Abstract: Characteristics of photosynthesis and water use of resistant (R-1) and susceptible (I-1) alfalfa clones damaged by thrips were tested to determine compensatory photosynthesis at the budding and early flowering stages in the field. The results showed that after the plants were damaged by thrips at the budding stage, the chlorophyll content and the net photosynthetic rate (Pn) of the resistant alfalfa clone R-1 increased significantly, while light compensation point (Lcp) and dark respiration rate (Rd) decreased significantly (P<0.05) compared with those of healthy plants (CK). However, the chlorophyll content of the susceptible alfalfa clone I-1 decreased (P<0.05), and the Pn had no significant difference from that of CK. Pn of I-1 plants damaged by thrips was lower than that of CK, while Lcp and Rd increased significantly (P<0.05) under lower illumination intensity. The values of intercellular CO2 concentration (Ci), transpiration rate (Tr) and stomatal conductance (Gs) of R-1 and I-1 plants damaged by thrips increased significantly, while the water use efficiency (WUE) decreased dramatically compared with those of CK. When the plants were damaged by thrips at the early flowering stage, Pn of I-1 plants increased significantly under strong illumination while decreased under weak illumination (P<0.05), while other indices showed the similar tendency as at the budding stage. In conclusion, R-1 alfalfa has significant compensatory photosynthesis to thrips damage, but I-1 alfalfa does not.

Key words: Thrips, alfalfa clone, effective optical radiation, physiological response, compensatory photosynthesis