Unique contributions of chlorophyll and nitrogen to predict crop photosynthetic capacity from leaf spectroscopy.

The photosynthetic capacity or CO2-saturated photosynthetic rate (Vmax), chlorophyll, and nitrogen are closely linked leaf traits that determine C4 crop photosynthesis and yield. Accurate, timely, rapid, and nondestructive approaches to predict leaf photosynthetic traits from hyperspectral reflectance are urgently needed for high-throughput crop monitoring. Therefore, this study thoroughly evaluated the state-of-the-art physically-based radiative transfer models (RTMs), data-driven partial-least-squares regression (PLSR), and generalized PLSR (gPLSR) models to estimate leaf traits from leaf-clip hyperspectral reflectance, which was collected from maize (Zea mays L.) plots with diverse genotypes, growth stages, treatments of nitrogen fertilizers and ozone stresses in three growing seasons. Results show that leaf RTMs considering bidirectional effects can give accurate estimates of chlorophyll content (Pearson correlation r = 0.95), while gPLSR enabled retrieval of leaf nitrogen concentration (r = 0.85). Using PLSR with field measurements for training, the cross-validation indicates that Vmax can be well predicted from spectra (r = 0.81). The integration of chlorophyll content (strongly related to visible spectra) and nitrogen concentration (linked to shortwave infrared signals) can provide better predictions of Vmax (r = 0.71) than only using either chlorophyll or nitrogen individually. This study highlights leaf chlorophyll content and nitrogen concentration have key and unique contributions to Vmax prediction.