Mitigating heat stress in tomato seedlings: Comparative evaluation of bulk and nano chitosan applications

Heat stress poses a major threat to tomato ( Solanum lycopersicum L.) production due to negative effects on physiological and biochemical processes. Chemical priming with high efficacy agents is a valuable agronomic approach that boosts thermotolerance and ensures production under less favourable environmental conditions. This study aimed to evaluate the effectiveness of foliar-applied chitosan—both in bulk and nano formulations—at two concentrations (100 and 150 ppm) in mitigating heat-induced stress in tomato seedlings. Seedlings were subjected to optimal (25 °C) and high-temperature (45 °C) conditions, with heat exposure applied for four hours per day. Morphophysiological, and biochemical parameters, including shoot length, chlorophyll content, photosynthesis rate, relative water content (RWC), electrolyte leakage (EL), ferric reducing antioxidant power (FRAP), and total phenolics, were assessed. Heat stress significantly lowered shoot length, chlorophyll levels, net photosynthesis rate, RWC, and leaf chlorophyll content (Soil Plant Analysis Development, SPAD value), while increasing EL and transpiration. Under heat stress, nanochitosan at 100 ppm (NC100) showed the most consistent and integrated improvements, including a strong increase in chlorophyll a (+45.99%), enhanced antioxidant capacity (FRAP, +15.79%), higher SPAD values, and marked reductions in electrolyte leakage (−37%) and transpiration relative to heat-stressed controls. Bulk chitosan at 100 ppm enhanced chlorophyll b and net photosynthesis rate, while Nanochitosan at 150 ppm increased RWC. Nanochitosan treatments notably improved the chlorophyll stability index (CSI%) indicating enhanced protection against heat-induced stress and overall heat stress resilience in tomato seedlings. Multivariate analyses confirmed the superior performance of Nanochitosan 100 ppm treatments under heat stress. Overall, chitosan application especially in nano-form proved effective in enhancing thermotolerance in tomato seedlings, suggesting its potential as a sustainable approach to mitigate heat stress in horticultural crops. • Nano-chitosan improved tomato heat tolerance and enhanced stress resilience. • Chitosan boosted photosynthesis, RWC and antioxidants under heat stress. • Low-dose nano-chitosan (100 ppm) was most effective in mitigating heat damage.