Abstract: 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 (P_n) 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 P_n. Elevated CO₂ decreased stomatal conduc-tance (g_s) and transpiration rate (T_r), and the decreases in sunny days were greater than that in cloudy days. The P_n, g_s, T_r, WUE and stomatal limit (L_s) 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 P_n, g_s, and T_r 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.

Key words: weather condition, hybrid rice, FACE (Free Air CO₂ Enrichment), carbon dioxide (CO₂), photosynthesis