Mitigating drought stress in bitter gourd (Momordica charantia L.) through foliar application of potassium, zinc, calcium, and silicon

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.