Abstract: By using a Li-6400 portable photosynthesis system, the diurnal dynamics of the photosynthetic characteristics of three alfalfa varieties Medicago sativa L. cv. Aohan, M. sativa L. cv. Rangelander, and M. sativa L. cv. Algonquin were studied. The results showed that for the test alfalfa varieties, the diurnal variation curves of their P_n, T_r, and WUE were not all presented double-peak. For Aohan and Algonquin, the maximum P_n and T_r occurred at 9:00-11:00; while for Rangelander, they appeared at 11:00. The minimum P_n and T_r of the three varieties all appeared at 7:00 and 19:00. The maximum WUE of Rangelander and Algonquin appeared at 7:00 and that of Aohan was at 9:00, and the minimum WUE of the three varieties all appeared at 15:00-17:00. Comprehensive analysis on the photosynthetic performance indices P_n, T_r, and WUE indicated that 2 years old alfalfa was in the best state, followed by 1 year old, and 4 years old. Correlation analysis showed that air temperature (T_a) was the main environmental factor affecting P_n and T_r, followed by water vapor pressure deficit (VPD) and relative humidity (RH).

Key words: standardized precipitation index (SPI), spring soybean, time-series characteristics of drought and flood, Heilongjiang Province, relative meteorological yield