The Functional Anatomy of Rice Leaves: Implications for Refixation of Photorespiratory CO2 and Efforts to Engineer C4 Photosynthesis into Rice

One mechanism to enhance global food stocks radically is to introduce C-4 photosynthesis into C-3 crops from warm climates, notably rice. To accomplish this, an understand-ing of leaf structure and function is essential. The chloren-chyma structure of rice and related warm-climate C-3 grasses is distinct from that of cool temperate C-3 grasses. In temperate C-3 grasses, vacuoles occupy the majority of the cell, while chloroplasts, peroxisomes and mitochondria are pressed against the cell periphery. In rice, 66 of protoplast volume is occupied by chloroplasts, and chloroplastsstromules cover 95 of the cell periphery. Mitochondria and peroxisomes occur in the cell interior and are intimately associated with chloroplastsstromules. We hypothesize that the chlorenchyma architecture of rice enhances diffusive CO2 conductance and max-imizes scavenging of photorespired CO2. The extensive chloroplaststromule sheath forces photorespired CO2 to exit cells via the stroma, where it can be refixed by Rubisco. Deep cell lobing and small cell size, coupled with chloroplast sheaths, creates high surface area exposure of stroma to intercellular spaces, thereby enhancing mesophyll transfer conductance. In support of this, rice exhibits higher mesophyll transfer conductance, greater stromal CO2 content, lower CO2 compensation points at warm temperature and less oxygen sensitivity of photosynthesis than cool temperate grasses. Rice vein length per leaf, mesophyll thickness and intercellular space volume are intermediate between those of most C-3 and C-4 grasses, indicating that the introduction of Kranz anatomy into rice may not require radical changes in leaf anatomy; however, deep lobing of chlorenchyma cells may constrain efforts to engineer C-4 photosynthesis into rice.