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Day and Night Temperature Effects on Nitrogen and Soluble Carbohydrate Allocation During Early Reproductive Growth in Soybeans¹

By altering the timing of reproductive events, temperatures may affect allocation of N and C within soybean [Glycine max (L.) Merrill]. The objective of this study was to determine the changes in partitioning associated with early development of the reproductive sinks. Plants of the determinate cultivar ‘Ransom’ growing in controlled environments were transferred to inductive shortday photoperiods at the V3 stage and grown under 25 combinations of day/night temperatures over a range of day temperatures of 14 to 30 C and night temperatures of 10 to 26 C. Plants from each combination were sampled at 0, 20, 30, and 50 days after the first inductive photoperiod. Plants were separated into leaves, stems, and pods, freeze-dried, and analyzed for total N and soluble carbohydrates.

During the first 20 days after plants were transferred to short days and before pods appeared, increases in rates of accumulation of dry matter, N and soluble carbohydrates in leaves and stems were related to increases in both day and night temperatures. Time of pod appearance was inversely related to temperature. As pod growth became an active sink, accumulation rates of dry matter, N and soluble carbohydrates decreased in leaves and stems, but even at higher temperatures with the greatest rates of pod development, net accumulation of dry matter and N (and soluble carbohydrates in some instances) were measured during the final sampling interval. The most advanced plants at Day 50 were approaching full seed (R6) development stage. Nitrogen and soluble carbohydrate concentrations in leaves and stems were different among temperature treatments at the initial sampling on 20 days after treatments were imposed and before pods had appeared. During the subsequent growth intervals, there were no further variations of N concentration in leaves and stems within temperature treatments in association with stage of pod development through R6. However, concentrations of soluble carbohydrates had begun to decline as the stage of pod development approached R6. These results indicate that until the stage of seed fill, the greatest effect of reproductive growth on N reserves in vegetative tissues may be in limiting the amount of vegetative tissues rather than in net remobilization from these pools.

Key Words: Anthesis • Glycine max (L.) • Merrill • Partitioning • Pod growth • Thermoperiod

² Assistant professor, Dep. of Botany, associate professor, Dep. of Soil Science, and associate professor, Dep. of Crop Science, North Carolina State Univ., Raleigh, NC 27650.

Received for publication June 26, 1980.