The role of leaf width and conductances to CO 2 in determining water use efficiency in C 4 grasses

Summary C 4 plants achieve higher photosynthesis ( A n ) and intrinsic water use efficiency ( iWUE ) than C 3 plants, but processes underpinning the variability in A n and iWUE across the three C 4 subtypes remain unclear, partly because we lack an integrated framework for quantifying the contribution of diffusional and biochemical limitations to C 4 photosynthesis. We exploited the natural diversity among C 4 grasses to develop an original mathematical approach for estimating eight key processes of C 4 photosynthesis and their relative limitations to A n . We also developed a new formulation to estimate mesophyll conductance ( g m ) based on actual hydration rates of CO 2 by carbonic anhydrases. We found a positive relationship between g m and iWUE and an inverse correlation with g sw among C 4 grasses. We also revealed subtype‐specific regulatory processes of iWUE that may be related to known anatomical traits characterising each C 4 subtype. Leaf width was an important determinant of iWUE and showed significant correlations with key limitations of A n , especially among NADP ‐ ME species. In conclusion, incorporating leaf width in breeding trials may unlock new opportunities for C 4 crops because the revealed negative relationship between leaf width and iWUE may translate into higher crop and canopy WUE .