The role of leaf width and conductances to CO in determining water use efficiency in C grasses.

C plants achieve higher photosynthesis (A ) and intrinsic water use efficiency (iWUE) than C plants, but processes underpinning the variability in A and iWUE across the three C subtypes remain unclear, partly because we lack an integrated framework for quantifying the contribution of diffusional and biochemical limitations to C photosynthesis. We exploited the natural diversity among C grasses to develop an original mathematical approach for estimating eight key processes of C photosynthesis and their relative limitations to A . We also developed a new formulation to estimate mesophyll conductance (g ) based on actual hydration rates of CO by carbonic anhydrases. We found a positive relationship between g and iWUE and an inverse correlation with g among C grasses. We also revealed subtype-specific regulatory processes of iWUE that may be related to known anatomical traits characterising each C subtype. Leaf width was an important determinant of iWUE and showed significant correlations with key limitations of A , especially among NADP-ME species. In conclusion, incorporating leaf width in breeding trials may unlock new opportunities for C crops because the revealed negative relationship between leaf width and iWUE may translate into higher crop and canopy WUE.