Effects of Preconditioning on Photosynthesis of Rice Seedlings under Water Stress

Rice seedlings (Oryza sativa L.) were grown in a controlled environment and divided into control seedlings (CK1: 80% field capacity was always held), preconditioned seedlings (PT, 6 days mild drought for preconditioning–3 days re-watering–intermediate drought stress) and non-preconditioned seedlings (CK2, 9 days 80% field capacity and immediately followed by intermediate drought). Photosynthetic CO2 exchange, photosynthetic pigment content were measured in CK1 and PT after preconditioning and re-watering as well as CK1, PT and CK2 after intermediate drought. After exposure to 6 days mild drought preconditioning, stomatal conductance (gs), transpiration rate (E) and chlorophyll content were lower, while Car content and Car/Chl ratio were higher in PT than those in CK1. PT had no significant differences in net photosynthetic rate (Pn) compared to CK1. After re-watering, Chla content was higher but Car/Chl ratio was lower in PT than those in CK1, and other parameters of PT were similar to CK1. After exposure to intermediate drought stress for 6 days, PT showed high water use efficiency (WUE) after intermediate drought stress. CK2 suffered more serious injuries than PT as indicated by lower Pn and pigment content, suggesting that the differences in response to intermediate drought stress in CK2 and PT seedlings are closely related to the effect of mild drought preconditioning. It may be concluded that preconditioning made rice seedlings modulate their metabolism such that they could acclimatize more successfully to the fluctuating water stress environment.