Relation of Leaf Nitrogen Content and Other Traits with Seed Yield of Soybean

Abstract

Soybean redistributes or"mobilizes"a large amount of its vegetative nitrogen in support of synthesis of seed storage protein. Most of this is from leaf tissue. Our objective was to elucidate the relationship between potentially mobilizable leaf nitrogen and seed yielding capacity of soybean. In each of two years, we grew 63 diverse soybean genotypes in a replicated field experiment. Whole plants were harvested from 1 m² of plot area. Leaf mass, leaf area index, and leaf nitrogen concentration were determined at beginning of seed growth (R5). Seed yield was obtained by combine harvest. Seed yields ranged from 2400 to 4400 kg ha^-1 the first year and from 2200 to 3800 kg ha^-1 the second. Total leaf nitrogen content at beginning of seed growth, after adjustment for differences in reproductive duration and lodging, accourited for 40 and 34% of genotypic variation in seed yield for the two years. Leaf mass, leaf area index, and leaf nitrogen concentration were less well related to yield. Reproductive duration, estimated as the time from beginning bloom to maturation, accounted for 30 and 20% of genotypic variation in seed yield, and lodging accounted for 21 and 23% for the two years. The three most important factors, total leaf nitrogen content at R5, reproductive duration, and lodging, as determined by multiple regression, accounted for 64 and 59% of genotypic variation in yield the two years. Hypothetical selection for total leaf nitrogen content at beginning of seed growth revealed a lack of consistency in genotypic performance between years, suggesting low heritability of the trait and therefore a low probability for improving the trait through plant breeding. We conclude that the failure of genotypes to perform similarly between years implies that the vegetative nitrogen pool is a very important, albeit secondary, source that is drawn upon variably in different environments depending upon the plant's capacity to assimilate nitrogen directly.