Abstract: In order to compare the performance and adaptive mechanisms of two varieties of alfalfa under low inorganic phosphate (low-Pi) conditions, and to provide more information for improvement of Pi utilization efficiency and absorption efficiency in alfalfa, the varieties Aohan and Victoria were grown under low-Pi (5 μmol·L^-1 KH₂PO₄) and normal Pi (500 μmol·L^-1 KH₂PO₄) conditions. Morphological and physiological traits were measured at 10, 15 and 20 days. It was found that, the height, aboveground biomass and Pi content of both Aohan and Victoria were decreased under low-Pi stress; while the underground biomass, root-shoot ratio, total root length, root surface area and Pi utilization were increased. The net photosynthetic rate, stomatal conductance and transpiration rate were decreased significantly under low-Pi (P<0.05), but the decrease was much smaller in cultivar Aohan than in cultivar Victoria. The acid phosphatase activity of both was significantly enhanced (P<0.01), and for Aohan was significantly higher than that of Victoria (P<0.01). To sum up, low-Pi stress inhibited the growth of the above ground plant organs and promoted growth of the underground parts. The greater low-Pi tolerance of Aohan compared to Victoria is associated with greater increase in acid phosphatase activity under low-Pi in Aohan, and a lower reduction in photosynthesis under low-Pi. The contrasting responses of the two varieties under low-Pi stress indicate a potential mechanism for alfalfa tolerance to low-Pi.

Key words: Medicago sativa, low-Pi tolerance, Pi content, net photosynthetic rate, acid phosphatase activity