Shoot growth and endogenous nitrogen turnover in alfalfa taproots
Abstract
Alfalfa (Medicago sativa L.) is a perennial legume utilized as a high-protein forage crop. As a forage crop, the plants are frequently defoliated. Since initial rate of regrowth is an important determinant of final yield per shoot, this research is directed towards understanding the limitations on regrowth imposed by: (1) bud shoot position on crowns and (2) endogenous nitrogen (N) supply. Shoots originating from different positions on the crown display different regrowth rates. Two shoot positions were identified on alfalfa crowns, basal and stubble. Basal shoots originate from buds sessile on the crown, while stubble shoots originate from leaf axils present on shoot bases after defoliation. Basal shoots were found to elongate 1.5 times faster and had greater leaf area expansion rate than stubble shoots. When specific leaf positions of basal and stubble shoots were labelled with $\sp{14}$CO$\sb2$, significant position did not effect distribution $\sp{14}$C-labelled compounds. However, radioactivity was higher in elongating internodes than in expanding leaves. Partitioning of radiolabel was not affected by mass or sink activity as elongating internodes of stubble shoots accumulated more radioactivity did basal shoots. In a second set of experiments, the utilization of N accumulated in taproots to support shoot regrowth was examined because N$\sb2$ fixation diminishes after defoliation. We suspect endogenous N redistribution may meet part of the N required by shoots during early regrowth. Total N in bark and wood tissues of taproots declined during early regrowth (0-21 days) before reaccumulating during late regrowth. The magnitude of the depletion was greater in root bark tissues than in root wood tissues. Buffer-soluble proteins and TCA-soluble amino-N declined and reaccumulate during regrowth as well. Protease activity increased to a maximum midway through regrowth before declining. Denaturing (SDS) polyacrylamide gel electrophoresis of buffer-soluble proteins indicated that certain proteins were preferentially degraded and reaccumulated. In this species, N availability and metabolism of specific pools in taproots might be an important factor in determining rate of shoot regrowth.
Degree
Ph.D.
Advisors
Volenec, Purdue University.
Subject Area
Botany|Agronomy
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