Bioenergy Sorghum Crop Model Predicts VPD-Limited Transpiration Traits Enhance Biomass Yield in Water-Limited Environments

Sorghum bicolor is a versatile, drought resilient crop used for production of grain, forage, sugar, and biomass. Bioenergy sorghum is targeted for production in drought-prone annual cropland not optimal for food production, and traits that improve plant water capture and water use are necessary to maximize productivity. A crop modeling framework, APSIM, used extensively for analysis of grain sorghum, was adapted to predict the growth and biomass yield of energy sorghum. APSIM simulations of energy sorghum development and biomass accumulation replicated results from field experiments across multiple years, patterns of rainfall, and irrigation schemes. Additionally, APSIM was extended to enable modeling of VPD-limited transpiration traits that limit crop water use under high vapor pressure deficits (VPDs). The response of transpiration rate to increasing VPD was modeled as a linear response until a VPD threshold was reached, at which the slope of the response changes, representing the range of responses to VPD observed in sorghum germplasm. Simulation results indicated that the VPD-limited transpiration trait is most beneficial in hot and dry regions of production where crops are exposed to extended periods without rainfall or terminal drought. In these settings, slower but more efficient transpiration increases biomass yield and prevents or delays the onset of plant dehydration and leaf senescence. The VPD-limited transpiration responses observed in sorghum germplasm increased biomass accumulation by 20% in dry years, and the ability to drastically reduce transpiration under high VPD conditions could increase biomass by 6% on average across all years. This work indicates that the productivity and resilience of bioenergy sorghum grown in water-limited environments could be further enhanced by optimal deployment of VPD-limited transpiration traits. The energy sorghum model and VPD-limited transpiration trait implementation are made available to simulate performance in other target environments.