Cytokinin modulates photosynthesis, antioxidant profiles and fatty acid composition in sunflower plants subjected to extreme heat stress

Exposure to extreme heat can severely impair crop growth, development and yield quality through manifold interacting physiological, biochemical and molecular processes. Plant growth regulators such as kinetin can help alleviate heat stress by amplifying antioxidant defenses, stabilizing photosynthesis, and regulating stress signals. Here, our purpose was to evaluate effects of high temperature exposure and foliar kinetin application on the physiological, biochemical and fatty acid profiles of five sunflower cultivars (Lakomka, Progress, Shams, Oskar, and Qasem) across two years. Terminal extreme heat exposure caused by late sowing time significantly altered crop phenology by truncating the reproductive growth period. Delayed sowing altered fatty acid composition and reduced grain-filling duration, seed yield, and oil content due to terminal heat stress. Foliar application of kinetin (50 µM) enhanced heat stress tolerance by improving chlorophyll retention, antioxidant activity, and stomatal conductance, while reducing hydrogen peroxide accumulation and lipid peroxidation. Exposure to extreme heat increased saturated fatty acids, palmitic and stearic acids by up to 58% and 21%, respectively, while reducing linoleic acid content across all cultivars. Oleic acid increased under delayed sowing, but kinetin application modulated this response via influence on desaturase enzyme activity involved in fatty acid biosynthesis. Kinetin reduced accumulation of saturated fatty acids and mitigated heat-induced perturbation of fatty acids, improving oil quality and stability. Our findings underscore the importance of interactive effects between sowing time and hormone treatment on crop physiology and oil quality, emphasizing the need to integrate genetic selection with bespoke heat stress mitigation practices to enhance resilience and sustain productivity under climatic adversity.