Effects of elevated ozone on physiology, growth, yield and grain quality of rice (Oryza sativa L.): An ozone gradient experiment

Rice (Oryza sativa L.) as the staple food is widely grown, but the current high tropospheric ozone (O3) pollution has seriously threatened the rice production. We performed an experiment on four rice cultivars widely grown in the Yangtze River Delta region of China, which were exposed to five O3 treatments under open top chambers. The effects of O3 on leaf physiology, growth, yield and grain quality of rice were comprehensively explored, and the results showed that (i) elevated O3 reduced the light-saturated rate of CO2 assimilation (Asat) and stomatal conductance (gs), and the O3-induced decrease in Asat was mainly attributed to stomatal limitation under low O3 treatment whereas stomatal and biochemical co-limitation under high O3 treatment; (ii) elevated O3 did not affected the harvest index since the inhibition of shoot biomass and yield was the same extent, which indicated that O3 does not alter the remobilization of carbon assimilates from vegetative organs to grains; (iii) only ambient air plus 60 ppb O3 treatment (NF60) significantly reduced the yield of rice, which was caused by the combined reduction of individual grain weight, grain number, and panicle number; (iv) elevated O3 did not significantly affect the concentration of mineral nutrients in grains, while the effect of O3 on the concentration of amino acids was consistent with biphasic O3-response profile. NF60 had a greater reduction in the concentration of essential amino acids than non-essential amino acids; (v) NF60 significantly reduced the accumulation of mineral nutrients and amino acid in grain due to a decrease in grain yield. These results are conducive to a comprehensive understanding of the impacts of O3 on rice, and also indicated that the risk assessment of O3 on crops would need to consider both crop yield and grain quality.