The functional anatomy of rice leaves: Implications for refixation of photorespiratory CO2 and efforts to engineer C4 photosynthesis into rice
2009
Sage, T.L.(University of Toronto, Ontario (Canada)) | Sage, R.F.
One mechanism to enhance global food stocks radically is to introduce Csub(4) photosynthesis into Csub(3) crops from warm climates, notably rice. To accomplish this, an understanding of leaf structure and function is essential. The chlorenchyma structure of rice and related warm-climate Csub(3) grasses is distinct from that of cool temperate Csub(3) grasses. In temperate Csub(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 chloroplasts/stromules cover 95% of the cell periphery. Mitochondria and peroxisomes occur in the cell interior and are intimately associated with chloroplasts/stromules. We hypothesize that the chlorenchyma architecture of rice enhances diffusive CO2 conductance and maximizes scavenging of photorespired CO2. The extensive chloroplast/stromule 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 Csub(3) and Csub(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 Csub(4) photosynthesis into rice.
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