Taproot Carbohydrate Concentrations and Stress Tolerance of Contrasting Alfalfa Genotypes

High concentrations of starch in taproots are believed to be essential for tolerance of alfalfa ( Medicago sativa L.) to defoliation and for winterhardiness. We used genotypes selected for high (HSt) and low (LSt) taproot starch concentrations to determine if high taproot starch concentrations were necessary to tolerate these stresses. In Exp. 1, roots from transplanted ramets were removed from the field at monthly intervals throughout late fall, winter, and early spring and analyzed for total sugar and starch. In Exp. 2, these genotypes were grown in the field and response to defoliation was examined. In both studies, starch concentrations in roots of the HSt genotype were twofold higher than those of LSt roots at most harvests. In Exp. I, only 5 of 12 LSt ramets regrew in spring of Year 1, while in Year 2 shoot regrowth in spring was lower for the LSt genotype compared with its herbage growth the previous fall. In Exp. 2, root starch concentrations had declined by Day 14 of regrowth to ≈20 and 40 g kg −1 for the LSt and HSt genotypes, respectively, when half of the remaining plants of both lines were defoliated a second time. Taproot starch concentrations of the LSt genotypes were consistently lower than those of the HSt genotype whether defoliated once or twice, but shoot regrowth of LSt plants was always greater than that of HSt plants. Constituents of taproots, in addition to starch, may influence tolerance to defoliation and winterhardiness of alfalfa.