Measuring and modelling CO2 effects on sugarcane

In order to fully capture the benefits of rising CO₂ in adapting agriculture to climate change, we first need to understand how CO₂ affects crop growth. Several recent studies reported unexpected increases in sugarcane (C₄) yields under elevated CO₂, but it is difficult to distinguish direct leaf-level effects of rising CO₂ on photosynthesis from indirect water-related responses. A simulation model of CO₂ effects, based purely on changes in stomatal conductance (indirect mechanism), showed transpiration was reduced by 30% (initially) to 10% (closed canopy) and yield increased by 3% even in a well-irrigated crop. The model incorporated the results of a field experiment, and a glasshouse experiment designed to disentangle the mechanisms of CO₂ response: whole-plant transpiration and stomatal conductance were both 28% lower for plants growing with high-frequency demand-based watering at 720 vs 390 ppm CO₂, but there was no increase in biomass, indicating that indirect mechanisms dominate CO₂ responses in sugarcane.