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

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Title: Seed Nitrogen Content of Soybean: Mobilization of Nitrogen Reserves and Its Relationship to Seedling Growth.
Author: Naegle, Erin Rochelle
Advisors: Thomas Rufty, Committee Chair
James Holland, Committee Member
Judith Thomas, Committee Member
Prachuab Kwanyuen, Committee Member
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.
Date: 2002-06-17
Degree: MS
Discipline: Crop Science

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