Plant Growth Regulators in Cucurbitaceae: A Systematic Review of Their Role in Vegetative Growth, Sex Expression, Flowering, Yield and Fruit Quality

Abstract

The Cucurbitaceae family encompasses some of the world's most economically significant vegetable and fruit crops, including cucumber, melon, watermelon, pumpkin, squash, and bitter gourd. Plant growth regulators (PGRs)—naturally occurring or synthetically applied compounds that modulate plant developmental processes—exert a profound and multifaceted influence on virtually every aspect of cucurbit crop physiology. This review synthesises current knowledge of how major PGR classes, namely auxins, gibberellins, cytokinins, ethylene and ethylene-releasing agents, abscisic acid, and brassinosteroids, regulate vegetative growth, sex expression, flowering, yield, and fruit quality across Cucurbitaceae. The central role of ethylene in determining floral sexuality through the ACC synthase gene network—CsACS2 in cucumber, CmACS7 in melon, CpACO1A in squash, and CitACS4 in watermelon—is examined alongside the promotive role of gibberellins in inducing male flower formation. The practical applications of exogenous ethephon for feminisation, silver compounds for masculinisation, and plant growth retardants for manipulating vegetative architecture are discussed with reference to their agronomic relevance. Parthenocarpy, mediated primarily by auxins, cytokinins, gibberellins, and brassinosteroids, emerges as a key strategy for improving fruit set under suboptimal pollination conditions. The contribution of abscisic acid to non-climacteric fruit ripening in watermelon and the complex hormonal crosstalk governing postharvest quality are also addressed. This review reveals both the scientific advances and the remaining knowledge gaps, and highlights the potential of integrating molecular and applied PGR research to improve cucurbit productivity and quality in the face of climate variability and changing agricultural demands.