Winter Hardiness and Changes in Bud and Taproot Physiology of Alfalfa Cultivars Selected for Contrasting Fall Dormancy

S. M. Cunningham¹, J. J. Volenec¹, and L. R. Teuber^2
1Dept. of Agronomy, Purdue Univ., West Lafayette, IN 47907
²Dept. of Agronomy and Range Science, Univ. of California, Davis, CA 95616

Winter hardiness is essential for alfalfa (Medicago sativa L.) grown in northern latitudes of the U. S. Winter hardiness has long been associated with fall dormancy, the reduction in shoot growth in autumn. Understanding the physiological and biochemical bases for genetic differences in fall dormancy and freezing tolerance could provide opportunities to genetically enhance winter hardiness of alfalfa. Our objectives were to 1) determine how divergent selection for fall dormancy influenced alfalfa winter survival; and 2) examine if changes in winter survival were associated with altered metabolism of sugars, starch, and protein in taproots and crown buds. Cultivars used as parents in the selection scheme were 'Wadi-Qurayat' (frost-sensitive, fall nondormant), 'CUF 101' (nondormant), 'Lahontan' (semi-dormant), and 'Norseman' (fall dormant). The third cycles from selection for enhanced fall dormancy (less shoot growth) and less fall dormancy (greater shoot growth) and their respective parent cultivars were established in the field in two successive years. Plants were harvested in Sept., Oct., Nov., Dec., and March. Taproots were separated from crowns, and divided into the top 2.5 cm of the taproot, and the remaining root tissue. White buds and green buds (those containing chlorophyll) were removed from crowns and analyzed separately.

Selection for less fall dormancy did not increase fall height of 'Wadi-Qurayat' or 'CUF 101', whereas, fall height of 'Norseman' and 'Lahontan' were increased by selection for less fall dormancy. Selection for greater fall dormancy reduced fall height of 'Lahontan' and 'CUF 101', but not 'Norseman' or 'Wadi-Qurayat'. Wadi-Qurayat and selections from it did not survive winter. Selection for greater fall dormancy improved winter survival of 'CUF 101' from 1% in original 'CUF 101' plants to over 90% in 'CUF 101' plants selected for greater fall dormancy. Winter survival of other cultivars was not affected by selection.

Buffer-soluble proteins in root tops of parental cultivars and selections that survived winter increased from Sept. to Dec., and were about 20% higher in Dec. than that of plants that winter killed. Sugar levels increased 3.5 fold from Sept. to Dec. in 'Norseman' root tops. Starch levels remained highest throughout autumn in the root tops of 'Wadi-Qurayat.' Sugar levels in green and white buds increased throughout autumn in all cultivars, but reached higher levels by Dec. in buds of plants that survived winter. In contrast white and green buds of more winter hardy cultivars contained more starch in late fall than the buds of the less winter hardy cultivars.

CUF 101 and its selections provide opportunity to characterize the physiological and biochemical bases for fall dormancy and its relationship to winter hardiness in alfalfa. Further studies will include HPLC analysis of sugars and qualitative analysis of membrane and buffer-soluble proteins from root tops and white buds throughout autumn to elucidate specific changes occurring in alfalfa as it hardens for winter.

1995 Central Alfalfa Improvement Conference Proceedings