Seed Nitrogen Content of Soybean: Mobilization of Nitrogen Reserves and Its Relationship to Seedling Growth.

Abstract

Leguminous crops such as soybean are commonly grown in the relatively infertile soils of the southeastern U.S. The primary source of N for soybean growth and development in these environments is N2-fixation, which requires a symbiotic relationship that does not develop until 3 to 4 weeks after germination. Prior to N2-fixation, plants are largely dependent on seed reserves and they often experience a period of N stress. The purpose of this thesis was to investigate mobilization of seed N and its impact on soybean seedling development.

Sixteen soybean lines differing in seed N content were grown hydroponically for 27 days without external N. Higher seed N was associated with increased seedling growth and reduced expression of N deficiency symptoms. Three of the 16 lines were selected for detailed characterization of seed protein degradation and N mobilization, and their relationship with seedling developmental responses during progression into and recovery from N stress. Leaf expansion and initiation were restricted more severely in soybean lines with lower seed N content. Depressed canopy development was the primary factor leading to decreased shoot:root growth ratios in all 3 lines. The soybean line with the lowest seed N content had a higher S/R ratio as the N stress progressed. The shoot and root growth changes were different than those in previous N deficiency studies, where adjustments have been proportional to the severity of N stress. When external N was supplied to plants deprived of N for 15 or 23 days, the induction period of the nitrate uptake process was extended and growth recovery rates were correlated with initial seed N contents. There was no delay, however, in stimulation of leaf initiation rates, which responded rapidly to the presence of external N. Individual leaf expansion during the recovery from N stress was dependent upon a lea'?s developmental stage.

The majority of N was mobilized out of cotyledons within 12 days in three soybean lines with differing seed N contents. Mobilization was complete before differences in seedling growth were measurable. Mobilization rates were lower when external N was present, suggesting the involvement of source/sink relations on the mobilization process. Differences in proteolysis of glycinin and beta-conglycnin, the main storage proteins in soybean seeds, between N treatments were not detectable. Storage protein content and proteolysis rates were proportional to differences in seed N content.