RESUMEN
To evaluate daytime and nighttime carbon balance and assimilate export in soybean (Glycine max [L.] Merrill) leaves at different photon flux densities, rates of CO(2) exchange, specific leaf weights, and concentrations of sucrose and starch were measured at intervals in leaves of pod-bearing ;Amsoy 71' and ;Wells II' plants grown in a controlled environment room. Assimilate export was estimated from CO(2) exchange and change in specific leaf weight. Total diurnal assimilate export was similar for both cultivars. Large cultivar differences existed, however, in the partitioning of carbon into starch reserves and the relative amounts of assimilate exported during the day and the night. Total amounts of both daytime and nighttime export increased with increasing photon flux density, as did sucrose and starch concentrations, specific leaf weight, and rate of respiratory carbohydrate loss at night. Cultivar differences in nighttime rate of export were more closely related to the differences in amount of assimilate available at the end of the day than to differences in daytime rate of net CO(2) assimilation. Daytime rates of export, however, were closely related to daytime rates of net CO(2) assimilation within each cultivar. The total amount of starch depleted during the 10-hour night increased as starch concentration at the beginning of the night increased.
RESUMEN
To evaluate assimilate export from soybean (Glycine max [L.] Merrill) leaves at night, rates of respiratory CO(2) 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 CO(2) 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 CO(2) 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.
RESUMEN
The relationships between various carbohydrate pools of the soybean (Glycine max [L.] Merrill) fruit and growth rate of seeds were evaluated. Plants during midpod-fill were subjected to various CO(2) concentrations or light intensities for 7 days to generate different rates of seed growth. Dry matter accumulation rates of seeds and pod wall, along with glucose, sucrose, and starch concentrations in the pod wall, seed coat, and embryo were measured in three-seeded fruits located from nodes six through ten. Seed growth rates ranged from 4 to 37 milligrams.day(-1).fruit(-1). When seed growth rates were greater than 12 milligrams.day(-1).fruit(-1), sucrose concentration remained relatively constant in the pod wall (1.5 milligrams.100 milligrams dry weight(-1)), seed coat (8.5 milligrams.100 milligrams dry weight(-1)), and embryo (5.0 milligrams.100 milligrams dry weight(-1)). However, sucrose concentrations decreased in all three parts of the fruit as growth rate of the seeds fell below 12 milligrams.day(-1).fruit(-1). This relationship suggests that at high seed growth rates, flux of sucrose through the sucrose pools of the fruit was more important than pool size for growth. Starch concentration in the pod wall remained relatively constant (2 milligrams.100 milligrams dry weight(-1)) at higher rates of seed growth but decreased as seed growth rates fell below 12 milligrams.day(-1).fruit(-1). This suggests that pod wall starch may buffer seed growth under conditions of limiting assimilate availability. There was no indication that carbohydrate pools of the fruit were a limitation to transport or growth processes of the soybean fruit.
RESUMEN
Relationships between respiration rate and adenylate and carbohydrate pools of the soybean (Glycine max L. Merrill) fruit during rapid seed growth were evaluated. Plants at mid pod-fill were subjected to different concentrations of CO(2) to alter the amount of photosynthate produced and, thus, available to the fruit. Respiration rate of the intact fruits was measured, along with glucose, sucrose, and starch concentrations, adenylate energy charge (AEC), and total adenylate pool (SigmaAdN) in the pod wall, seed coat, and cotyledons. The concentration of sucrose remained relatively constant in the pod wall (1.0 milligram per 100 milligrams dry weight), seed coat (6.5 milligrams per 100 milligrams dry weight), and cotyledons (4.5 milligrams per 100 milligrams dry weight) at moderate and high respiration rates. Furthermore, AEC remained relatively constant in the pod wall (0.55), seed coat (0.24), and cotyledons (0.44) during changes in respiration rate. This suggests that the amount of assimilate transported to the fruit, and its flux through the sucrose pools of the fruit parts, were important in the regulation of the respiration rate of the fruit. The average SigmaAdN in the seed coat (1300 picomoles per milligram dry weight) was significantly greater than in the cotyledons (750 picomoles per milligram dry weight) and pod wall (300 picomoles per milligram dry weight). In addition, the SigmaAdN in the seed coat and cotyledons increased with increasing respiration rate of the fruit. The high SigmaAdN in the seed coat and its increase with increases in respiration rate of the fruit suggest that an energy-requiring process is involved in the movement of sucrose through the seed coat.
RESUMEN
To evaluate leaf carbon balance during rapid pod-fill in soybean (Glycine max [L.] Merrill), measurements were made of CO(2) assimilation at mid-day and changes in specific leaf weight, starch, and sucrose concentrations over a 9-hour interval. Assimilate export was estimated from CO(2) assimilation and leaf dry matter accumulation. Chamber-grown ;Amsoy 71' and ;Wells' plants were subjected on the day of the measurements to one of six photosynthetic photon flux densities in order to vary CO(2) assimilation rates.Rate of accumulation of leaf dry matter and rate of export both increased as CO(2) assimilation rate increased in each cultivar.Starch concentrations were greater in Amsoy 71 than in Wells at all CO(2) assimilation rates. At low CO(2) assimilation rates, export rates in Amsoy 71 were maintained in excess of 1.0 milligram CH(2)O per square decimeter leaf area per hour at the expense of leaf reserves. In Wells, however, export rate continued to decline with decreasing CO(2) assimilation rate. The low leaf starch concentration in Wells at low CO(2) assimilation rates may have limited export by limiting carbon from starch remobilization.Both cultivars exhibited positive correlations between CO(2) assimilation rate and sucrose concentration, and between sucrose concentration and export rate. Carbon fixation and carbon partitioning both influenced export rate via effects on sucrose concentration.
RESUMEN
Net photosynthetic rate, CO(2) compensation concentration, and starch and soluble sugar concentrations were measured in soybean (Glycine max [L.] Merrill) leaves in an attempt to evaluate the effect of carbohydrate concentration on rate of CO(2) assimilation.Plants were grown in a controlled environment room at 23.5 C, 50% relative humidity, 16-hour photoperiod, and quantum flux (400-700 nm) of 510 mueinsteins/m(2).sec (30,090 lux) at plant level. On the 21st day after seeding, plants were subjected for 12.5 hours to one of three CO(2) concentrations (50, 300, or 2000 mul/l) in an attempt to alter leaf carbohydrate levels. Following the CO(2) treatment, gas exchange measurements were made at a CO(2) concentration of 300 mul/l on the lowermost trifoliolate leaf. Immediately after measurement, the leaf was removed and stored at -20 C until carbohydrate analyses were performed.Increasing the CO(2) concentration for 12.5 hours significantly increased leaf starch concentration but not soluble sugar concentration. There was a strong negative correlation between net photosynthetic rate and starch concentration. Net photosynthetic rate declined from approximately 38 to 22 mg CO(2)/dm(2) leaf area.hr as starch concentration increased from 0.5 to 3 mg/cm(2) leaf area. Carbohydrate concentrations had no effect on compensation concentration.The decrease in net photosynthetic rate as starch concentration increased resulted from an increase in mesophyll (liquid phase) CO(2) diffusion resistance. This suggests that starch accumulation may reduce net photosynthetic rate by impeding intracellular CO(2) transport.
RESUMEN
Rates of net photosynthesis and translocation, CO(2) diffusive resistances, levels of carbohydrates, total protein, chlorophyll, and inorganic phosphate, and ribulose 1,5-diphosphate carboxylase activity were measured in soybean (Glycine max L. Merrill) leaves to ascertain the effect of altered assimilate demand. To increase assimilate demand, the pods, stems, and all but one leaf (the "source leaf") of potted plants were completely shaded for 6 or 8 days and the responses of the illuminated source leaf were monitored. Rate of net photosynthesis in the source leaf of the shaded plants was found to increase curvilinearly to a maximum on the 8th day. The source leaf of the control plants (no sink shading) maintained a constant photosynthetic rate during this period. Vapor-phase resistance to CO(2) diffusion did not vary with treatment, but mesophyll (liquid phase) resistance was significantly lower in the source leaf of the shaded plants.Starch concentration in the source leaf of shaded plants decreased more than 10-fold during the 8-day shading period. In this same period, sucrose concentration rose nearly 3-fold. Conversely, in the source leaf of the unshaded plants, starch concentration remained high (23% of leaf dry weight) and sucrose concentration remained very low (1.2%). When measured on the 8th day of treatment, translocation rate, ribulose 1,5-diphosphate carboxylase activity, and inorganic phosphate concentration were found to be significantly higher in the source leaf of the shaded plants than in the control source leaf.When shaded plants were again illuminated, all measured response trends in the source leaf were reversed. These data indicate that assimilate demand has a marked influence on source-leaf photosynthesis and carbohydrate formation and export.
RESUMEN
An apparatus to produce continuous gas mixtures for use in measurements of plant gas exchange is described. A wide range of CO(2) and water vapor concentrations can be provided and O(2) concentration can be varied from 0 to 21%. Changes in the concentrations of the components are accomplished conveniently, rapidly, and independently. With occasional adjustments, CO(2) and O(2) concentrations can be maintained to within +/- 1 mul/l and +/- 0.1%, respectively. Dew point of the gas mixture can be maintained to within +/- 0.05 C.
RESUMEN
A study was made of diurnal trends in net photosynthetic rate and carbohydrate levels of unifoliolate leaves of soybean (Glycine max L. Merrill) under constant environmental conditions (50,000-lux light intensity, 24.5 C air temperature, 60% relative humidity, and 300 microliters of CO(2) per liter of air).Net photosynthetic rate remained relatively constant between 4 and 10 hours after the lights were turned on but then gradually declined to 85% of this rate by the end of the 16-hour photoperiod. The decline in net photosynthetic rate was due to increases in both stomatal diffusion resistance and residual resistance to CO(2).The decline in net photosynthetic rate began when the rate of starch accumulation began to decline rapidly. At this time, there also appeared to be an increase in soluble carbohydrate level. The results suggest that when a high starch level was reached, further starch synthesis was impaired, leading to an increase in soluble carbohydrate level and, consequently, a reduction in net photosynthetic rate.