Response of Gas Exchange, Chlorophyll α Fluorescence, and Activities of Antioxidation Enzymes to Osmotic Stress in YF2-1 Derived from Oryza sativa × Echinochloa caudata

YF2-1 is obtained by distant cross between rice (Oryza sativa) and Echinochloa caudate. To assess its osmotic stress resistance at physiological level, the gas exchange, chlorophyll a fluorescence, and antioxidation enzyme activities under osmotic stress simulated by PEG-6000 were studied in seedlings of upland rice YF2-1 and H65. The results showed that YF2-1 maintained higher net photosynthetic rate and stomatal conductance under osmotic stress condition, indicating that YF2-1 suffered less inhibition in photosynthesis. This may be related to its high water retaining capacity and its effective functions for high excessive light energy dispersing (high non-photochemical quenching) and high activities of antioxidation enzymes effectively clearing reactive oxygen species) produced by excessive light energy. The result shows that distant crossing may increase the resistance to osmotic stress in rice.