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Effect of Pod Number on Dry Matter and Nitrogen Accumulation and Distribution in Soybean¹

Greenhouse and field studies were undertaken to document the effects of a decreased pod number on dry matter and N accumulation and distribution during grain filling in soybean [Glycine max (L.) Merr.]. Removal of about 40% of the pods significantly shifted dry matter partitioning in favor of vegetative tissues. Total dry matter and N accumulation were not affected, indicating that partial depodding did not cause feedback inhibition of photosynthesis. The crop compensated for the lower pod number with an increased 100- seed weight, so that final grain yield was depressed only 7 to 14% below controls.

Vegetative tissue N of control field-grown plants decreased rapidly during grain filling, and the majority of this loss was attributed to redistribution of N to the growing seed. Partial depodding greatly decreased vegetative N loss, and stem N concentrations actually increased toward the end of grain filling. Apparently, current N₂ fixation roughly equalled N demand by the seed in these plants so that little net redistribution occurred. The decreased pod load may also have made more photosynthate available to support greater late-season N assimilation than occurred in the controls.

Key Words: Grain yield • 100-seed weight • Remobilization • Assimilate partitioning • Nitrogen partitioning • Source-sink relationships • Flowering • Grain filling • Senescence • Glycine max (L.) Merr.

² Respectively, research plant physiologist, research plant physiologist, and laboratory technician, Plant Physiology Dep., Shell Agric. Chemical Co., P.O. Box 4248, Modesto, CA.

Received for publication October 7, 1985.