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Leaf N and Photosynthesis during Reproductive Growth in Soybeans¹

The redistribution of N from the vegetative plant parts and pod walls to the seed of soybeans [Glycine max (L.) Merr.] during reproductive growth can be a major source of N for the developing seed. It has been suggested that the redistribution of N may be associated with leaf senescence. Experiments were conducted to evaluate the redistribution of N and the changes in photosynthesis in two soybean cultivars that differed in the duration of seed fill. ‘Williams’ and ‘Lincoln’ were grown in the field using conventional cultural practices for 2 years and total N, protein (1 N NaOH extractable), and CO₂ uptake rate (measured with a ¹⁴CO₂ technique) were determined for two leaf positions (last fully expanded and 10th node) during reproductive growth. Total N (mgdm^-2) in the last fully expanded leaves began to decline at growth stage R6 and declined steadily until leaf abscission occurred. Total N in the leaves from the 10th node began to decline sooner but the levels in the leaves at abscission were the same as in the last fully expanded leaves. The decline in protein generally followed a pattern similar to total N. The CO₂ uptake rate increased to a maximum during the beginning of seed fill (growth stage R5) and started to decline early in growth stage R6. There was no difference between cultivars in the relationship between CO₂ uptake rate and the levels of leaf protein. The earlier senescence of the short filling period cultivar (Lincoln) was associated with more rapid decline in CO₂ uptake rate compared with Williams.

Key Words: Seed-filling period • Glycine max (L.) Merr. • Senescence • N redistribution

² Former graduate research assistant (currently assistant professor, Chulalongkorn Univ., Bangkok, Thailand) and professors of Agronomy, Univ. of Kentucky, Lexington, KY 40546-0091.

Received for publication June 17, 1982.