Impact of elevated atmospheric carbon dioxide and ozone concentrations on leaf photosynthesis of 'Shanyou 63' hybrid rice

Elevated atmospheric carbon dioxide (CO₂) concentration increases plant photosynthesis while elevated tropospheric ozone (O₃) concentration could have the opposite effect. The interactive effects of the two gaseous processes have remained largely unclear. By using a new sola-illuminated gas fumigation platform, an Indica hybrid rice cultivar 'Shanyou 63' was exposed to these gases in four gas treatments. The treatments included the control (real time ambient CO₂/O₃ condition), elevated CO₂ (CO₂) (200 μmol·mol^-1 above ambient CO₂), elevated O₃ (O₃) (60% higher than ambient value) and combined elevated CO₂+O₃ (CO₂+O₃). Photosynthesis of rice grown in the chambers was determined at jointing stage, heading stage and filling stage, respectively. Over the growing season, the target achievement ratios of CO₂ and O₃ concentrations were 1.04, 1.00, respectively. Compared with the control, CO₂ increased net photosynthetic rate (Pn) of rice by 15%, 11% and 28% at jointing, heading and grain-filling stages, respectively. However, O₃ decreased Pn by 32%, 32% and 88% at the respective growth stages. Compared with the control, leaf Pn was not altered by the CO₂+O₃ treatment at jointing and heading stages, but was 48% lower during grain-filling stage. CO₂ significantly decreased stomatal conductance (Gs) and transpiration rate (Tr) at jointing and heading stages, but the two variables remained unaffected at grain-filling stage. The magnitude of O₃ effect on Gs and Tr was larger than that of CO₂, with the highest effect of O₃ detected during grain filling. The effects of CO₂+O₃ treatment on Gs and Tr were generally smaller than those of individual O₃ treatment. Compared with the control, intercellular CO₂ concentration (Ci) in rice substantially increased with CO₂ or CO₂+O₃ treatment while limited response of Ci was observed in individual O₃ treatments. Leaf water use efficiency (WUE) increased with CO₂ treatment, while reverse trend was noted under O₃, especially during filling stage. CO₂+O₃ increased leaf WUE by about 15% at jointing and heading stages, while the reverse trend was noted in the late season, due mainly to cumulative O₃ damage. The above findings suggested that the projected rise of atmospheric CO₂ level increased the photosynthetic capacity, while the elevated tropospheric O₃ concentration decreased it at a progressive severity over the growing season. The concurrent increases in CO₂ and O₃ ameliorated the severity of deleterious effects of O₃ on leaf photosynthesis of 'Shanyou 63'.