Enhancing growth, yield, and fruit quality of Kulai chilli (Capsicum annuum L.) using selected plant growth regulators

Abstract

Plant growth regulators (PGRs) offer a potential means of improving chilli productivity by regulating vegetative growth, flowering, fruit development, and biochemical quality. This study evaluated the responses of Kulai chilli (Capsicum annuum L.) to foliar applications of naphthaleneacetic acid (NAA; 20 and 40 mg L⁻¹), gibberellic acid (GA₃; 10 and 20 mg L⁻¹), and indole-3-acetic acid (IAA; 25 and 50 mg L⁻¹) under rain-shelter conditions in Sandakan, Sabah, Malaysia. The experiment was arranged in a completely randomised design comprising seven treatments, including a water-sprayed control, with five replications. Treatments were applied at 30 and 45 days after transplanting. Plant growth, flowering, fruit maturity, yield, fruit characteristics, capsaicin content, and vitamin C content were evaluated. GA₃ at 20 mg L⁻¹ produced the tallest plants throughout the cultivation period, reaching 79.4 cm at 90 days after transplanting. IAA at 50 mg L⁻¹ induced the earliest flowering and fruit maturity, reducing the time to the final maturity index from 50.4 days in the control to 41.8 days after anthesis. However, NAA at 40 mg L⁻¹ produced the most favourable overall response, increasing cumulative yield per plant by 61.9% relative to the control and producing the greatest fruit weight and length. This treatment also resulted in the highest capsaicin and vitamin C contents. Seed number per fruit was not significantly affected by the treatments. These findings demonstrate that PGR responses are trait-specific: GA₃ primarily enhanced vegetative growth, IAA accelerated crop development, and NAA at 40 mg L⁻¹ provided the best balance between earliness, yield, and fruit quality. Further multi-season and multi-location validation is required before commercial application.