Photoperiod effect on phytochrome and abscisic acid in alfalfa varieties differing in fall dormancy

Short daylength (SD) is the main environment-induced factor leading to fall dormancy (FD) in alfalfa (Medicago sativa L.). However, the physiological mechanisms causing varietal differences in shoot growth and fall dormancy of alfalfa crop are not fully understood. The objective of this research was to explore the physiological principles regulating FD in alfalfa by examining phytochrome B (PhyB) and abscisic acid (ABA) contents in response to photoperiodic reaction and fall dormancy. Three alfalfa varieties having different levels of fall dormancy were examined for ABA and PhyB concentrations under three photoperiod conditions (8, 12, and 16 h photoperiod). Our results showed that the leaf PhyB concentration was the greatest in the highly dormant variety, intermediate in semi-dormant, and the lowest in non-dormant varieties. Across all the varieties, PhyB concentration under 8-h photoperiod was 9.2% and 37.4% higher than those under 12-h and 16-h photoperiod (LD), respectively. Accordingly, the ABA concentration was also greater in highly dormant varieties. When subjected to LD conditions, all varieties reached flowering although it took additional 2-4 d for dormant varieties to blossom than non-dormant ones. It was evident that SD treatment induced the biosynthesis of PhyB and ABA. Our data indicate that large differences in PhyB and ABA concentrations exist among alfalfa varieties differing in fall dormancy.