Ecophysiological Characteristics of High-Photosynthetic-Efficiency Rice Varieties and Their Environmental Regulation

Improving photosynthetic efficiency is considered to be one of the most promising approaches for further boosting rice yield. However, the ecophysiological characteristics of high-photosynthetic-efficiency rice varieties, whether at the leaf or canopy level, remain poorly understood. In this work, the ecophysiological characteristics of high-photosynthetic-efficiency rice varieties at leaf and canopy levels, as well as their regulation by environmental factors, were investigated. Furthermore, approaches for further improving rice photosynthesis were discussed. We concluded that high-photosynthetic-efficiency rice varieties are characterized by specific leaf traits, including high Rubisco and nitrogen content, high stomatal density with large stomata, large intercellular airspaces and high chloroplast coverage, and thin cell walls. They also exhibit high leaf vein density, large vein diameter, and large root surface area and root diameter. In addition, they have erect and dark green leaves, more leaves in the lower canopy, and a high degree of light-nitrogen matching in the canopy. Improving the response speed of rice photosynthesis to radiation changes, enhancing its adaptation to temperatures, increasing its tolerance to drought, and raising the CO₂ concentration can all effectively improve rice photosynthetic capacity. Increasing N input enhances high photosynthetic traits and promotes gas exchange. However, excessive N application decreases the N use efficiency (NUE) and causes a series of environmental problems. High NUE rice varieties should be bred to coordinately improve photosynthesis and protect the environment. This study provides a theoretical guidance for high-photosynthetic-efficiency rice breeding and cultivation.