Whole-plant growth and N allocation in transgenic rice plants with decreased content of ribulose-1,5-bisphosphate carboxylase under different CO2 partial pressures

Growth of transgenic rice (Oryza sativa L.) with an antisense gene to the small subunit of Rubisco was analysed under 36 and 100 Pa CO2 during a 14-h photoperiod (1000 mol quanta m–2 s–1). Two lines of the antisense plants were used; one with 65% wild-type Rubisco and the other with 40% wild-type Rubisco. The plants were grown hydroponically for 70 d. The final biomass of the antisense plants grown in 36 Pa CO2 was much smaller than that of the wild-type plant. However, several compen-sation phenomena were found in the antisense plants. Increased biomass allocation to leaf blades and preferential N investment in leaf blades were observed. Leaf senescence was also delayed. Elevated CO2 levels up to 100 Pa caused the antisense plants to achieve a size similar to that of the wild-type plant. However, although the antisense plant with 65% wild-type Rubisco was selected as a plant with optimal Rubisco content for CO2 -saturated photosynthesis, its final biomass was not greater than that of the wild-type plant. This may have been caused by a relatively strong Rubisco-antisense effect during the early stage of growth (21–42 d). N-use efficiency for growth after d 42 was greater in the selected antisense plant. Thus, improvement of N-use efficiency at the level of a single leaf did not necessarily lead to greater production of biomass at the whole-plant level.