Using foliar spectral properties to assess the effects of drought on plant water potential

Drought frequency is predicted to increase in future environments. Leaf water potential (Psi(LW)) is commonly used to evaluate plant water status, but traditional measurements can be logistically difficult and require destructive sampling. We used reflectance spectroscopy to characterize variation in Psi(LW) of Quercus oleoides Schltdl. & Cham. under differential water availability and tested the ability to predict pre-dawn Psi(LW) (PD Psi(LW)) using spectral data collected hours after pressure chamber measurements on dark-acclimated leaves. Psi(LW) was measured with a Scholander pressure chamber. Leaf reflectance was collected at one or both of two time points: immediately (Psi(LW)) and similar to 5 h after pressure chamber measurements (PD Psi(LW)). Predictive models were constructed using partial least-squares regression. Model performance was evaluated using coefficient of determination (R-2), root-mean-square error (RMSE), bias, and the percent RMSE of the data range (% RMSE). Psi(LW) and PD Psi(LW) were well predicted using spectroscopic models and successfully estimated a wide variation in Psi(LW) (light-or dark-acclimated leaves) as well as PD.LW (dark-acclimated leaves only). Mean Psi(LW) R-2, RMSE and bias values were 0.65, 0.51 MPa and 0.09, respectively, with a % RMSE between 8% and 20%, while mean PD Psi(LW) R-2, RMSE and bias values were 0.60, 0.44 MPa and 0.01, respectively, with a % RMSE between 9% and 20%. Estimates of PD Psi(LW) produced similar statistical outcomes when analyzing treatment effects on PD Psi(LW) as those found using reference pressure chamber measurements. These findings highlight a promising approach to evaluate plant responses to environmental change by providing rapid measurements that can be used to estimate plant water status as well as demonstrating that spectroscopic measurements can be used as a surrogate for standard, reference measurements in a statistical framework.