Is stomatal conductance optimized over both time and space in plant crowns? A field test in grapevine (Vitis vinifera)

Crown carbon gain is maximized for a given total water loss if stomatal conductance (g(s)) varies such that the marginal carbon product of water (A/E) remains invariant both over time and among leaves in a plant crown, provided the curvature of assimilation rate (A) versus transpiration rate (E) is negative. We tested this prediction across distinct crown positions in situ for the first time by parameterizing a biophysical model across 14 positions in four grapevine crowns (Vitis vinifera), computing optimal patterns of g(s) and E over a day and comparing these to the observed patterns. Observed water use was higher than optimal for leaves in the crown interior, but lower than optimal in most other positions. Crown carbon gain was 18% lower under measured g(s) than under optimal g(s). Positive curvature occurred in 39.6% of cases due to low boundary layer conductance (g(bw)), and optimal g(s) was zero in 11% of cases because A/E was below the target value at all g(s). Some conclusions changed if we assumed infinite g(bw), but optimal and measured E still diverged systematically in time and space. We conclude that the theory's spatial dimension and assumption of positive curvature require further experimental testing. Carbon gain is maximised for a given total transpiration rate if the marginal carbon product of water is invariant in time and space, provided the curvature of the relationship between net CO2 assimilation rate and transpiration rate. We tested the spatial dimension of this prediction, as well as the requirement for negative curvature, for the first time, in 14 positions across four grapevine canopies over one day. We found substantial systematic deviations between the observed and optimal spatial patterns of water loss, and we also found that positive curvature occurred in 40% of leaves, largely due to low boundary layer conductance. Our results highlight the importance of diffusive resistances other than stomatal in the economics of carbon-water balance, and they suggest that optimisation theory may need to be revised to account for the economic significance of differences among crown positions in hydraulic limitations to transpiration rate.