confirmed that increasing water deficit leads to a switch, from net sucrose degradation to net sucrose synthesis. (iv) In parallel incubations containing identical concentrations of sugars but differing in which sugar was labeled, degradation of [¹⁴C]sucrose and labeling of sucrose from [¹⁴C]glucose and fructose was found at each mannitol concentration. This shows that there is a cycle of sucrose degradation and resynthesis in these tuber discs. Increasing the extent of water stress changed the relation between sucrose breakdown and sucrose synthesis, in favour of synthesis. (v) Analysis of metabolites showed a biphasic response to increasing water deficit. Moderate water stress (0–200 mM mannitol) led to a decrease of the phosphorylated intermediates, especially 3-phosphoglycerate (3PGA). The decrease of metabolites at moderate water stress was not seen when high concentrations of glucose were supplied to the discs. More extreme water stress (300–500 mM mannitol) was accompanied by an accumulation of metabolites at low and high glucose. (vi) Moderate water stress led to an activation of sucrose phosphate synthase (SPS) in discs, and in intact tubers. The stimulation involved a change in the kinetic properties of SPS, and was blocked␣by protein phosphatase inhibitors. (vii) The amount of ADP-glucose (ADPGlc) decreased when discs were incubated on 100 or 200 mM mannitol. There was a strong correlation between the in vivo levels of ADPGlc and 3PGA when discs were subjected to moderate water stress, and when the sugar supply was varied. (viii) The level of ADPGlc increased and starch synthesis was further inhibited when discs were incubated in 300–500 mM mannitol. (ix) It is proposed that moderate water stress leads to an activation of SPS and stimulates sucrose synthesis. The resulting decline of 3PGA leads to a partial inhibition of ADP-glucose pyrophosphorylase and starch synthesis. More-extreme water stress leads to a further alteration of partitioning, because it inhibits the activities of one or more of the enzymes involved in the terminal reactions of starch synthesis. Received: 26 August 1996 / Accepted: 5 November 1996     

Effects of low water potentials on some aspects of carbohydrate metabolism in Chlorella pyrenoidosa

Summary Chlorella pyrenoidosa was subjected to low water potentials and the resulting changes in carbohydrate metabolism were measured.Water deficit reduced the incorporation of ¹⁴C-glucose into methanol insoluble compounds, principally starch and increased that into sucrose. Even moderate water deficit, for example potentials of -2.5 and -5 atm, greatly reduced the incorporation of ¹⁴C-glucose into uridine diphosphate glucose, while ¹⁴C levels of the hexose monophosphates changed little, indicating a direct stimulus of sucrose synthesis. This increased sucrose synthesis was one of the earliest effect of water deficit, because potentials of -2.5 and -5 atm did not reduce respiration and glucose uptake.At lower water potentials (-10 atm or less) there was reduced ¹⁴C incorporation into all sugar phosphates. This resulted from a combination of reduced ¹⁴C-glucose uptake and increased sucrose synthesis.Water potentials as low as -20 atm had little effect on acetate uptake, or on the ¹⁴C levels in the intermediates of the TCA cycle. This confirmed that low water potentials do not directly inhibit respiratory pathways in Chlorella.The results are discussed in relation to the effect of water deficit on levels of various metabolites in vascular plants, which have been reported by other workers.