Performance of a potential C4 rice: overview from quantum yield to grain yield

Abstract

The primary determinant of yield in relation to solar radiation and photosynthesis is the radiation conversion factor (RCF, also called radiation-use efficiency): the amount of aboveground dry matter produced from each megajoule of photosynthetically active radiation (PAR) intercepted by the crop. The RCF of rice (2.2 g MJ –1 ) is lower than that of wheat (2.7 g MJ –1 ) or maize (3.3 g MJ –1 ). If genetic engineering could produce rice with C 4 photosynthesis that possessed the RCF of maize, a 50% increase in yield would be conceivable. Analysis of losses of potential fixed carbon when scaling up from quantum yield to RCF identifies the differences between C 3 and C 4 plants. The scaling-up progresses from photochemistry in the cell (nanoseconds) through leaf and canopy photosynthesis (seconds to hours) to crop growth (weeks to months). Rice and maize differ in several respects. Rice has losses from photorespiration but maize has a lower theoretical quantum yield because of the energy costs of the C 4 pathway. In the hierarchy of scale, there are losses at the cell level from inactive absorption of PAR and these are smaller in maize than in rice. The loss at the transition between leaf and canopy photosynthesis is also smaller in maize. Scaling up from leaf to canopy includes changing from leaf photosynthesis unsaturated for PAR (in which each additional unit of absorbed PAR produces the same additional amount of fixed carbon) to photosynthesis by the canopy with leaves at varying saturation depending on PAR incident on the leaf and on leaf age and photosynthetic capability. The analysis shows that a C 4 rice with improved RCF, approaching that of maize, must have higher quantum yield and higher rates of leaf photosynthesis. Suppression of photorespiration will increase quantum yield provided that the energy costs of the C 4 pathway, including leakage of carbon dioxide from the bundle sheath, are not much above the minimum observed in C 4 plants. Higher rates of leaf photosynthesis, arising from increased concentration of carbon dioxide around Rubisco, increase canopy photosynthesis and make it less prone to saturation by PAR. Many features of C 4 photosynthesis must be introduced into a C 4 rice, and must operate with high efficiency and coordination, if RCF and yield are to be improved significantly.