Flexible regulation of photosynthesis to low temperature in alfalfa contributed to spring rejuvenation.

Alfalfa (Medicago sativa L.) often suffers from low temperature during spring rejuvenation. Photosynthetic cold resistance, which is associated with fall dormancy (FD), is crucial for successful rejuvenation. Different dormancy levels of alfalfa ('Xinmu No. 4', FD = 3 and 'Ganong No. 5', FD = 8) were investigated to study the contribution of photosynthesis to spring rejuvenation of alfalfa, and elucidate the relationship between FD level and cold resistance. The results showed that lower photosynthesis which contributed to the yield and nutritive value of alfalfa, including destroyed chloroplast ultrastructure, limited enzyme activity of Calvin cycle, and the photoinhibition of both PSI and PSII, was showed in alfalfa during the rejuvenation than that in alfalfa aftrer rejuvenation. But cold-induced damage was lighter in low FD alfalfa during rejuvenation. The yield and nutritive value of the first crop were higher in low FD alfalfa. In low FD alfalfa during the rejuvenation, more stable photosynthesis was indicated by the higher stomatal opening, Rubisco activity, Rubisco activating enzyme activity, and net photosynthetic rate, as well as more stable chloroplast ultrastructure. Morover, ETR(I) and ETR (II) were decreased, while the expression of Lhcb2 and D1 proteins were reduced, NPQ and Y(NO) were up-regulated, which indicated electron transfer from PSII to PSI was decreased. Y(ND) and CEF were up-regulated, and the expression of Lhca1 protein was reduced which would reduce the accumulation of excess electrons in PSI. The reduced electron flow from PSII to PSI and increased energy dissipation on the donor side of PSI, as well as a relatively efficient Calvin cycle alleviated PSI photoinhibition. In a word, stable chloroplast ultrastructure, light PSI photoinhibition and relatively efficient Calvin cycle were contributed to cold resistance of photosynthsis in low FD alfalfa during spring rejuvenation.