Water scarcity is one of the most critical constraints in arid and semi-arid regions, directly limiting plant growth, yield, and quality through its effects on physiological and biochemical processes. In Iran, where most areas experience arid or semi-arid conditions, enhancing crop resilience to drought is a pressing need. Bitter gourd (Momordica charantia L.), a medicinal and nutritionally valuable cucurbit, is particularly sensitive to water deficits. This study evaluated the effects of foliar-applied potassium (K), zinc (Zn), calcium (Ca), and silicon (Si) on vegetative growth, physiological responses, and nutrient accumulation under drought stress. The experiment followed a factorial design with two factors: foliar treatments (Zn, K, Ca, Si) and drought stress levels (50%, 75%, and 100% field capacity, FC) with five repetitions. Drought stress increased ion leakage and reduced relative water content (RWC), especially at 50% FC, while Ca and Si effectively alleviated these effects. Si-treated plants showed the highest RWC at 100% FC, whereas Ca was most effective at 75% FC. Chlorophyll and carotenoid levels declined under drought; however, Si and Zn preserved chlorophylls, and Ca and Si enhanced carotenoids. Photosystem II performance was maintained by K and Ca, with Si showing particular benefits at 50% FC. Biochemical adjustments included greater carbohydrate and proline accumulation, stimulated mainly by K and Si, while Ca and Si enhanced phenolic content and antioxidant activity. Ca was most effective in sustaining ascorbic acid and peroxidase activity, while Si and K improved catalase activity. Growth and yield traits declined significantly under drought but were best preserved by Ca, followed by K and Si, with Zn showing limited benefits. Nutrient uptake (Ca, Mg, K) in roots and shoots improved under Ca and K treatments, with Si contributing under severe stress. Overall, Ca emerged as the most versatile element, Si excelled under severe drought, and K supported osmotic and photosynthetic functions.
