Autumn defoliation effects on alfalfa winter survival, root physiology, and gene expression

Harvesting alfalfa (Medicago sativa L.) after mid-September in the North-Central USA often reduces plant winter survival, but the physiological mechanisms associated with poor winter survival are not understood. Our objective was to determine how autumn harvesting affects alfalfa root physiology, gene expression, and plant winter survival. In Exp. 1, seven fall harvest dates were used to identify I to 15 October as a critical interval where significant changes in alfalfa winter survival and root physiology occur in Indiana. In Exp. 2, rows of six alfalfa cultivars possessing contrasting fall dormancy (FD) were established in May. Plants in one-half of each row were defoliated in mid-October, and roots were sampled at this defoliation and again in December. Winter injury was determined in mid-April. Shoot removal in mid-October increased winter injury and reduced plant vigor in spring. As expected, the October defoliation reduced root protein and starch concentrations in December, but unexpectedly increased root sugar concentrations. In addition, defoliation did not reduce the steady state transcript levels of several cold-acclimation responsive (car) genes that are associated with genetic variation in winter survival. Although positively associated with genetic differences in winter hardiness, factors other than root sugar accumulation and expression of these car genes regulate defoliation-induced changes in winter survival of these alfalfa cultivars.