Genetic basis of autumn growth and winter hardiness in alfalfa
Alfalfa (Medicago sativa L.) has a broad geographic distribution, with populations adapted to both cold and warm climates. Despite considerable research on winter hardiness, the mechanisms and genetic control of winter survival are poorly understood. We conducted two experiments designed to identify and quantify morphological, physiological, and biochemical traits associated with winter hardiness and to estimate the genetic relationship of these traits with winter hardiness. In Experiment 1, ten alfalfa genotypes were planted in June 2000 and in May 2001 at Ames, IA, and traits were measured in August, November, and the following April in each year. Concentrations of palmitate, palmitoleate, linoleate, linolenate and total fatty acids increased in crowns and roots of all genotypes from August to November and declined by April. Winter injury showed a negative phenotypic correlation with concentrations of palmitate, linoleate, linolenate, total fatty acids in both roots and crowns. These data suggest an important role of fatty acids in winter hardiness.;In Experiment 2, we established an F1 segregating population of 200 individuals in the field in successive years at Ames, IA and measured agronomic and physiological traits in August and November. Although differences among the individuals in the F1 population were present 2 for all traits, and transgressive segregation present for many, the population mean tended to fall intermediate to the two parents. Broad sense heritabilities on an entry mean basis were 0.58 for winter injury, 0.60 for plant height in November, and ranged from 0.44 to 0.63 for shoot, crown, and root mass in August and November. Total fatty acid concentration had an H2 of 0.72 in August and 0.44 in November. Autumn plant height had weak positive genetic correlations with winter injury, in contrast to other research. Shoot, crown, and root biomass in both August and November showed strong negative genetic correlations with winter injury, suggesting that in this population, winter survival was related to plant vigor. Preliminary genetic mapping identified several chromosomal locations associated with most traits. These results suggest that the determinants of winter survival in this population formed from semidormant parents differ from those in populations with nondormant parental genotypes.