Rubisco and nitrogen relationships in rice: Leaf photosynthesis and plant growth

This review describes how ribulose-1,5-bisphosphate carboxylase (Rubisco) is related to photosynthesis, N economy, and whole plant growth in rice (Oryza sativa L.). Rubisco is a rate-limiting factor for potential photosynthesis under the present atmospheric air conditions. Although there is no variation in the enzymic properties of Rubisco among rice varieties including old and modern cultivars, the specific activity is 30 to 40% lower than that in other higher plants such as wheat, spinach, and tobacco. In the future, the use of recombinant DNA technology in the chloroplast may enable to improve the kinetic efficiency of rice leaf Rubisco. Rubisco content is correlated with leaf-N content, but its amount in senescent leaves is often much lower than predicted from leaf-N content. Degradation of Rubisco is the major determinant of Rubisco content in senescent leaves. In sub-optimal environments characterized by a moderately low temperature, low irradiance, and CO2 enrichment, Rubisco content does not limit photosynthesis and remains excessive. Even when rice is adapted to such environments, rice does not optimize Rubisco content. Transgenic rice with theoretically optimal Rubisco content at elevated [CO2] by antisense rbcS shows higher rates of photosynthesis only under elevated [CO2] conditions. However, such an improvement at the level of a single leaf does not necessarily lead to a higher production of plant biomass. Decrease in Rubisco content induces the operation of the cyclic electron flow around PS I, which enables to prevent photoinhibition as well as to downregulate the quantum yield of PS II. Thus, the photosynthetic processes are highly integrated and strictly regulated. It is considered that there is a limit to improvement which can be achieved only by alteration of Rubisco.