Projected increase in temperature, prolonged severe drought, and frequent extreme weather events due to climate change are likely to impact plant productivity and global food supply. For several decades, there have been efforts to identify hydraulic constraints on photosynthesis and growth, and to predict plant mortality from soil water scarcity. However, our mechanistic understanding of how xylematic water limitations might impact biochemical processes remains limited. Here, we examined leaf water potential, relative water content, biomass, leaf gas exchange, and chlorophyll a fluorescence (ChlF) on the youngest fully expanded leaves of Brassica rapa (Oilseed) growing in greenhouse conditions under progressive water limitation. The rapid ChlF light response curves from the drought experiment were used to parameterize the responses of the quantum yield of photosystem II (ΦPSII) to light intensity and plant water status in a photosynthesis model incorporating PSII mechanisms (βPSII model) within a whole plant process model, the Terrestrial Regional Ecosystem Exchange Simulator (TREES). Using the updated model, we predicted plant hydraulic traits and growth under water limiting conditions until death.
Results/Conclusions
The updated TREES model incorporating the PSII mechanism showed improved predictions of photosynthesis (from RMSE of 6.1010 to 5.9934 μmol m-2 s-1), stomatal conductance (from RMSE of 0.2979 to 0.2974 mol m-2 s-1), and leaf water potential (from RMSE of 1.3409 to 0.5512 MPa) in water limiting conditions. The model also predicted the above- and belowground carbon content within the same order of magnitude during progressive water limitations, from fully watered plants to dead, and it successfully captured the patterns of above- and belowground carbon partitioning. Importantly, the updated model showed negative feedback between stomatal conductance and leaf water potential, which allowed a slower decrease in leaf water potential over time. Compared to the percent loss of conductivity from the base model, the new estimates from the updated model were at least 10% lower with the decreasing water over an extended period ( > 10 days). This might provide a buffer to reduce the risk of premature hydraulic failure before the observed timing of mortality. These results suggest a pivotal role of PSII in response to water limitations and provide a tool for improved prediction of integrated responses of photosynthetic metabolism to water stress.
