Mannitol metabolism in cultured plant cells

Non-structural storage carbohydrates were measured in 9-day-old barley ( Hordeum vulgare L. cv. Brant) primary leaves. Accumulation rates of starch, sucrose and total non-structural carbohydrates (TNC) were approximately linear when measured between 2- and 12-h of light. Progressively higher TNC accumulation rates were observed at higher irradiance levels (i.e., comparing 250, 550 and 1050 ·mol m⁻² s⁻¹). Synthesis of a low-molecular-weight fructan also was enhanced by high irradiances. Low irradiance treatments decreased leaf sucrose levels and there was a corresponding increase in the lag period preceding starch synthesis in the light. Increased starch accumulation rates were usually observed when sucrose concentrations were high. These and other results suggested that cytosolic sucrose concentrations affected starch metabolism in the chloroplast. However, sucrose accumulation rates increased and starch storage decreased when barley seedlings were transferred from 20 to 10°C during the light period. Lowering the night temperature from 20 to 10°C for a single dark period 8-days after planting increased the TNC content of barley primary leaves at the beginning of day nine. In this experiment, TNC accumulation rates of treated and untreated leaves were similar. Changes in the accumulation rate of TNC were usually observed within 2- to 4-h after barley seedlings were exposed to altered environmental conditions. Monitoring rapid changes in leaf carbohydrate levels is a sensitive method for assessing the effects of environmental treatments on photosynthetic metabolism.     

Changes in Nonstructural Carbohydrates in Different Parts of Soybean (Glycine max [L.] Merr.) Plants during a Light/Dark Cycle and in Extended Darkness

Diurnal patterns of nonstructural carbohydrate (starch, sucrose, and hexose sugars) concentration were characterized in different parts (leaves, petioles, stems, and roots) of vegetative soybean (Glycine max [L.] Merr.) plants. Pronounced changes in all carbohydrate pools were observed in all plant parts during the normal photosynthetic period; however, starch accumulation within leaves accounted for more than 80% of the nonstructural carbohydrate accumulated by the plant during the light period. Efficiency of utilization of starch and sucrose during the normal dark period differed among organs, with leaves being most efficient in mobilizing starch reserves and roots being most efficient in utilizing sucrose reserves. The vast majority (about 85%) of the whole plant carbohydrate reserves present at the end of the photosynthetic period were utilized during the normal dark period. Sink leaf expansion ceased in plants transferred to extended darkness and the cessation in leaf expansion corresponded with carbohydrate depletion in the subtending source leaf and the remainder of the plant. Collectively, the results indicated that under the conditions employed, leaves are the whole plant's primary source of carbon at night as well as during the day.     

Diurnal fluctuations in cotton leaf carbon export, carbohydrate content, and sucrose synthesizing enzymes

In fully expanded leaves of greenhouse-grown cotton (Gossypium hirsutum L., cv Coker 100) plants, carbon export, starch accumulation rate, and carbon exchange rate exhibited different behavior during the light period. Starch accumulation rates were relatively constant during the light period, whereas carbon export rate was greater in the afternoon than in the morning even though the carbon exchange rate peaked about noon. Sucrose levels increased throughout the light period and dropped sharply with the onset of darkness; hexose levels were relatively constant except for a slight peak in the early morning. Sucrose synthase, usually thought to be a degradative enzyme, was found in unusually high activities in cotton leaf. Both sucrose synthase and sucrose phosphate synthetase activities were found to fluctuate diurnally in cotton leaves but with different rhythms. Diurnal fluctuations in the rate of sucrose export were generally aligned with sucrose phosphate synthase activity during the light period but not with sucrose synthase activity; neither enzyme activity correlated with carbon export during the dark. Cotton leaf sucrose phosphate synthase activity was sufficient to account for the observed carbon export rates; there is no need to invoke sucrose synthase as a synthetic enzyme in mature cotton leaves. During the dark a significant correlation was found between starch degradation rate and leaf carbon export. These results indicate that carbon partitioning in cotton leaf is somewhat independent of the carbon exchange rate and that leaf carbon export rate may be linked to sucrose formation and content during the light period and to starch breakdown in the dark.     

Trends in carbohydrate depletion, respiratory carbon loss, and assimilate export from soybean leaves at night

To evaluate assimilate export from soybean (Glycine max [L.] Merrill) leaves at night, rates of respiratory CO₂ loss, specific leaf weight loss, starch mobilization, and changes in sucrose concentration were measured during a 10-hour dark period in leaves of pod-bearing `Amsoy 71' and `Wells II' plants in a controlled environment. Lateral leaflets were removed at various times between 2200 hours (beginning dark period) and 0800 hours (ending dark period) for dry weight determination and carbohydrate analyses. Respiratory CO₂ loss was measured throughout the 10-hour dark period. Rate of export was estimated from the rate of loss in specific leaf weight and rate of CO₂ efflux. Rate of assimilate export was not constant. Rate of export was relatively low during the beginning of the dark period, peaked during the middle of the dark period, and then decreased to near zero by the end of darkness. Rate of assimilate export was associated with rate of starch mobilization and amount of starch reserves available for export. Leaves of Amsoy 71 had a higher maximum export rate in conjunction with a greater total change in starch concentration than did leaves of Wells II. Sucrose concentration rapidly declined during the first hour of darkness and then remained constant throughout the rest of the night in leaves of both cultivars. Rate of assimilate export was not associated with leaf sucrose concentration.     

Diurnal changes in allocation of newly fixed carbon in exporting sugar beet leaves

Storage of newly fixed carbon as starch and sucrose follows a regular daily pattern in exporting sugar beet leaves under constant day length and level of illumination. Up to the final two hours of the light period, when starch storage declines, a nearly constant proportion of newly fixed carbon was allocated to carbohydrate storage, principally starch. Sucrose is stored only early in the light period, when there is little accumulation of starch. Pulse labeling with ¹⁴CO₂ revealed that considerable starch synthesis was taking place at this time. Starch made the previous day was not mobilized during this period but breakdown of newly synthesized starch may occur when carbon flow into sucrose synthesis increases early in the day. At the end of the day, starch storage declined from the constant level observed during most of the day, but no diversion of label into export of specific alternative compounds could be detected. Lowered storage of starch persisted when the 14-hour light period was lengthened. Changed allocation of recently fixed carbon to sucrose and starch at the beginning and end of the light period was not the result of outright inactivation of pathways but of regulation of carbon flow.     

Carbohydrate partitioning and metabolism during acclimatization of micropropagated Calathea

Regulation of carbohydrate metabolism and compartmentation were studied during the acclimatization of tissue cultured Calathea plantlets. At transplantation plants were characterised by a heterotrophic metabolism with roots and stems as the main storage organs for carbohydrates. As acclimatization proceeded, a switch to autotrophic growth was observed: leaves became source organs, which was among others reflected by significant increases of invertase, sucrose synthase and sucrose-P synthase activities. Mobilization of reserves in roots and stems was also observed during the same period. Sucrose and starch accumulation in leaves was positively correlated with increasing light intensity.     

Diurnal Changes in Maize Leaf Photosynthesis : III. Leaf Elongation Rate in Relation to Carbohydrates and Activities of Sucrose Metabolizing Enzymes in Elongating Leaf Tissue

Maize (Zea mays L. cv. Pioneer 3184) leaf elongation rate was measured diurnally and was related to diurnal changes in the activities of sucrose metabolizing enzymes and carbohydrate content in the elongating portion of the leaf. The rate of leaf elongation was greatest at midday (1300 hours) and was coincident with the maximum assimilate export rate from the distal portion of the leaf. Leaf elongation during the light period accounted for 70% of the total observed increase in leaf length per 24 hour period. Pronounced diurnal fluctuations were observed in the activities of acid and neutral invertase and sucrose phosphate synthase. Maximum activities of sucrose phosphate synthase and acid invertase were observed at 0900 hours, after which activity declined rapidly. The activity of sucrose phosphate synthase was substantially lower than that observed in maize leaf source tissue. Neutral invertase activity was greatest at midday (1200 hours) and was correlated positively with diurnal changes in leaf elongation rate. There was no significant change in the activity of sucrose synthase over the light/dark cycle. Sucrose accumulation rate increased during a period when leaf elongation rate was maximal and beginning to decline. Maximum sucrose concentration was observed at 1500 hours, when the activities of sucrose metabolizing enzymes were low. At no time was there a significant accumulation of hexose sugars. The rate of starch accumulation increased after the maximum sucrose concentration was observed, continuing until the end of the light period. There was no delay in the onset of starch mobilization at the beginning of the dark period, and essentially all of the starch was depleted by the end of the night. Mobilization of starch in the elongating tissue at night could account for a significant proportion of the calculated increase in the tissue dry weight due to growth. Collectively, the results suggested that leaf growth may be controlled by the activities of certain sucrose metabolizing enzymes and may be coordinated with assimilate export from the distal, source portion of the leaf. Results are discussed with reference to diurnal photoassimilation and export in the distal, source portion of the leaf.     

Diurnal Pattern of Translocation and Carbohydrate Metabolism in Source Leaves of Beta vulgaris L

Transitions in carbohydrate metabolism and translocation rate were studied for evidence of control of export by the sugar beet (Beta vulgaris L. Klein E.) source leaf. Steady-state labeling was carried out for two consecutive 14-hour light periods and various quantities related to translocation were measured throughout two 24-hour periods. Starch accumulation following illumination was delayed. Near the end of the light period, starch stopped accumulating, whereas photosynthesis rate and sucrose level remained unchanged. At the beginning of the dark period there was a 75-minute delay before starch was mobilized. The rate of import to the developing sink leaves at night was similar to that during the day, whereas export decreased considerably at night.

Starch accumulation and degradation seemed to be initiated in response to the level of illumination. Cessation of starch accumulation before the end of the light period was initiated endogenously. Exogenous control appeared to be mediated by the level of sucrose in the source leaf while endogenous control seemed to be keyed to photoperiod or photosynthetic duration.

     

Primary partitioning and storage of photosynthate in sucrose and starch in leaves of C4 plants

A procedure involving pulse labelling of leaves with ¹⁴CO₂ was developed to measure the primary (initial) partitioning of photosynthate between sucrose and starch. Partitioning of photosynthate into sucrose and starch was determined in leaves of C₄ plants and compared with the patterns of storage of carbon in these products during the light period. The ratio of primary partitioning into sucrose and starch varied from about 0.5 in those species that accumulated mostly starch in the leaves (Amaranthus edulis L., Atriplex spongiosa F. Muell. and Flaveria trinervia (Spreng.) C. Mohr) to about 8 in Eleusine indica (L.) Gaertn., which accumulated mostly sucrose. No label was detected in free glucose or fructose. Generally there was a reasonable link between the primary partitioning of photosynthate and the type of carbohydrate stored in the leaf during the day. However, the ratio of carbon initially partitioned into sucrose versus starch was about 3 to 4 times higher in leaves of NADP-malic enzyme-type monocotyledonous species compared with phosphoenolpyruvate carboxykinase-type species, although the ratio of sucrose to starch accumulated in leaves during the day was very similar in the two groups. Sucrose and starch were the principal carbohydrates accumulated in leaves during the day. None of the species examined contained significant amounts of fructan and only one species, Atriplex spongiosa, contained substantial amounts of hexose sugars. In most of the species studied, the proportion of photosynthate partitioned into starch was greater at the end of the day than at the beginning. With the exception of Flaveria trinervia, the rate of CO₂ assimilation did not decline during the day, showing that, under our conditions, accumulation of carbohydrate in the leaves did not lead to feedback inhibition of photosynthesis in these C₄ species.Abbreviations Chl chlorophyll - NAD-ME NAD-malic enzyme - NADP-ME NADP-malic enzyme - PCK phosphoenolpyruvate carboxykinase We thank Prof. H.W. Heldt (Pflanzenphysiologisches Institut, Universität Göttingen) for discussions and advice during the course of this work.     

A comparative study of metabolite levels in plant leaf material in the dark

Metabolite levels have been compared in the dark and during photosynthesis in leaves and protoplasts from spinach, pea, wheat and barley. In protoplasts the subcellular distribution was also studied. The levels of triose phosphates and sugar bisphosphates were high in the light and low in the dark. The hexose phosphates and 3-phosphoglycerate levels in the dark were very variable depending on the plant material. In most conditions, hexose phosphates and triose phosphates were mainly in the extrachloroplast compartment, while 3-phosphoglycerate and the sugar bisphosphates were mainly in the chloroplast compartment. Leaves always had a very low triose phosphate: 3-phosphoglycerate ratio in the dark, but in protoplasts this ratio was higher. Detailed studies with spinach showed that metabolite levels were very dependent on the availability of carbohydrate in the leaf, particularly starch. Starch mobilisation is not controlled just by the availability of inorganic phosphate and accumulation of phosphorylated intermediates. Hydrolysis of starch may provide precursors for sucrose synthesis while phosphorolysis leads to provision of substrates for respiration. Starch breakdown generates high enough levels of hexose phosphate to support substantial rates of sucrose synthesis in the dark. Respiration is not greatly increased when metabolite levels are high during starch mobilisation. Higher levels of metabolites shorten the length of the induction phase of photosynthesis.Abbreviations Chl chlorophyll - DHAP dihydroxyacetone phosphate - Fru2,6bisP fructose-2,6-bisphosphate - NMR nuclear magnetic resonance - PGA 3-phosphoglyceric acid - Pi inorganic phosphate - RuBP ribulose-1,5-bisphosphate - UDPGlc uridine-5-diphosphate glucose     

Alterations in leaf carbohydrate metabolism in response to nitrogen stress

A series of experiments was conducted to characterize alterations in carbohydrate utilization in leaves of nitrogen stressed plants. Two-week-old, nonnodulated soybean plants (Glycine max [L.] Merrill, `Ransom'), grown previously on complete nutrient solutions with 1.0 millimolar NO₃⁻, were transferred to solutions without a nitrogen source at the beginning of a dark period. Daily changes in starch and sucrose levels of leaves were monitored over the following 5 to 8 days in three experiments. Starch accumulation increased relative to controls throughout the leaf canopy during the initial two light periods after plant exposure to N-free solutions, but not after that time as photosynthesis declined. The additional increments of carbon incorporated into starch appeared to be quantitatively similar to the amounts of carbon diverted from amino acid synthesis in the same tissues. Since additional accumulated starch was not degraded in darkness, starch levels at the beginning of light periods also were elevated. In contrast to the starch effects, leaf sucrose concentration was markedly higher than controls at the beginning of the first light period after the N-limitation was imposed. In the days which followed, diurnal turnover patterns were similar to controls. In source leaves, the activity of sucrose-P synthase did not decrease until after day 3 of the N-limitation treatment, whereas the concentration of fructose-2,6-bisphosphate was decreased on day 2. Restricted growth of sink leaves was evident with N-limited plants within 2 days, having been preceeded by a sharp decline in levels of fructose-2,6 bisphosphate on the first day of treatment. The results suggest that changes in photosynthate partitioning in source leaves of N-stressed plants resulted largely from a stable but limited capacity for sucrose formation, and that decreased sucrose utilization in sink leaves contributed to the whole-plant diversion of carbohydrate from the shoot to the root.     

Diurnal patterns of growth and transient reserves of sink and source tissues are affected by cold nights in barley

Barley is described to mostly use sucrose for night carbon requirements. To understand how the transient carbon is accumulated and utilized in response to cold, barley plants were grown in a combination of cold days and/or nights. Both daytime and night cold reduced growth. Sucrose was the main carbohydrate supplying growth at night, representing 50–60% of the carbon consumed. Under warm days and nights, starch was the second contributor with 26% and malate the third with 15%. Under cold nights, the contribution of starch was severely reduced, due to an inhibition of its synthesis, including under warm days, and malate was the second contributor to C requirements with 24–28% of the total amount of carbon consumed. We propose that malate plays a critical role as an alternative carbon source to sucrose and starch in barley. Hexoses, malate, and sucrose mobilization and starch accumulation were affected in barley elf3 clock mutants, suggesting a clock regulation of their metabolism, without affecting growth and photosynthesis however. Altogether, our data suggest that the mobilization of sucrose and malate and/or barley growth machinery are sensitive to cold.     

Influence of magnesium deficiency on rates of leaf expansion, starch and sucrose accumulation, and net assimilation in Phaseolus vulgaris

Twenty one-day-old Phaseolus vulgaris 'Saxa'plants were cultured in a growth chamber and the plants supplied with either a complete or a Mg-free nutrient solution. From 6 days after transfer to the Mg-free solution, the rate of increase of the area of the second trifoliate leaf was considerably reduced; by day 11 the sucrose concentration in the first trifoliate leaf had increased 6. 2-fold at the end of the dark period and 4. 6-fold after the light period as compared with the control plants. Corresponding starch concentrations increased 6. 6-fold and 2. 9-fold respectively. After days 5 to 6 the assimilation rates declined in the first trifoliate leaf of the plants showing deficiency, in comparison with the plants fully supplied with nutrients; respiration increased during darkness. The reduction in net assimilation rate was to a great extent reversible after resupply of magnesium.
The reduction of magnesium concentration in the deficient plants was much more marked in the expanding leaves than in the mature primary leaves and roots. Sucrose and starch accumulation did not occur when the first trifoliate leaf was partially shaded, although magnesium concentration, as in the unshaded leaves, was reduced to 13% of that of the control plants. The consequences of magnesium deficiency in the expanding first trifoliate leaf are discussed in terms of the possibility of sink limitation.     

A comparative analysis of fructose 2,6-bisphosphate levels and photosynthate partitioning in the leaves of some agronomically important crop species

Starch, sucrose, and fructose 2,6-bisphosphate (F2, 6BP) levels were measured in pea (Pisum sativum L.), maize (Zea mays L.), onion (Allium cepa L.) and soybean (Glycine max L.) leaves throughout a light/dark cycle. Leaf starch accumulated in pea, maize, and soybean but not in onion. Sucrose was a major leaf storage reserve in pea, maize, and onion but was only found at low levels in soybean. In all species examined, the most dramatic changes in F2,6BP concentration coincided with light/dark transitions. During the light period F2,6BP levels were about 0.1 nanomole/milligram chlorophyll in soybean source leaves and there was a small increase in effector concentration in the dark. Levels of F2,6BP were also low in pea and maize leaves during the light period but then increased 10- or 20-fold in the dark. Dark onion leaf F2,6BP levels were about 1.1 to 1.3 nanomole/milligram chlorophyll and these values decreased by 20 to 30% in the light. Thus, three different patterns were identified that describe diurnal F2,6BP levels in source leaves. These results support the suggestion that F2,6BP is involved in the regulation of sucrose biosynthesis. However, it was not possible to demonstrate that high levels of F2,6BP are essential for starch synthesis in the chloroplast.     

Evidence for circadian regulation of starch and sucrose synthesis in sugar beet leaves

Starch accumulation and sucrose synthesis and export were measured in leaves of sugar beet (Beta vulgaris L.) during a period of prolonged irradiance in which illumination was extended beyond the usual 14-hour day period. During much of the 14-hour day period, approximately 50% of the newly fixed carbon was distributed to sucrose, about 40% to starch, and less than 10% to hexose. Beginning about 2 hours before the end of the usual light period, the portion of newly fixed carbon allocated to sucrose gradually increased, and correspondingly less carbon went to starch. By the time the transition ended, about 4 hours into the extension of the light period, nearly 90% of newly fixed carbon was incorporated into sucrose and little or none into starch. Most of the additional sucrose was exported. Gradual cessation of starch accumulation was not the result of a futile cycle of simultaneous starch synthesis and degradation. Neither was it the result of a decrease in the extractable activity of adenosine diphosphoglucose pyrophosphorylase or phosphoglucose isomerase, enzymes important in starch synthesis. Nor was there a notable change in control metabolites considered to be important in regulating starch synthesis. Starch accumulation appeared to decrease markedly because of an endogenous circadian shift in carbon allocation, which occurred in preparation for the usual night period and which diverted carbon from the chloroplast to the cytosol and sucrose synthesis.     

Control of photosynthetic sucrose synthesis in barley primary leaves: role of fructose 2,6-bisphosphate

Levels of fructose 2,6-bisphosphate (F2,6BP) and related metabolites were measured in 8- or 9-day-old barley (Hordeum vulgare L.) primary leaves throughout a 24 hour cycle. Young barley leaves contained about 0.4 nanomole F2,6BP per milligram chlorophyll at the end of a 12 hour dark period. F2,6BP levels increased rapidly following a dark-to-light transition and then decreased to about 0.1 nanomole per milligram chlorophyll after 5 or 10 minutes of light. Low levels of F2,6BP were detected in barley primary leaves throughout the day. A 10-fold increase in F2,6BP was observed during the first hour of the dark period and then levels of this metabolite decreased slowly for the next several hours. Only small diurnal fluctuations were noted in barley leaf glucose 6-phosphate and uridine 5′-diphosphoglucose levels. There were rapid changes in whole leaf F2,6BP levels when the light intensity was altered. High F2,6BP levels in the dark were not observed after short photosynthetic periods. Results obtained with barley primary leaves support the suggestion that F2,6BP is involved in regulating the flow of photosynthate from the chloroplast to sucrose. Extractable sucrose-phosphate synthase activity was inversely related to barley primary leaf F2,6BP levels. This finding may indicate that the activities of sucrose-phosphate synthase and cytosolic fructose 1,6-bisphosphatase in barley primary leaves are metabolically coordinated.     

Photosynthetic Carbon Metabolism and Translocation in Wild-Type and Starch-Deficient Mutant Nicotiana sylvestris L

A high rate of daytime export of assimilated carbon from leaves of a starch-deficient mutant tobacco (Nicotiana sylvestris L.) was found to be a key factor that enabled shoots to grow at rates comparable to those in wild-type plants under a 14-h light period. Much of the newly fixed carbon that would be used for starch synthesis in leaves of wild-type plants was used instead for sucrose synthesis in the mutant. As a result, export doubled and accumulation of sucrose and hexoses increased markedly during the day in leaves of the mutant plants. The increased rate of export to sink leaves appeared to be responsible for the increase in the proportion of their growth that occurred during the day compared to wild-type plants. Daytime growth of source leaves also increased, presumably as a result of the increased accumulation of recently assimilated soluble carbon in the leaves. Even though starch accumulation did not occur in the leaves of mutant plants, nearly all the sugar that accumulated during the day was exported in the period of decreasing irradiance at the end of the diurnal light period. Changes in carbon allocation that occurred in leaves of wild-type and mutant plants near the end of the light period appeared to result from endogenous diurnal regulation associated with the day-night transition.     

Role of carbohydrates in diurnal chilling sensitivity of tomato seedlings

Tomato seedlings (Lycopersicon esculentum Mill.) chilled starting at different times during the light/dark cycle were most chilling-sensitive at the end of the dark period (AI King, MS Reid, BD Patterson 1982 Plant Physiol 70: 211-214). Low-temperature tolerance was regained with as little as 10 minutes of light exposure. Low light intensities were less effective than high light intensities in reducing sensitivity, and the length of exposure to light directly influenced sensitivity. Seedlings kept at low night temperatures prior to chilling were also less injured following chilling. Light also restored chilling tolerance to seedlings whose roots were removed. Supplying cut shoots with sucrose, glucose, or fructose reduced chilling sensitivity and largely eliminated the diurnal difference in sensitivity. Endogenous carbohydrate content was correlated with changes in chilling sensitivity; starch and sugar content fell markedly during the dark period. Increased concentrations of sugars were detected 15 minutes after the start of the light period. This evidence all suggests that changes in chilling sensitivity over the diurnal period are regulated by the light cycle. It also suggests that increased sensitivity at the end of the dark period could be due to carbohydrate depletion, and that chilling tolerance following light exposure is likely due to carbohydrate accumulation or closely related events.     

Effect of Day Length and Night Temperature on Starch Accumulation and Degradation in Soybean

The effect of the day length on the accumulation and the degradationof the starch in leaf, stem and root tissues of prefloweringsoybean plants was determined by growing plants under a 7 or14 h light regime. As has been reported previously, the rateof starch accumulation by leaves was inversely related to daylength. High sucrose content was associated with a high rateof starch accumulation. Stem tissue showed diurnal fluctuationsin starch content and the rate of accumulation was also inverselyrelated to day length. This starch resulted from photosynthesiswithin the stem itself. A negligible amount of starch was foundin root tissue of both sets of plants. The rate of starch breakdown in leaves of 7 h plants was significantlyless than that in 14 h plants. Nevertheless, leaf starch inshort day length plants was depleted at least 4 h prior to theend of the dark period. In both sets of plants, degradationof stem starch started simultaneously with that in the leavesand continued throughout the dark period, although at a muchlower rate than that of leaves. Thus, stem starch acted as abuffer once leaf starch was depleted, providing carbohydratesto the plant, although in small quantities. To determine if soybean leaves adjust their rate of starch accumulationduring the light period to different dark period temperatures,plants were grown under temperature regimes of 30/20 ^°Cand 30/30 ^°C. Plants did not differ in rate of starch accumulationor CO₂ exchange rate, but did show large differences in growthcharacteristics. High temperature plants had significantly greaterleaf area and tended to have greater leaf area ratio. Thus,despite similar rates of starch accumulation on a leaf areabasis, high temperature plants accumulated greater amounts ofstarch on a per plant basis. Glycine max(L.)Merr., soybean reserve carbohydrates, remobilization, source-sink realtionships     

Differential usage of storage carbohydrates in the CAM bromeliad Aechmea 'Maya' during acclimation to drought and recovery from dehydration

CAM requires a substantial investment of resources into storage carbohydrates to account for nocturnal CO₂ uptake, thereby restricting carbohydrate partitioning to other metabolic activities, including dark respiration, growth and acclimation to abiotic stress. Flexible modulation of carbon flow to the different competing sinks under changing environmental conditions is considered a key determinant for the growth, productivity and ecological success of the CAM pathway. The aim of the present study was to examine how shifts in carbohydrate partitioning could assure maintenance of photosynthetic integrity and a positive carbon balance under conditions of increasing water deprivation in CAM species. Measurements of gas exchange, leaf water relations, malate, starch and soluble sugar (glucose, fructose and sucrose) contents were made in leaves of the CAM bromeliad Aechmea ‘Maya’ over a 6‐month period of drought and subsequently over a 2‐month period of recovery from drought. Results indicated that short‐term influences of water stress were minimized by elevating the level of respiratory recycling, and carbohydrate pools were maintained at the expense of export for growth while providing a comparable nocturnal carbon gain to that in well‐watered control plants. Longer term drought resulted in a disproportionate depletion of key carbohydrate reserves. Sucrose, which was of minor importance for providing substrate for the dark reactions under well‐watered conditions, became the major source of carbohydrate for nocturnal carboxylation as drought progressed. Flexibility in terms of the major carbohydrate source used to sustain dark CO₂ uptake is therefore considered a crucial factor in meeting the carbon and energy demands under limiting environmental conditions. Recovery from CAM‐idling was found to be dependent on the restoration of the starch pool, which was used predominantly for provision of substrate for nocturnal carboxylation, while net carbon export was limited. The conservation of starch for the nocturnal reactions might be adaptive with regard to responding efficiently to a return of water stress.