Taproot nitrogen accumulation and use in overwintering alfalfa ( Medicago sativa L.)

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Alfalfa (Medicago sativa L.) taproots accumulate organic reserves that are important for winter survival and subsequent growth in spring. Our objective was to determine if specific nitrogen (N) pools accumulate in taproot tissues prior to winter that may subsequently be used during initiation of herbage growth in spring. Taproots were obtained at approximately monthly intervals during fall and winter, and biweekly in early spring. Taproots were separated at the cambium into bark and wood tissues. Bark tissues consistently contained higher N concentrations than did wood tissues. N concentrations of both tissues gradually increased between early and late fall and declined in early spring when growth was initiated. Both soluble amino-N and buffer-soluble proteins increased during autumn and declined extensively during early spring in both tissues. A nonwinterhardy alfalfa line accumulated less soluble protein in taproot tissue when compared to a hardy genotype. Specific proteins with molecular masses of 32, 19, and 15 kDa were identified as major components of the buffer-soluble protein pool. These proteins rapidly disappeared from taproot tissues in spring as buffer-soluble protein concentrations declined. Protease activity in bark tissues declined gradually during late autumn and winter before increasmg over two-fold in early spring. Protease activity in wood tissues was approximately one-half that of bark tissues and also increased in spring when growth resumed. Our results indicate that high concentrations of soluble amino compounds and specific proteins accumulate in taproots during autumn and early winter. These N pools decline markedly in spring coincident with the onset of herbage growth.