Toward C4 rice: learning from the acclimation of photosynthesis in the C3 leaf

Previous studies of photosynthesis made in the field under irrigated conditions have shown that there are several potential sites of limitation at the leaf level, including light saturation of photosynthesis in upper leaves, mid-morning depression of photosynthetic capacity, and photoinhibition of photosynthesis (reduction in quantum yield). The response of rice photosynthesis in the field to a high CO2 concentration (measured in a leaf chamber) indicates the central importance of Rubisco chemistry and photorespiration in limiting the assimilatory capacity and potential biomass production of rice under tropical conditions in the field. In this paper, we discuss the relationship between C3 photosynthesis and rice leaf morphology and how this research may be incorporated into a program to produce a rice plant with C4 features. Rice crops typically demand high inputs of N fertilizer to achieve high grain yield and this is reflected in the Rubisco concentrations observed in field-grown rice leaves. We have described an inconsistent relationship in the field between Rubisco content and in situ rates of rice leaf photosynthesis in some genotypes and postulated the role of Rubisco in forming part of an N store for later remobilization to the grain. Rice leaf morphology (in this case thickness and area) is a feature of rice crops important for canopy efficiency and integrity, photosynthetic rate, and N content. However, the relationship among leaf thickness, N content, and photosynthesis is not clear. We have adopted a number of lines of research that explore the factors responsible for leaf thickness determination in rice. First, using differences in morphology induced by acclimation to irradiance, we suggest that this results from a signal provided by mature leaves. We postulate that these changes are a ”fine-tuning” of cellular morphology and that the establishment of Kranz anatomy in rice may not require such signals. Second, we are exploiting genotype differences and rice mutant collections. The exploitation of new mutant resources for rice will be essential if the goal of C4 rice is to be achieves. Although high-throughput screening of rice mutant populations is still largely impracticable, this may not apply to IRRI's IR64 deletion mutant collection.