and Carbohydrate and Protein Metabolism in Alfalfa Roots

D. M. Haagenson, S. M. Cunningham, and J. J. Volenec

Department of Agronomy, Purdue University,

West Lafayette, IN 47907-1150, dhaagens@purdue.edu

Harvesting alfalfa after mid-September in the Midwest US often reduces alfalfa winter survival. Untimely autumn defoliation is of great concern today with the increased interest in planting higher yielding, less fall dormant varieties. The lush fall growth produced by a less dormant variety will tempt producers to harvest between mid-September and November; a practice that might compromise winter survival. We wanted to determine the effects of untimely autumn defoliation on root physiology and winter survival. Six alfalfa (Medicago sativa L.) cultivars with contrasting fall dormancy (FD), were selected for this experiment: fall dormant, winter hardy 53Q60, and 54H69 (FD=3, 4), semidormant Z57NO2, 96P51PSI, and 96P55PS1 (FD= 5 to 7), and nondormant, non-winter hardy 5939 (FD=9). Plant heights were measured in early October to estimate fall dormancy, then one-half of the plants in each plot were defoliated in order to interrupt cold acclimation. Roots were sampled in mid-October and in December for laboratory analysis, and winter survival was determined in mid-April. As expected, autumn defoliation increased winter injury and decreased spring vigor. Defoliation decreased root starch and protein concentrations. Surprisingly, sugar concentrations in December were 60% greater in roots of plants defoliated in early October when compared to intact plants (Fig. 1). This is the first time that accumulation of sugars in roots has been associated with enhanced winter injury. Western blot analysis was conducted to evaluate the effects of defoliation on vegetative storage protein (VSP) accumulation. Defoliation reduced VSP accumulation in December. Associated with this reduction in VSP accumulation was reduced expression of a high molecular weight VSP transcript. Changes in steady-state mRNA transcript levels of three cold acclimation responsive genes were also measured. Surprisingly, defoliation in October did not reduce expression of these cold hardiness genes. Clearly, the role of sugar accumulation and the expression of genes associated with winter survival requires re-analysis before mechanisms of alfalfa winter survival are completely understood.

Figure 1. Effects of untimely autumn defoliation on root sugar concentration. Error bars represent the standard error of the mean (n=4).