Agronomic and Hyperspectral Characterization of Onion (Allium cepa L.) in Response to Foliar Salicylic Acid Under Arid Conditions

Salicylic acid (SA) is a plant signalling compound that may enhance crop growth and productivity, while hyperspectral reflectance provides a non-destructive approach for characterizing plant optical and physiological variability. A two-season field experiment was conducted to evaluate the effects of foliar SA application on the growth and yield of red onion (Allium cepa L. cv. Giza Red) under arid field conditions, within the broader context of agricultural sustainability. Five foliar treatments, consisting of a water-sprayed control and SA at 100, 200, 300, and 400 mg L−1, were arranged in a randomized complete block design with three replicates. Plant growth, bulb characteristics, and marketable and non-marketable yields were evaluated across the 2023/2024 and 2024/2025 growing seasons using a combined analysis that included the effects of growing season, SA concentration, and their interaction. Leaf reflectance spectra were also measured from seven representative samples during each season, and 15 vegetation indices associated with vegetation vigor, pigment status, red-edge behavior, and physiological stress were calculated. The season × SA interaction was non-significant for most agronomic traits, indicating generally consistent SA responses across seasons, except for bulb fresh weight at harvest, for which a significant interaction was detected. The highest numerical values were generally recorded at 400 mg L−1, although 300 mg L−1 produced near-maximum responses for several traits, particularly marketable yield. The spectral datasets displayed the typical reflectance pattern of green vegetation, characterized by relatively low visible reflectance, a marked red-edge transition, and higher near-infrared reflectance. Considerable variation was observed among individual leaf samples in NDVI, GNDVI, NDRE, CIred-edge, MTCI, REIP, PSRI, PRI, and other indices. However, because the spectral samples were not accompanied by verified treatment codes, the observed spectral variability was interpreted at the individual-sample level and was not statistically attributed to specific SA concentrations. The results demonstrate the agronomic potential of foliar SA application in onion and highlight the complementary value of hyperspectral measurements for the non-destructive characterization of leaf optical variability.