Soy flour derived from low-oligosaccharide soybeans resulted in less gas production than that derived from conventional soybeans.

terized by higher rates of CO2 fixation, sucrose formation and assimilate export compared with C3 plants (2). Carbon assimilation by the C4 pathway is not completely light saturated at full sunlight and therefore the rate of CO2 uptake is subject to large changes with natural fluctuations in light intensity during the day. In contrast, carbon assimilation in C3 plants is usually saturated at a lower irradiance and therefore does not fluctuate with changes in natural illumination to the same extent as C4 plants.Recently, photosynthesis by the C3 plant soybean was characterized diurnally. In this study (10), the activity of SPS4 was shown to oscillate with an endogenous rhythm. SPS activity was correlated positively with assimilate export rate during the day and maximum SPS activity was similar to the measured assimilate export rate. Maize (11) and barley (12) leafSPS have recently been shown to be light activated; however, it is not known whether SPS from these sources may be controlled by an endogenous rhythm in activity as well as by short-term light modulation.Our goal in conducting the present study was to characterize diurnal changes in maize leaf carbon assimilation, partitioning and export, in relation to changes in the activities of SPS and key enzymes of the C4 pathway. To determine whether diurnal fluctuations in enzyme activities were caused by an endogenous rhythm, enzyme activities were also measured in plants after reciprocal light/dark transfers at different times during the diurnal cycle.

Experiments were conducted with vegetative soybean plants (Glycine max [L.] Meff., '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.Sucrose is the primary transport carbohydrate in most higher plants (1 1). In leaves of C3 plants, synthesis of sucrose occurs in the cytoplasm of mesophyll cells (3,25,30). The principal precursors for the sucrose biosynthetic pathway are triose phosphates which are generated within the chloroplast and exported to the cytoplasm by way of the phosphate translocator ( 17, 30). In the light, triose phosphates originate from concurrent photosynthesis and in the dark from degradation of starch. Decreased export of assimilates and lower sucrose levels in leaf tissue (8,10,12,16)

Experiments were conducted with soybean (Glycine max [L.] Merr. cv 'Ransom') plants to determine if diurnal rhythms in net carbon dioxide exchange rate (CER), stomatal resistance, and sucrose-phosphate synthase (SPS) activity persisted in constant environmental conditions (constant light, LL constant dark DD) and to assess the importance of these rhythms to the production of nonstructural carbohydrates (starch, sucrose, and hexose). Rhythms in CER, stomatal resistance, and SPS activity were observed in constant environmental conditions but the rhythms differed in period length, amplitude, and phase. The results indicated that these photosynthetic parameters are not controlled in a coordinated manner. The activity of UDPG pyrophosphorylase, another enzyme involved in sucrose formation, did not fluctuate rhythmically in constant conditions but increased with time in plants in LL. In LL, the rhythm in CER was correlated positively with fluctuations in total chlorophyll (r = 0.810) and chlorophyll a (r = 0.791) concentrations which suggested that changes in pigment concentration were associated with, but not necessarily the underlying mechanism of, the rhythm in photosynthetic rate. Assimilate export rate, net starch accumulation rate, and leaf sucrose concentration also fluctuated in constant light. No single photosynthetic parameter was closely correlated with fluctuations in assimilate export during LL thus, assimilate export may have been controlled by interactions among the endogenous rhythms in CER, SPS activity, or other metabolic factors which were not measured in the present study.Circadian rhythms of certain physiological and biochemical processes have been observed in many unicellular and multicellular plant species (3,6,8,24). Rhythmic processes include phtosynthetic parameters, such as 02 evolution (16-18, 23, 26), net C02 fixation (2,19,20), Chl a concentration (2), whole chain electron flow (16, 18), and electron flow through PSII (26). Although rhythms in C3 photosynthesis apparently are not caused by changes in the activity of enzymes of the Calvin cycle (7,17), rhythms in CAM photosynthesis are, in part, attributed to the rhythm in PEP carboxylase activity (24). Thus, it is evident that enzymes of carbon metabolism, as well as components of Recently, the activity of SPS,2 a key regulatory enzyme of sucrose formation (9, 10, 12), was reported to change diurnally in soybean leaves (25). The plants from which these observations were made had not been cultured nor measured in constant environmental conditions, so it is not known if the rhythm is endogenously controlled or attributable to some exogenous stimulus (i.e. light or temperature). Although the metabolic implications of rhythms in SPS activity and net CO2 fixation are not entirely clear at present, a synchronization (or lack thereof) between these rhythms could conceivably affect carbohydrate formation.To determine if rhythms in SPS activity and net CO2 fixation are endogenously controlled and to assess the potential significance o...

No abstract