Characterization of diurnal changes in activities of enzymes involved in sucrose biosynthesis

Experiments were conducted with vegetative soybean plants (Glycine max [L.] Merr., `Ransom') to determine whether the activities in leaf extracts of key enzymes in sucrose metabolism changed during the daily light/dark cycle. The activity of sucrose-phosphate synthase (SPS) exhibited a distinct diurnal rhythm, whereas the activities of UDP-glucose pyrophosphorylase, cytoplasmic fructose-1,6-bisphosphatase, and sucrose synthase did not. The changes in extractable SPS activity were not related directly to photosynthetic rates or light/dark changes. Hence, it was postulated that the oscillations were under the control of an endogenous clock. During the light period, the activity of SPS was similar to the estimated rate of sucrose formation. In the dark, however, SPS activity declined sharply and then increased even though degradation of starch was linear. The activity of SPS always exceeded the estimated maximum rate of sucrose formation in the dark. Transfer of plants into light during the normal dark period (when SPS activity was low) resulted in increased partitioning of photosynthate into starch compared to partitioning observed during the normal light period. These results were consistent with the hypothesis that SPS activity in situ was a factor regulating the rate of sucrose synthesis and partitioning of fixed carbon between starch and sucrose in the light.     

Photosynthesis, Carbohydrate Metabolism, and Export in Beta vulgaris L. and Phaseolus vulgaris L. during Square and Sinusoidal Light Regimes

Rates of photosynthesis, sucrose synthesis, starch accumulation and degradation were measured in sugar beet (Beta vulgaris L.) and bean (Phaseolus vulgaris L.) plants under a square-wave light regime and under a sinusoidal regime that simulated the natural daylight period. Photosynthesis rate increased in a measured manner in direct proportion to the increasing light level. In contrast to this close correspondence between photosynthesis and light, a lag in photosynthesis rate was seen during the initial hour under square-wave illumination. The leaf appeared to be responding to limits set by carbon metabolism rather than by gas exchange or light reactions. Under the sinusoidal regime starch degradation occurred during the first and last 2 hours of the photoperiod, likely in response to photosynthesis rate rather than directly to light level. Starch broke down when photosynthesis was below a threshold rate and accumulated above this rate. Under square-wave illumination, accumulation of starch did not begin until irradiance was at full level for an hour or more and photosynthesis was at or near its maximum. Under a sinusoidal light regime, sucrose synthesis rate comprised carbon that was newly fixed throughout the day plus that from starch degradation at the beginning and end of the day. Synthesis of sucrose from recently fixed carbon increased with increasing net carbon fixation rate while its formation from degradation of starch decreased correspondingly. The complementary sources of carbon maintained a relatively steady rate of sucrose synthesis under the changing daytime irradiance.     

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.     

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.     

Synchronous Growth and Plastid Replication in the Naturally Wall-less Alga Olisthodiscus luteus

Olisthodiscus luteus is a unicellular biflagellate alga which contains many small discoidal chloroplasts. This naturally wall-less organism can be axenically maintained on a defined nonprecipitating artificial seawater medium. Sufficient light, the presence of bicarbonate, minimum mechanical turbulence, and the addition of vitamin B₁₂ to the culture medium are important factors in the maintenance of a good growth response. Cells can be induced to divide synchronously when subject to a 12-hour light/12-hour dark cycle. The chronology of cell division, DNA synthesis, and plastid replication has been studied during this synchronous growth cycle. Cell division begins at hour 4 in the dark and terminates at hour 3 in the light, whereas DNA synthesis initiates 3 hours prior to cell division and terminates at hour 10 in the dark. Synchronous replication of the cell's numerous chloroplasts begins at hour 10 in the light and terminates almost 8 hours before cell division is completed. The average number of chloroplasts found in an exponentially growing synchronous culture is rather stringently maintained at 20 to 21 plastids per cell, although a large variability in plastid complement (4-50) is observed within individual cells of the population. A change in the physiological condition of an Olisthodiscus cell may cause an alteration of this chloroplast complement. For example, during the linear growth period, chloroplast number is reduced to 14 plastids per cell. In addition, when Olisthodiscus cells are grown in medium lacking vitamin B₁₂, plastid replication continues in the absence of cell division thereby increasing the cell's plastid complement significantly.     

A Quantification of the Significance of Assimilatory Starch for Growth of Arabidopsis thaliana L. Heynh

These studies use starch synthesis mutants to quantify the contribution of assimilatory starch to whole plant growth and form. Arabidopsis thaliana (L.) Heynh plants were used with null plastid phosphoglucomutase (T Caspar, SC Huber, CR Sommerville, [1986] Plant Physiol 79; 1-7) or 7% of wild-type ADP-glucose pyrophosphorylase (T-P Lin, T Caspar, CR Sommerville, J Preiss [1988] Plant Physiol 88; 1175-1179). The daily turnover of starch and the rate of biomass increase in the mutants and the wild type were investigated during growth in a 14 hour light/10 hour dark cycle in high irradiance (600 micromoles per square meter per second) and nitrogen (6 millimolar NH₄NO₃), in high irradiance and low nitrogen (0.1 millimolar NH₄NO₃) or in low irradiance (80 micromoles per square meter per second) and high nitrogen. There is some variability in the data, but the following conclusions can be drawn. Growth was slow in the absence of starch turnover. In high nitrogen conditions, about 1 mole of carbon per gram dry weight per day was incorporated additionally into structural biomass for every one mole of carbon turned over as starch per gram dry weight per day. In low nitrogen, the gain was much lower. This indicates that temporary storage of photosynthate is important for rapid growth in high nitrogen, but not in low nitrogen when carbohydrate is in excess. Starch-deficient plants showed the usual decrease of the shoot/root ratio in low nitrogen and increase of the ratio in low light. This shows that adjustment of plant form to nitrogen nutrition and irradiance is not mediated via regulation of photosynthate partitioning in the leaf. Starch deficient plants had lower shoot/root ratios than the wild type and the nitrogen concentration in their leaves was increased. It is discussed how interactions between carbohydrate allocation, respiration and growth at the organ and whole plant level generate these changes. We conclude that mutants with a decreased capacity to carry out a particular partial process provide a powerful tool to disect complex mutually interacting systems, and define and quantify causal interactions at the level of whole plant growth.     

Effect of shortened photosynthetic period on C-assimilate translocation and partitioning in reproductive soyeans

Starch accumulation rate in leaves of vegetative soybeans is inversely related to the length of the daily photosynthetic period. However, it is not known whether a similar response would be observed during reproductive growth. Soybeans (Glycine max L. Merr. cv Amsoy 71) were grown to three stages of reproductive growth (beginning seed, mid seed-fill, and late seed-fill) under 12-hour daylengths, and then shifted to 6-hour photosynthetic periods (12-hour photoperiods) for 4 days. One and 4 days after treatment, a mid-canopy leaf was pulsed with ¹⁴CO₂, and sampled for radiolabeled starch and water-soluble compounds at 0.5, 1, 3, 9, and 21 hours after labeling.

Plants exposed to the 6-hour photosynthetic periods at the beginning seed stage retained and incorporated significantly more label as starch than did those given 12-hour photosynthetic periods. However, plants exposed to the shortened photosynthetic periods at the late seed-fill stage partitioned less label into starch. Plants exposed at mid seed-fill gave a variable response.

Shortened photosynthetic periods resulted in preferential partitioning of recently fixed carbon to the seed at the expense of the pod wall. The results of these experiments suggest that the increased sink demand present during late reproductive growth may be of greater importance in control of leaf starch accumulation than is the length of the daily photosynthetic period.

     

Light-limited growth of two strains of the marine diatom Phaeodactylum tricornutum Bohlin: Chemical composition,carbon partitioning and the diel periodicity of physiological processes

Two isolates of the marine pennate diatom Phaeodactylum tricornutum Bohlin were grown in semi-continuous, nutrient-sufficient culture at varying irradiances on a 12-h light, 12-h dark illumination cycle. The reponse of the isolates to varying degrees of light limitation differed with respect to all of the compositional parameters measured, including growth rates, elemental composition, chlorophyll content, and the partitioning of cellular carbon into four biochemical classes: proteins, lipids, polysaccharides, and low-molecular weight intermediates. The isolates also differed with respect to the relative contributions of light-period and dark-period uptake to the total uptake of ammonium and phosphate ions, although in all cases uptake took place at a reduced rate in the dark. They did not differ with respect to the diel periodicity of cell division, chlorophyll synthesis, and biochemical synthesis. Slightly more cell division took place during the dark period than during the light period. The specific rate of chlorophyll synthesis in the light period, when expressed as a function of irradiance, saturated rapidly; the rate was nearly constant for all irradiances > 100 βE · m^?2 · s^?1. Chlorophyll synthesis in the dark was positively correlated with irradiance over the entire range of irradiances, except where photoinhibition was involved. Protein was synthesized in both the light and dark periods, but at a reduced rate in the dark. Polysaccharides were synthesized during the light period and consumed during the dark period. Lipids and low molecular weight intermediates were synthesized during the light period, but showed little net change during the dark period.     

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.     

Carbon Partitioning in a Flaveria linearis Mutant with Reduced Cytosolic Fructose Bisphosphatase

Oxygen sensitivity and partitioning of carbon was measured in a mutant line of Flaveria linearis that lacks most of the cytosolic fructose-1,6-bisphosphatase found in wild-type lines. Photosynthesis of leaves of the mutant line was nearly insensitive to O₂, as found before. The mutant plants partitioned 2.5 times less carbon into sucrose than the wild type in a pulse chase experiment, with the extra carbon going mainly to starch but also to amino acids. From 10 to 50 min postlabeling, radioactivity chased out of the amino acid fraction to starch in both lines. In the middle of the light period, starch grains were larger in the mutant than in the wild type and covered 30% of the chloroplast area as seen with an electron microscope. Starch grains were found in both mesophyll and bundle sheath chloroplasts in both lines in these C₃-C₄ intermediate plants. At the end of the dark period, the starch levels were considerably reduced from what they were in the middle of the light in both lines. The concentration of sucrose was higher in the mutant line despite the lack of cytosolic fructose-1,6-bisphosphatase. The amino acid fraction accounted for about 30% of all label following a 10-min chase period. In the mutant line, most of the label was in the glycine + serine fraction, with 10% in the alanine fraction. In wild-type leaves, 35% of the label in amino acids was in alanine. These results indicate that this mutant survives the reduced cytosolic fructose-1,6-bisphosphatase activity by partitioning more carbon to starch and less to sucrose during the day and remobilizing the excess starch at night. However, these results raise two other questions about this mutant. First, why is the sucrose concentration high in a plant that partitions less carbon to sucrose, and second, why is alanine heavily labeled in the wild-type plants but not in the mutant plants?     

Comparative proteomic study reveals the involvement of diurnal cycle in cell division, enlargement, and starch accumulation in developing endosperm of Oryza sativa

The development and starch accumulation of cereal endosperms rely on the sugar supply of leaves, which is subject to diurnal cycles, and the endosperm itself also experiences a light/dark switch. However, revealing how the cereal endosperm responds to diurnal input remains a major challenge. We used comparative proteomic approaches to probe diurnally affected processes in rice endosperm (Oryza sativa) 10 days after flowering under 12-h light/12-h dark. Starch granules in rice endosperm showed a growth ring structure under a normal light/dark cycle but not under constant light. Sucrose showed a high level in light and low level in dark. Two-dimensional (2-D) differential in-gel electrophoresis-based proteomic analysis revealed 101 protein spots diurnally changed and 91 identities, which were involved in diverse processes with preferred distribution in stress response, protein synthesis/destination and metabolism. Proteins involved in cell division showed high expression in light and those in cell enlargement and cell wall synthesis high in dark, while starch synthesis proteins were light-downregulated and dark-upregulated. Redox homeostasis-associated proteins showed in-phase peaks under light and dark. These data demonstrate diurnal input-regulated diverse cellular and metabolic processes in rice endosperm, and coordination among these processes is essential for development and starch accumulation with diurnal input.     

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.     

Regulation of photosynthate partitioning into starch in soybean leaves : response to natural daylight

Studies conducted in controlled environments indicate that daylength affects the proportion of photosynthate stored in leaves as starch or sucrose. To examine the response of partitioning to natural daylight, soybeans (Glycine max [L.] Merr. cv Williams) were grown at 12 different times between May and November in a constant temperature greenhouse without supplemental lighting. Plants were transferred from the greenhouse to a controlled environment chamber at the end of civil twilight at a set developmental stage (expanding seventh trifoliolate leaf, counting acropetally). Net photosynthesis and the accumulation of starch and sugar in fully expanded fourth trifoliolate leaves were determined the following day under standard conditions in the chamber (lights-on synchronized with sunrise). Photosynthesis on a leaf area basis decreased about 10% between midsummer and early autumn. Leaf soluble sugar accumulation was low at all harvests. However, a twofold increase in photosynthate partitioning into starch occurred over the same time period, resulting in an 80% increase in absolute starch accumulation rate. Starch was responsible for about 78% of the increase in leaf dry matter during the light at all harvests, indicating that starch accumulation as affected by prior daylight conditions will alter export of photosynthate during the light period. Photosynthate partitioning into starch was linearly correlated with daylength at harvest, prior average peak solar irradiance, and other parameters that correlated with daylength and solar radiation such as harvested top dry matter. The relation between growth and seasonal changes in daylight (including daylength, irradiance, and light integral) are discussed in relation to photosynthate partitioning under field conditions.     

Influence of Carbon Dioxide Concentration on Growth, Carbohydrate Content, Translocation and Photosynthesis of White Clover

White clover ramets were grown at various carbon dioxide concentrations(200, 350 and 1000 µl 1^–1), defoliated and regrownat the same concentrations. Morphological characteristics, dryweights and non-structural carbohydrate contents of plant organs,diurnal variation of sugar and starch content of leaves, translocationof assimilates and photosynthesis were determined. Carbon dioxide concentration influenced the dry weights, butnot the number and size of the plant organs. However, defoliationof plants at low carbon dioxide concentration resulted in decreasedleaf size and stolon length. Carbon dioxide concentration influencedthe content and diurnal variation of starch and sugar in theleaves. Starch was accumulated at medium carbon dioxide concentrationand sugar at a higher concentration when the storage capacityfor starch seemed to be exceeded. Starch was preferentiallyaccumulated in the first and sugar in the second half of thelight period. Translocation was decreased during the periodsof accumulation. Sugar accumulation in the leaves seemed tobe a consequence of the imbalance between sink and source, whereasstarch accumulation seemed to follow an in-built diurnal pattern.Accumulation of both starch and sugar during the photoperiodwas followed by degradation and export during the dark period.Decreased dark export occurred at low carbon dioxide concentrationwhen neither starch nor sugar was accumulated during the photoperiod. Carbon dioxide, white clover, Trifolium repens L., growth, carbohydrates, starch, sugar, translocation, photosynthesis     

Alteration of C-assimilate partitioning in leaves of soybeans having increased reproductive loads at one node

The objectives of this study were to determine if the partitioning of recently fixed carbon between starch and water-soluble compounds could be altered by increasing the pod load in the leaf axil, and if the presence of source leaves acropetal to such a node would influence the partitioning of carbon within the subtending leaf. Soybeans (Glycine max L. Merr. cv Hodgson 78) were grown to full-bloom in a controlled environment chamber, and then deflowered at all nodes except the eighth. This treatment resulted in an 83% increase in the number of pods at the eighth node. At 24 days after flowering, one-half of the treated plants were girdled above the untreated node. Forty-two hours later, the eighth trifoliolate was pulsed with ¹⁴CO₂ and sampled for radiolabeled starch and water-soluble compounds (WSC) at 0.5, 2, 4, 8, 12, and 24th after labeling.

When no girdling was applied above the increased pod load at the eighth node more label was accumulated by the pod walls (+6.9%) and seeds (+6.3%) when compared to the controls. Starch accumulation was not altered in the labeled leaf of the nongirdled plants. When the stem was girdled above the eighth node, significantly less starch was retained in the labeled leaf. Girdling also resulted in an increase in label accumulation by the pod walls (+5.4%) and seeds (+6.6%). These data suggest that the plant will change the distribution patterns of assimilate to supply added sink demand before altering the partitioning of recently fixed carbon in the subtending leaf.

     

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.     

Light-Induced Chloroplast Differentiation in Soybean Cells in Suspension Culture : Ultrastructural Changes during the Bleaching and Greening Cycles

Suspension cultures of SB-P cells of soybean (Glycine max) provide a novel, reproducible, and readily manipulable greening system useful for inducing chloroplast differentiation. The cells are subcultured and grown heterotrophically (3% sucrose) in the dark for at least three successive 14-day periods, subcultured and grown in the dark for 7 days more, and finally placed under white light and grown photoautotrophically. Chlorophyll begins to accumulate by 1 hour of light and continues up to 12 days. The chlorophyll a:chlorophyll b ratio is 3:1. Dark-grown cells contain a small amount of total carotenoids which increase 10-fold during greening. Chloroplast differentiation is strictly light dependent, with photosynthetic pigments accumulating in the light and being lost from cells returned to the dark. In the dark, the chloroplasts dedifferentiate to amyloplasts as the organized thylakoid network is lost and starch accumulates. Under continuous light, the amyloplasts differentiate into mature chloroplasts as the organelle elongates, becomes spanned by several bands of thylakoids, and undergoes grana formation. Chloroplast differentiation in SB-P cells is similar to that in intact angiosperms developing under normal light-dark cycles.     

Photosynthetic Carbon Metabolism in Leaves and Isolated Chloroplasts from Spinach Plants Grown under Short and Intermediate Photosynthetic Periods

Responses of foliar and isolated intact chloroplast photosynthetic carbon metabolism observed in spinach (Spinacia oleracea cv Wisconsin Bloomsdale) plants exposed to a shortened photosynthetic period (7-hour light/17-hour dark cycle), were used as probes to examine in vivo metabolic factors that exerted rate determination on photosynthesis (PS) and on starch synthesis. Compared with control plants propagated continuously on a 12-hour light/12-hour dark cycle, 14 to 15 days were required, subsequent to a shift from 12 to 7 hours daylength, for 7-hour plants to begin to grow at rates comparable to those of 12-hour daylength plants. Because of shorter daily durations of PS, daily demand for photosynthate by growth processes appeared to be greater in the 7-hour than in the 12-hour plants. The result was that 7-hour plants established a 1.5- to 2.0-fold higher total PS rate than 12-hour plants.

Intact chloroplasts isolated from the leaves of 7-hour plants (7-h PLD) displayed 1.5- to 2.0-fold higher PS rates than plastids isolated from 12-hour plants (12-h PLD). Plastid lamellae prepared from 7- and 12-h PLD isolates displayed equivalent rates of ferredoxin-dependent ATP and NADPH photoformation indicating that electron transport processes were not factors in the establishment of higher 7-h PLD PS rates. Analyses, both in leaves as well as intact PLD isolates, of dark to light transitional increases in Calvin cycle intermediates, e.g., ribulose-1,5-bisphosphate (RuBP) and 3-phosphoglycerate (3-PGA), as well as estimations of activities of RuBP carboxylase and fructose-1,6-bisphosphate phosphatase, indicated that 7-hour plant leaves displayed higher PS rates (than 12-hour plants), because there was a higher magnitude of activity of the Calvin cycle.

Although both the foliar level of starch and sucrose, as well as starch synthesis rate, often was higher in 7-hour compared with 12-hour plant foliage, the higher 7-hour plant total PS rates indicated that maximal sucrose and starch levels did not mediate any `feedback' inhibition of PS. The higher 7-hour plant foliar and PLD PS rates resulted in higher glucose-1-P levels as well as a higher ratio of 3-PGA:Pi, both factors of which would enhance the activity of chloroplast ADP-glucose pyrophosphorylase, and which were attributed to be causal to the higher starch synthesis rates observed in 7-hour plant foliage and PLD isolates.

     

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.