Light and carbon dioxide (CO2) are two essential components for plant photosynthesis. To understand the effects of elevated CO2 concentration on photosynthetic characteristics of hybrid rice under different light conditions, two hybrid rice varieties (YLY900 and YY538) were grown in the field using a free-air CO2 enrichment facility (FACE) in 2017 with two CO2 concentration treatments (ambient CO2 and elevated 200 μmol·mol-1 above ambient CO2), the photosynthesis traits of top full expansion leaves were measured in both sunny and cloudy days at jointing and grain filling stages. Elevated CO2 increased net photosynthetic rate (Pn) of two rice varieties. The increase in sunny days (31%) was greater than in cloudy days (21%), and the increase at jointing stage (37%) was greater than at grain filling stage (21%). There were significant interactions between CO2 with weather, and between CO2 with growth stage. Water use efficiency (WUE) of leaves in response to elevated CO2 showed the similar trend as Pn. Elevated CO2 decreased stomatal conduc-tance (gs) and transpiration rate (Tr), and the decreases in sunny days were greater than that in cloudy days. The Pn, gs, Tr, WUE and stomatal limit (Ls) measured in cloudy days were significantly lower than that measured in sunny days by 41%, 18%, 41%, 26% and 27%, respectively. Results from the correlation analyses showed that the Pn, gs, and Tr in sunny days were significantly positively correlated with the corresponding parameters in cloudy days. The results indicated that cloudy weather conditions reduced photosynthesis and its response to elevated CO2 of two hybrids rice varieties at middle and late growth stages. Therefore, weather variation should be considered when assess rice yield potential in the future environment.

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