Physiological analysis of alfalfa (Medicago sativa L.) winter hardiness among cultivars exhibiting contrasting fall dormancy
Abstract
Fall dormant cultivars exhibit prostrate growth in autumn, slow shoot elongation after hay harvest in summer, and they are winter hardy. Non-dormant cultivars are desirable because of the potential for additional harvests due to increased shoot elongation rates in summer and erect growth habit in autumn, however, they have poor winter survival. The physiological mechanisms causing fall dormancy-induced differences in shoot growth in autumn and winter hardiness are not understood. The goal of this research was to investigate the physiological, biochemical, biophysical, and molecular changes associated with genetic and management-induced differences in alfalfa winter hardiness. Increased root sugar, amino N, and protein concentrations in December were closely associated with fall dormancy and decreased winter injury of winter hardy experimental germplasms selected for decreased fall dormancy. However, this study did not reveal an association between improved winter hardiness and cold hardiness gene expression. Many freezing-tolerant species have altered water relations during cold acclimation, increased osmolality and sugar concentrations, and accumulate cold-regulated proteins such as dehydrins. In our study, root water content in autumn was not associated with genetic variation in winter survival, but soluble sugar concentrations and osmolality of cell sap from roots and suspension cell cultures increased with cold acclimation. Alfalfa dehydrin transcript abundance from field-grown plants and suspension cell cultures increased with cold acclimation. Dehydrin transcript abundance was closely associated with improved winter survival. However, the association between dehydrin protein abundance and improved alfalfa freezing tolerance is not clear as the level of dehydrin increased after cold acclimation, regardless of genetic variation in winter hardiness. Cold acclimated suspension cell cultures had increased dehydrin transcript abundance, but large cell line-specific differences in induction were observed. Mid-October defoliation increased winter injury and reduced spring vigor. October defoliation significantly reduced root protein and starch concentrations in December. Surprisingly, October defoliation increased root sugar concentrations; a trait that until now has been consistently associated with improved winter survival. In addition, October defoliation did not affect the expression of several alfalfa cold hardiness genes in December. Clearly, more studies are needed to identify additional mechanisms controlling the complex association of fall dormancy and winter survival.
Degree
Ph.D.
Advisors
Volenec, Purdue University.
Subject Area
Agronomy|Botany|Plant propagation|Genetics
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