Ribulose-1,5-bisphosphate carboxylase (Rubisco) content and nitrogen allocation in rice leaves under CO2 enrichment: Optimization of Rubisco content by ‘antisense’ rbcS

Abstract

Ribulose-1,5-bisphosphate carboxylase (Rubisco) efficiency for CO2-saturated photosynthesis was estimated in leaves of rice (Oryza sativa L.) grown under conditions of normal CO2 air (36 Pa). The light-saturated rate of photosynthesis under CO2 enrichment (e.g. above 100 Pa) was not limited by Rubisco, and its amount was 30 to 55% excessive. Subsequently, the long-term effects on N allocation in a leaf were investigated. CO2 enrichment led to decreases in photosynthesis and the amount of Rubisco protein. In addition, a greater than proportional decrease in Rubisco relative to leaf N was also found in the elevated-CO2-grown plants. However, this was not the result of an optimization of N allocation at elevated CO2, but simply caused by an decrease in total leaf N content by CO2 enrichment. Thus, rice plants did not optimize N allocation at elevated CO2. Transgenic rice plants with optimal Rubisco content at elevated CO2 were obtained by transformation with the rice rbcS antisense gene. The transformant with 65% wild-type Rubisco was selected as a plant set with optimal Rubisco content. Although the plants with 65% wild-type Rubisco showed 20% lower rates of photosynthesis at normal air (36 Pa), they had 5 to 15% higher rates of photosynthesis at elevated CO2 (100 Pa) than the wild-type plants for a given leaf N content. We concluded that the rice plants with optimal Rubisco content at elevated CO2 have a greater photosynthesis under the conditions of the elevated CO2.