Spectral and Physiological Responses of Pisum sativum and Datura stramonium to Light Intensity and Far-Red Radiation Under Controlled Hydroponic Conditions

Light quantity and spectral quality regulate plant growth and may influence crop–weed interactions by altering plant morphology, biomass allocation, and physiology. This study evaluated the combined effects of photosynthetic photon flux density (PPFD) and far-red (FR) radiation on the morphological, biomass, physiological, and hyperspectral responses of Pisum sativum L. and Datura stramonium L. grown separately under controlled hydroponic conditions. A 2 × 2 × 2 factorial experiment was conducted using two species, two PPFD levels, and two FR treatments. Morphological traits, biomass production, root mass fraction, SPAD chlorophyll content, hyperspectral reflectance (350–2500 nm), and vegetation indices were analyzed using multivariate statistics, principal component analysis, and Random Forest classification. Species identity explained most of the variation in morphology, biomass allocation, and spectral characteristics. FR supplementation increased mean shoot height in D. stramonium by 62.5% under high PPFD and 101.2% under low PPFD, whereas shoot height in P. sativum was not significantly affected. Red wavelengths between 649 and 668 nm provided the strongest discriminatory information, while short-wave infrared wavelengths contributed complementary biochemical information. The Random Forest model achieved an overall accuracy of 75.7%, an AUC of 0.839, and a Cohen’s κ of 0.489. These findings demonstrate that D. stramonium exhibits greater FR-induced morphological plasticity than P. sativum and that hyperspectral sensing provides useful, although not yet field-validated, crop–weed discrimination.