Ammonium nutrition increases photosynthesis rate under water stress at early development stage of rice (Oryza sativa L.)

An hydroponic experiment with a simulated water stress induced by PEG (6000) was conducted in a greenhouse to study the effects of nitrate (NO3-), ammonium (NH4+) and the mixture of NO3- and NH4+, on water stress tolerance of rice seedlings. Rice (Shanyou 63) was grown under non- or simulated water stress condition (10% (w/v) PEG, MW6000) with the 3 different N forms during 4 weeks. Under non-stressed condition no difference was observed among the N treatments. Under simulated water stress, seedlings grown on N-NO3- were stunted. Addition of PEG did not affect rice seedling growth in the treatment of only NH4+ supply but slightly inhibited the rice seedling growth in the treatment of mixed supply of NO3- and NH4+. Simulated water stress, when only N-NH4+ was present, did not affect leaf area and photosynthesis rate, however, both parameters decreased significantly in the NO3- containing solutions. Under water stress, Rubisco content in newly expanded leaves significantly increased in the sole NH4+ supplied plants as compared to that in plants of the other two N treatments. Under water stress, the ratio of carboxylation efficiency to Rubisco content was, respectively, decreased by 13 and 23% in NH4+ and NO3- treatments, respectively. It is concluded that, water stress influenced the Rubisco activity than stomatal limitation, and this effects could be regulated by N forms.