Coordinated physiological reorganization of growth and reproductive modulation in watermelon (Citrullus) lanatus L.) induced by exogenous application of auxins under humid tropical conditions

Abstract

The commercial production of watermelon (Citrullus lanatus L.) increasingly demands strategies capable of improving fruit quality while reducing seed content. The present study evaluated the effects of exogenous auxin application on fruit growth, seed development and physiological integration under humid tropical conditions of the Ecuadorian Amazon. A completely randomized design with three treatments and three replications was established, including a control without auxin application and two concentrations of Flower Tie® (1 and 2 cm³ L⁻¹). Fruit weight, diameter, length and seed number were quantified, and integrated physiological indicators were developed to characterize reproductive efficiency, fruit expansion and overall hormonal response. Auxin application significantly enhanced fruit development and partially reduced seed formation. The highest dose increased fruit weight from 9.88 to 15.32 kg (+55.1%), fruit diameter from 16.58 to 24.63 cm (+48.6%) and fruit length from 36.36 to 41.40 cm (+13.9%), while reducing seed number from 645.7 to 548.1 seeds per fruit, corresponding to a 15.1% reduction relative to the control. Analysis of variance revealed highly significant treatment effects (p < 0.0001), with very large effect sizes across morphometric, reproductive and integrated physiological variables (η² = 0.7891–0.9817; ω² = 0.7674–0.9790). The Integrated Hormonal Response Index (IHRI) increased from 0.00 in the control treatment to 33.15 in the highest auxin concentration, indicating a coordinated enhancement of fruit growth and reproductive modulation. Dose–response modeling showed strong quadratic relationships (R² = 0.9024–0.9817), while principal component analysis explained 97.86% of total variability and revealed a clear physiological gradient linking biomass accumulation and fruit expansion with reduced seed development. These findings demonstrate that exogenous auxin application promotes coordinated physiological reorganization of fruit growth and reproductive processes, representing a promising strategy for improving watermelon fruit quality under tropical production systems.