Simulating the field-scale potential of natural variation in soybean leaf optical properties on carbon assimilation and water use

Agricultural lands provide a near-term opportunity to address climate change, food, and water security challenges. Addressing these complex challenges will require the integration of empirical data on the natural variation of vegetation traits with computational approaches that can integrate this empirical information toward an understanding of canopy-scale impacts. The relationships between leaf chlorophyll content, leaf optical properties, and biochemical photosynthetic capacity were measured in a collection of soybean (Glycine max) accessions showing large variations in leaf chlorophyll content (Chl). These relationships were integrated into a biophysical model of canopy-scale photosynthesis to simulate the within-canopy light environment, carbon dioxide assimilation and water use. Simulations were conducted for each accession to identify possible opportunities for improving canopy photosynthesis through foliar chlorophyll modification. These simulations demonstrate that canopy photosynthesis may not increase as Chl is reduced due to increases in leaf reflectance and nonoptimal distribution of canopy nitrogen. However, similar rates of canopy photosynthesis can be maintained with a 9% savings in leaf nitrogen resulting from decreased Chl. Additionally, analysis of these simulations indicates that the inability of Chl reductions to increase photosynthesis arises primarily from the connection between Chl and leaf reflectance and secondarily from the mismatch between the vertical distribution of leaf nitrogen and the light absorption profile. These simulations motivated the use of numerical optimization to quantify the extent to which low Chl mutations can be used to improve canopy performance by adapting the distribution of the “saved” nitrogen within the canopy to take greater advantage of the more deeply penetrating light. We conclude with a discussion of the impact of the re-distribution of nitrogen vertically through the canopy to improve photosynthesis and canopy water use.