Cytokinin-based bioregulators improved heat stress tolerance by violaxanthin cycle activation and enhanced photoprotection in wheat (Triticum aestivum L.).

The study investigates the role of bioregulatory molecules in improving photoprotection and thermotolerance in wheat (Triticum aestivum L.) during terminal heat stress, a major threat to wheat production that drastically decreases yield and quality. The experiment was conducted on two genotypes, Chirya 3 (stay green) and HD 2329 (non stay green), grown under normal (11th November 2022) and late (4th January 2023) planting conditions to simulate heat stress. Foliar applications of bioregulatory compounds (BA, TDZ, GABA, BA + GABA, TDZ + GABA) and a water spray (control) were done at anthesis, with observations recorded at 7-day intervals, while carotenoid profiling and gene expression analyses were executed 14 days post-anthesis. Heat stress reduced total carotenoids while elevating non-photochemical quenching (NPQ), particularly in Chirya 3, signifying higher photoprotective energy dissipation. Bioregulatory treatments, including BA + GABA, reduced carotenoid degradation and improved NPQ, with TDZ exhibiting the highest NPQ in both cases. Carotenoid analysis indicated elevated zeaxanthin levels and reduced violaxanthin levels under stress, indicating the activation of the violaxanthin cycle. The expression of the TaVDE gene, essential for the violaxanthin cycle, enhanced during heat stress, with Chirya 3 demonstrating elevated expression levels. In particular, BA + GABA treatments enhanced TaVDE expression and photoprotective mechanisms. This work is the first investigation demonstrating that BA + GABA treatment can synergistically enhance wheat heat tolerance by modulating the violaxanthin cycle thus offering a promising strategy to mitigate the adverse effects of terminal heat stress.