Rates of net photosynthesis and translocation, C02 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) were planted in 1-liter plastic pots containing a fertile greenhouse soil-vermiculite mix (5:2 v/v). Plants were thinned to one per pot 1 week after seeding and were watered daily. Plants were grown in a controlled environment room with a 16-hr photoperiod and a 23.5 ± 1.0 C temperature. A mixture of fluorescent and incandescent lamps supplied a quantum flux (400-700 nm) of 510 ± 50 Meinsteins/m2-sec (30,090 lux). Relative humidity was maintained at about 50%.CO2 Treatments. Twenty-one days after seeding, eight plants were randomly selected for each treatment. At the beginning of the photoperiod, the plants were placed in a glass chamber (30 x 30 x 60 cm) equipped with a circulating fan. Plants were watered and a clear acrylic lid was placed over the chamber. The chamber was then placed into a growth cabinet, and the temperature inside the plant chamber was maintained at 26 ± 1 C. A mixture of fluorescent and incandescent lamps provided a quantum flux (400-700 nm) of 400 ± 30 geinsteins/m2 -sec (23,600 lux). Relative humidity in the plant chamber was 65 ± 10%.
Leaf area (A) and leaf dry weight (LW) were determined throughout the growing season for lower, middle, and upper main stem segments and branches of a field soybean [Glycine max (L.) Merrill] community. The A/LW ratios were regressed on number of days after planting, using polynomial regression models. For the middle main‐stem segment, A was also regressed on LW.Mean seasonal A/LW decreased from the base to the top of the canopy. Mean A/LW of branches was higher than that of any main‐stem segment.The A/LW ralio of each main‐stem segment continually declined until just prior to leaf senescence. The trend subsequently reversed and A/LW increased rapidly during leaf abscission. The trend in A/LW of the branches differed in that there was an initial increase in A/LW, followed by a decline and then a second increase during leaf abscission.Due to time related changes in A/LW, models for the prediction of soybean leaf area from leaf weight should also include time as a variable if observations are made at different growth stages.
A study was made of diurnal trends in net photosynthetic rate and carbohydrate levels of unifoliolate leaves of soybean (Glycine ntax L. Merrill) under constant environmental conditions (50,000-lux light intensity, 24.5 C air temperature, 60% relative humidity, and 300 microliters of C02 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 C02. Tlhe subject of a recent review by Neales and Incoll (14) was the hypothesis that assimilate accumulation reduces leaf photosynthetic rate. It is notable that much of the evidence for the correlation between assimilate level and photosynthetic rate has been obtained in experiments employing treatments designed to increase the level of leaf assimilates above normal levels. In the study reported here, we evaluated the association between diurnal trends in net photosynthetic rate and carbohydrate level of young, unifoliolate leaves of soybean (Glycine max L. Merrill). The daily accumulation of leaf carbohydrates provided a range of nornal assimilate levels and therefore eliminated some of the risks imposed by artificial treatments.Tsuno and Fujise (17) performed similar experiments with sweet potato (Ipomoea batatas) and determined that there was no reduction in photosynthetic rate, in spite of an increase in leaf carbohydrates, up to 4:00 PM, at which time their measurements were terminated. We present evidence that later in the day there is a reduction in net CO2 assimilation rate of soybean leaves, which may be associated with leaf carbohydrate level.
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