Enhancing Antioxidant Activity and Osmolyte Accumulation in Heat-Stressed Maize Hybrids with Plant Growth Promoters and Thermotolerance-Induced Seeds

High-temperature stress significantly hampers maize production by reducing both yield and crop quality. This study investigated strategies to mitigate heat stress in maize under natural summer conditions. A split-plot design was used, with two maize hybrids (RCRMH-2 and 900MG) as the main treatments and several heat stress mitigation measures as sub-treatments, including foliar applications of salicylic acid (200 ppm), orthosilicic acid (200 ppm), calcium chloride (2000 ppm), benzyl adenine (20 ppm), and thermotolerance-induced seed treatment. The results showed that the hybrid RCRMH-2 outperformed 900MG, exhibiting higher osmolyte accumulation and enhanced antioxidant activities. RCRMH-2 accumulated 2.80% more proline than 900MG, with cytokinin treatment increasing proline by 46.6%. Calcium chloride led to a 47.16% increase in protein accumulation, while thermotolerance-induced seed treatment boosted free amino acids by 30.62%. In terms of sugar accumulation, seed treatment reduced sugar levels by 53.90%, while RCRMH-2 showed higher non-reducing sugar accumulation compared to 900MG. Salicylic acid was most effective in reducing reducing sugars, with a decrease of 17.71%. These findings suggest that specific mitigation strategies, including seed treatments and foliar applications, can significantly enhance maize heat tolerance by improving osmolyte synthesis and antioxidant defense. The study offers practical approaches to mitigate heat stress, with implications for improving maize resilience amid global climate change.