ASSIMILATE EXPORT FROM SOYBEAN LEAVES AT NIGHT
Abstract
To evaluate assimilate export from soybean (Glycine max (L.) Merrill) leaves at night, rates of respiratory CO$\sb2$ loss, specific leaf weight loss, starch mobilization, and changes in sucrose concentration were measured during a 10-h dark period in leaves of pod-bearing 'Amsoy 71' and 'Wells II' plants. Plants were grown in a controlled environment room with a 14-h light/10-h dark period, constant 25 $\pm$ 1$\sp\circ$C temperature, and approximately 50% RH. Rate of assimilate export during the dark period 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 with leaf starch concentration. Sucrose concentration rapidly declined during the first hour of darkness and then remained constant. Rate of assimilate export was not associated with leaf sucrose concentration. Cultivar differences in respiratory CO$\sb2$ loss were observed but these differences did not influence differences in export rates. To evaluate daytime and nighttime assimilate export at different photon flux densities, rates of daytime and nighttime CO$\sb2$ exchange and carbohydrate concentrations at 2200 and 0800 h were measured at different photon flux densities. Plants were grown in the same environmental conditions as described above. Total diurnal export was similar for both cultivars. Large cultivar differences existed, however, in the total amount of assimilates exported during both the day and the night. The amount of photosynthetically fixed carbon exported during the day versus that retained as starch in the leaf for export during the night differed between cultivars. Cultivar differences in rate of nighttime export were closely related to the differences in the amount of assimilates available at the end of the day. Rates of daytime export were closely related to net rates of daytime CO$\sb2$ assimilation in leaves of both cultivars. The total amount of starch depleted during the 10-h night increased as photon flux density increased. This indicates that the overall rate at which starch was mobilized was associated with the amount of starch present.
Degree
Ph.D.
Subject Area
Agronomy
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