Quantifying Water-Use Efficiency in Plant Canopies with Varying Leaf Angle and Density Distribution

Abstract Background and Aims Variation in architectural traits related to the spatial and angular distribution of leaf area can have considerable impacts on canopy-scale fluxes contributing to water-use efficiency (WUE). These architectural traits are frequent targets for crop improvement, and for improving understanding and predictions of net ecosystem carbon and water fluxes. Methods A three-dimensional, leaf-resolving model along with a range of virtually-generated hypothetical canopies were used to quantify interactions between canopy structure and WUE by examining its response to variation of leaf inclination independent of leaf azimuth, canopy heterogeneity, vegetation density, and physiological parameters. Key Results Overall, increasing leaf area index (LAI), increasing the daily-averaged fraction of leaf area projected in the sun direction (Gavg) via the leaf inclination or azimuth distribution, and increasing homogeneity had a similar effect on canopy-scale daily fluxes contributing to WUE. Increasing any of these parameters tended to increase daily light interception, increase daily net photosynthesis at low LAI and decrease at high LAI, increase daily transpiration, and decrease WUE. Isolated spherical crowns could decrease photosynthesis by about 60% but increase daily WUE up to 130% relative to a homogenous canopy with equivalent leaf area density. There was no observed optimum in daily canopy WUE as LAI, leaf angle distribution, or heterogeneity were varied. However, when the canopy was dense, a more vertical leaf angle distribution could simultaneously increase both photosynthesis and WUE. Conclusions Variation in leaf angle and density distributions can have a substantial impact on canopy-level carbon and water fluxes, with potential trade-offs between the two. These traits therefore may be viable target traits for increasing or maintaining crop productivity while using less water, and for improvement of simplified models. Increasing canopy density or decreasing canopy heterogeneity increases the impact of leaf angle on WUE and its dependent processes.