Fischer, Sabrina2026-07-152026-07-152026-042026-07-15http://hdl.handle.net/1993/39867Humic and fulvic acid-based biostimulants are increasingly used in agriculture to enhance plant physiological processes, improve crop performance, and, more recently, help tolerate abiotic stressors. Despite their widespread use, the optimal timing of application for greatest benefit remains poorly understood. Additionally, few studies have assessed the effects of organic acids on plants under low-temperature stress. This study examined the effects of humic and fulvic acid applications at different growth stages on physiological parameters, including photosynthetic metrics and plant nutrient levels, as well as agronomic measures in spring wheat (Triticum aestivum L.). A second objective was to study the effects of a humic acid seed treatment on wheat development and gene expression under low-temperature stress. The results revealed that the physiological responses to fulvic or humic acid application differed by growth stage, environmental conditions, and application timing. Fulvic acid applied during vegetative growth (wheat Zadok stage 25-37) resulted in the greatest yield increase of 9%, corresponding to 347 kg/ha of wheat, in small-plot randomized field trials. Wheat seeds treated with humic acid and subjected to low-temperature stress showed improved developmental characteristics, with cold-responsive gene expression varying by gene target and treatment. Taken together, these findings show that physiological traits and grain yield are strongly dependent on the timing of humic and fulvic acid application, with the greatest yield response occurring during early vegetative growth. Furthermore, humic acid can improve early developmental growth characteristics of wheat under low-temperature conditions, but specific gene expression should be further studied.engspring wheatTriticum aestivumfulvic acidhumic acidbiostimulants