PROSPECT-S: A Model for Spike Optical Properties Considering Winter Wheat Growth Dynamics

As the primary reproductive organ of winter wheat, the spike plays a crucial role in grain production, with its optical spectrum reflecting a combination of biochemical properties and morphological structures. Structural modifications, particularly spike width expansion, in wheat spikes induced by developmental progression, make it necessary to develop a spectral model that can respond to structural changes due to winter wheat growth dynamics and is applicable to the entire life cycle of wheat spikes. In this study, we developed PROSPECT-S, a spectral model that accounts for the effects of the dynamic growth of spikes on the structural characteristics, including structural parameter ( N s ) and refractive index ( nr ). Together with key biochemical parameters, including chlorophyll content ( C ab ), equivalent water thickness ( C w ), and dry matter content ( C m ), the model jointly characterized the optical properties of winter wheat spikes across the spectral range of 400 to 2,500 nm. The model’s performance was evaluated using actual spike measurements. The results demonstrated a strong agreement between the simulated and observed spike spectra across all phenological stages (validation accuracy: R 2 = 0.92, root mean square error = 0.052, normalized root mean square error = 0.205). Furthermore, our results indicate that PROSPECT-S can be effectively combined with measured spike spectra to accurately estimate key functional traits from the flowering stage to the grain filling stage, such as C ab , C w , and C m . Overall, the PROSPECT-S model, which could respond to the dynamic growth of the spike, provides a robust physics-based approach for accurately simulating spike spectra and retrieving key spike functional traits from hyperspectral measurements.