Xanthophyll cycle pigments and water-water cycle in transgenic rice with decreased amounts of ribulose-1,5-bisphosphate carboxylase and the wild-type rice grown under different N levels

Chlorophyll (Chl) fluorescence, gas exchange rates, the amounts of xanthophyll cycle pigments, and the activities of several antioxidant enzymes including superoxide dismutase, ascorbate peroxidase, glutathione reductase, and monodehydroascorbate radical reductase were examined in the leaves of wild-type rice (Oryza sativa L.) and rbcS antisense plants grown under different N levels. The decrease in the CO₂ assimilation capacity by the introduction of the rbcS antisense gene and N deficiency was closely related to the decrease in the quantum yield of photo system (PS) II (φPSII) and the enhancement of non-photochemical quenching (NPQ). No differences in the relationships between the electron transport rates from Chl fluorescence and those from gas exchange were found between the wild-type and rbcS antisense plants and there were no differences in the activities of all the antioxidant enzymes per unit of Chl examined. Although a remarkable increase in NPQ was found for the rbcS antisense and N-deficient wild-type plants, the amounts of the total xanthophyll cycle pigments per unit of Chl remained constant in all the plants. NPQ was highly correlated with only the ratio of antheraxanthin plus zeaxanthin to total xanthophyll cycle pigments. In addition, this ratio was negatively correlated with Rubisco content, irrespective of the genotype and N treatment. The results indicate that the low capacity for CO₂ assimilation and photorespiration by the introduction of the rbcS antisense gene and N deficiency did not affect the electron flow in the water-water cycle but enhanced the deepoxidation state of the xanthophyll cycle pigments.