Abstract: Medicago sativa is an important legume forage in China,and it is strong in drought resistance. However,water deficit is still the main stress factor affecting its yield.The photosynthetic characteristics and yield of M.sativa under different water stress conditions were studied by carrying out the field experiments.The results showed that:① Soil water stress affected significantly the photoresponse parameters of M.sativa(P<0.05).With the increase of soil water stress,the maximum net photosynthetic rate,apparent quantum yield and light saturation point were decreased gradually,but the dark respiration rate and light compensation point were gradually increased,thus the dyssophotic absorption and conversion efficiency of M.sativa in its photosynthesis were directly affected.② With the increase of soil water stress,the net photosynthetic rate (P_n) and transpiration rate (T_r) of M.sativa leaves were reduced significantly,but the stomatal conductance (G_s) was decreased at first and then increased,which revealed that the stomatal factors under slight water stress were the main factors resulting in the P_n reduction,and the non-stomatal factors under moderate and severe water stress were the main factors resulting in the P_n reduction.③ When the photosynthetically active radiation (PAR) value was 1 200 μmol·m^-2·s^-1,the water use efficiency (WUE) of slight water stress was significantly higher than that of full irrigation (P<0.05),indicating that moderate water stress could improve the water use efficiency of M.sativa.④ There was no significant difference of hay yield between slight water stress and fully irrigation,indicating that high forage yield and water-saving irrigation could be achieved under slight water stress.

Key words: Medicago sativa, drip irrigation, water use efficiency, soil water stress, photosynthesis, biomass, Yongchang, Gansu