Rice ( Oryza sativa L.) with reduced chlorophyll content exhibit higher photosynthetic rate and efficiency, improved canopy light distribution, and greater yields than normally pigmented plants

The net photosynthesis and photosynthetic N use efficiency (PNUE) of a rice mutant with reduced chlorophyll synthesis were much higher than those of its wild type (WT). To understand the physiological mechanism, we conducted a model analysis and pot and field experiments under different N rates. Both the model analysis and the experiments showed that low chlorophyll content did not impede the development of chloroplast in the mutant; the reduced chlorophyll content and high expression levels of chlorophyll binding protein genes suggested the smaller light-harvesting chlorophyll antenna size, and higher expression levels of thylakoid membrane protein genes indicated high potential for chloroplast electron transport in the mutant; the small antenna size avoided excessive light absorption, and increased PSII efficiency, resulting in high electron transport rate in the mutant. The higher Rubisco content in the mutant ensured a higher Rubisco capacity for carboxylation. The high CO2 conductivity of stomata and mesophyll ensured availability of CO2 at the site of carboxylation. All these contributed to the increased photosynthetic rate in the mutant. Leaf N content was found higher in the mutant than in the WT, however, the extent to which the photosynthetic rate was increased was much larger than that of leaf N content, leading to improved PNUE in mutant. Furthermore vertical light distribution was improved in the canopy of the mutant, as greater transmittance of sunlight within the canopy was observed in the density experiment in the field. The mutant showed yield advantages considering its shorter growth duration, indicating the higher photosynthetic capacity and improved canopy light distribution with reduced chlorophyll synthesis. The mutant plant could potentially be used to improve photosynthetic efficiency.