Integrated application of humic acid, urea, and microalgal biostimulant enhances drought tolerance in pea (Pisum sativum L.)

Drought stress severely restricts plant growth, productivity and the functioning of their physiological structures. This study investigated the effects of humic acid (2.5 mL/L), urea (200 kg/ha), and a microalgal biostimulant (MB) (3 mg/100 g extract) on the vegetative growth, yield, and physiological and biochemical characteristics of pea ( Pisum sativum L.) under varying drought conditions applied at control (100% FC), 70%, 40%, and 25% field capacity (FC). The experiment was conducted as a factorial arrangement in a randomized complete block design (RCBD) with three replications. Results showed significant effects of treatments on growth and physiological traits ( p ≤ 0.05). The treatments significantly enhanced growth and yield traits, where humic acid notably increased plant height and biomass production by 35.65%, while MB application improved leaf area by 22.96% and pod length by 60.40%. Urea treatments increased the chlorophyll levels of the plants (SPAD values increased by 35.24%) and MB treatments increased the amount of relative water content in the plants (9.54%) and the protein accumulation (38.88%). These improvements were accompanied by a significant increase in osmotic adjustment, as seen in the amount of proline accumulation (+ 16.40%), due to increased levels of phenolics and greater total antioxidant activity and enhancement of the antioxidant enzymes (catalase and superoxide dismutase), and indicating reduced oxidative damage. The combined application of humic acid, nitrogen, and microalgal biostimulants synergistically enhanced drought tolerance in pea plants.