Using hyperspectral leaf reflectance to estimate photosynthetic capacity and nitrogen content across eastern cottonwood and hybrid poplar taxa

Eastern cottonwood (Populus deltoides W. Bartram ex Marshall) and hybrid poplars are well-known bioenergy crops. With advances in tree breeding, it is increasingly necessary to find economical ways to identify high-performing Populus genotypes that can be planted under different environmental conditions. Photosynthesis and leaf nitrogen content are critical parameters for plant growth, however, measuring them is an expensive and time-consuming process. Instead, these parameters can be quickly estimated from hyperspectral leaf reflectance if robust statistical models can be developed. To this end, we measured photosynthetic capacity parameters (Rubisco-limited carboxylation rate (V(cmax)), electron transport-limited carboxylation rate (J(max)), and triose phosphate utilization-limited carboxylation rate (TPU)), nitrogen per unit leaf area (N(area)), and leaf reflectance of seven taxa and 62 genotypes of Populus from two study plantations in Mississippi. For statistical modeling, we used least absolute shrinkage and selection operator (LASSO) and principal component analysis (PCA). Our results showed that the predictive ability of LASSO and PCA models was comparable, except for N(area) in which LASSO was superior. In terms of model interpretability, LASSO outperformed PCA because the LASSO models needed 2 to 4 spectral reflectance wavelengths to estimate parameters. The LASSO models used reflectance values at 758 and 935 nm for estimating V(cmax) (R(2) = 0.51 and RMSPE = 31%) and J(max) (R(2) = 0.54 and RMSPE = 32%); 687, 746, and 757 nm for estimating TPU (R(2) = 0.56 and RMSPE = 31%); and 304, 712, 921, and 1021 nm for estimating N(area) (R(2) = 0.29 and RMSPE = 21%). The PCA model also identified 935 nm as a significant wavelength for estimating V(cmax) and J(max). Therefore, our results suggest that hyperspectral leaf reflectance modeling can be used as a cost-effective means for field phenotyping and rapid screening of Populus genotypes because of its capacity to estimate these physicochemical parameters.