Seedling Development and Deposition of Starch and Storage Proteins in Alfalfa Roots

Abstract

The physiological basis for defoliation tolerance in alfalfa (Medicago sativa L.) is associated with taproot organic reserves, including soluble sugars, starch, and vegetative storage protein (VSP). It is not known when organic reserve accumulation begins in developing seedlings or the factors that influence their deposition. The objectives of three greenhouse experiments were to determine (i) the onset starch and VSP accumulation in roots of alfalfa seedlings, (ii) if N fertilization of seedlings stimulated root VSP deposition, and (iii) seedling defoliation tolerance is associated with the presence of root organic reserves. In Exp. 1, roots and shoots of ‘Vernal’, ‘Resistar’, and ‘WL 605’ were sampled periodically beginning 19 d after planting (DAP) to determine the developmental paltern of starch and VSP deposition in roots. In Exp. 2, normal and ineffective nodulating ‘Saranac’ was grown with or without 10 mM N to determine if applied N stimulates early VSP deposition. In Exp. 3, alfalfa seedlings were completely defoliated (including cotyledons) 14, 28, 42, and 56 DAP, and shoot regrowth and seedling survival were used to measure defoliation tolerance. In all experiments, starch, soluble sugars, protein, and VSP concentrations in roots were determined. Root starch and VSP concentrations were low up to 30 DAP. Starch and VSP accumulation began by 39 DAP and coincided with declining root protein and soluble sugar concentrations. Addition of N increased herbage growth but did not stimulate early VSP deposition. In Exp. 3, ≈80% of seedlings survived complete defoliation 14, 42, and 56 DAP, while only 52% survived defoliation 28 DAP. Roots contained low starch and VSP concentrations 14 and 28 DAP. Although seedling survival 42 and 56 DAP was associated with presence of starch and VSP in seedling roots, defoliation tolerance of alfalfa seedlings 14 DAP appears to be influenced by factors other than root starch and VSP concentrations.