Physiological Factors Limiting Leaf Net Photosynthetic Rate in C₃ Crops like Rice and Approaches for Improving It

Improving leaf photosynthetic capacity is one of the most promising approaches to further boost crop yield. Clarifying factors limiting leaf photosynthetic capacity, especially in C₃ crops, is meaningful for designing strategies to improve it. Leaf net photosynthetic rate (<i>A</i>) is one of the parameters describing leaf photosynthetic capacity. In the present study, physiological factors limiting <i>A</i> in C₃ crops such as rice were discussed and different approaches for <i>A</i> improvement were summarized to provide theoretical guidance for increasing leaf photosynthetic capacity. <i>A</i> will be limited by both CO₂ availability and light intensity over periods from a few hours to several days, and by one of them over shorter intervals. Under current ambient atmospheric conditions, <i>A</i> of C₃ crops is mainly limited by Rubisco activity and the CO₂ concentration in chloroplasts. Leaf nitrogen content affects <i>A</i> by regulating Rubisco content and leaf anatomy; leaf morphological and anatomical traits limit <i>A</i> by impacting stomatal and mesophyll CO₂ diffusion. Further improvements of <i>A</i> in C₃ crops can be achieved by designing or introducing high-activity Rubisco; adjusting leaf nitrogen allocation to optimize leaf anatomy and leaf chemical composition; modifying leaf morphology and anatomy for greater CO₂ diffusion; improving the activity of proteins and enzymes associated with sugar transportation and utilization; introducing C₄ photosynthetic mechanisms and combining high photosynthetic traits by conventional breeding.