Impact of root C and N reserves on shoot regrowth of defoliated alfalfa cultivars differing in fall dormancy

Key determinants of regrowth rate following harvest in alfalfa (Medicago sativa L.) are the levels of root carbon (C) and nitrogen (N) reserves. This study was undertaken to assess the impact of alfalfa cultivars differing in fall dormancy on regrowth potential in relation to the quantitative changes in root C and N reserves at the onset of regrowth. Five contrasting alfalfa cultivars were grown in the field and were harvested three times. Shoot regrowth dry weight increased significantly with decreasing fall dormancy. Alfalfa cultivars with less dormancy tended to have higher concentrations of root C and N reserves, whereas significantly greater pools of root C and N reserves were found in roots at each harvest time because of greater root dry weight. Shoot regrowth was positively correlated to the pools of root soluble sugar, starch and total non-structural carbohydrates, but not correlated to their concentrations at each harvest time. Shoot regrowth was also linearly related to the pools of root total N and soluble proteins, but not consistently related to their concentrations. Our results suggest that the total amount of C and N organic reserves in alfalfa roots rather than their concentrations are determining factors of shoot regrowth.