Phenological Responses of Rice (Oryza sativa) to Heat Stress and their Regulation by Plant Growth Regulators

Abstract

Rising temperatures threaten rice productivity, particularly when heat stress coincides with sensitive developmental stages. This study evaluated stage-specific phenological, physiological, and yield responses of rice (Oryza sativa L.) cv. Jyothi and examined the mitigating effects of foliar plant growth regulators. A factorial completely randomised design was conducted during the 2025 kharif season using eight temperature-exposure regimes under polyhouse and ambient conditions and four foliar treatments: no spray, water spray, salicylic acid (400 ppm), and brassinosteroid (5 ppm). Heat stress was imposed across the active tillering, heading, and milking stages. Elevated temperature delayed phenological progression and, under severe reproductive-stage stress, prevented flowering, grain filling, and yield formation in several treatment regimes. Heat stress also reduced chlorophyll content and membrane stability index, whereas plant growth regulator treatments improved these physiological traits relative to untreated or water-sprayed plants. Brassinosteroid treatment generally produced the highest chlorophyll content, membrane stability, and yield-related performance, followed by salicylic acid. Yield components, including tiller and panicle number, filled grains, and grain yield, declined under heat stress, with particularly severe effects when reproductive development was exposed to high temperature. The findings indicate that heading is a highly heat-sensitive stage and that foliar brassinosteroid and salicylic acid applications can partially mitigate heat-related physiological and yield impairment in this rice variety under the conditions of the study.