Alfalfa Root Nitrogen Reserves and Regrowth Potential in Response to Fall Harvests

Abstract

The adverse effect of fall harvesting alfalfa (Medicago sativa L.) during a critical rest period on persistence and the following spring regrowth has been historically attributed to a reduction in the concentrations of organic reserves, especially total nonstructural carbohydrates. However, recent reports highlight the determinant role of N reserves in overwintering and spring regrowth of alfalfa. This study was undertaken to assess the impact of fall harvest management on regrowth potential in relation to the quantitative changes in N reserves in alfalfa taproots throughout fall and winter. The experiment was conducted under simulated winter conditions in an unheated greenhouse with two alfalfa cultivars (AC Caribou and WL 225). The fall harvest treatments were no additional fall harvest (two harvests = control) or a third fall harvest applied at 400, 500, or 600 growing degree days (GDD) after the second harvest. Total N concentrations were significantly reduced in plants harvested at 400 or 500 GDD as compared with plants harvested at 600 GDD or harvested only twice. The striking accumulation of proline, arginine, and histidine observed in fall and winter was depressed by a fall harvest, especially in plants harvested at 400 or 500 GDD. The abundance of a major soluble protein of 32 kDa was reduced by harvesting at 400 or 500 GDD. Concentrations of major N components were correlated with shoot regrowth in spring in AC Caribou, but not in WL 225. However, the total amounts of major N components in taproots were correlated with spring regrowth in both cultivars. Our results point out that N reserves available in roots are determinant for spring regrowth in alfalfa under various fall harvest treatments.