Seasonal carbohydrate and nitrogen metabolism in roots of contrasting alfalfa ( Medicago sativa L.) cultivars

Abstract

Summary Prostrate shoot growth of fall dormant alfalfa (Medicago sativa L.) cultivars in autumn is positively associated with winter survival. Our objective was to determine how carbohydrate and nitrogen pools in roots of alfalfa cultivars exhibiting contrasting fall dormancy change during winter hardening in autumn and when shoot growth resumes in spring. Sugars, buffer-soluble protein, low molecular weight-N, and vegetative storage proteins (VSPs) increased in roots of all cultivars in autumn, while root starch concentrations declined throughout autumn and winter. Sugar, protein, low molecular weight-N, and VSPs levels declined in spring as shoot growth resumed, then re-accumulated in roots as shoots began to flower on June 2. Defoliation on June 2 resulted in a loss of starch, protein, and VSPs from roots as shoots regrew. Roots of fall dormant, winter hardy cultivars contained higher concentrations of sugars and buffer soluble protein in November and December, whereas higher concentrations of starch and low molecular weight-N were found in roots of nondormant cultivars at these times. Concentrations of total N and VSPs were similar between dormant and nondormant cultivars indicating that N deficiency caused by low dinitrogen fixation during hardening is not a factor contributing to the poor winter survival of nondormant alfalfa. Efforts aimed at understanding fall dormancy and winter hardiness of alfalfa should focus on mechanisms controlling accumulation of sugars and specific (non-VSP) soluble proteins in roots in autumn.