Effect of bioagents on physiological and biochemical parameters in African Marigold(Tagetes erecta L.)under heat stress conditions

African marigold (Tagetes erecta L.) is an important commercial flower crop, widely cultivated for loose flower production throughout the year. It serves as a lucrative cash crop, providing farmers with high returns. Farmers grow a range of marigold, especially African marigold varieties for different seasons; however, high temperatures limit marigold production during the summer season particularly in Northern India. An investigation was conducted to analyse the influence of bioagents (Anabaena torulosa, Pseudomonas nitroreducens, Providencia spp., and Trichoderma viride) on the physiological and biochemical responses of three marigold varieties i.e. ‘Bidhan Marigold-1’, ‘Bidhan Marigold-2’, and ‘Punjab Gainda No.1’ under open field heat stress conditions. The results showed that bioagent treatments mitigated heat-induced cell injury, as evidenced by a reduction in electrolyte leakage, and significantly enhanced gas exchange parameters such as photosynthetic rate, stomatal conductance, and transpiration rate compared to the control in all marigold varieties. Additionally, application of bioagents significantly increased the levels of cellular metabolites, including sugars, phenols, and proteins in the leaves, compared to untreated plants. The findings revealed that, among the three marigold varieties, ‘Punjab Gainda No.1’ exhibited the highest heat resistance, followed by ‘Bidhan Marigold-2’ and ‘Bidhan Marigold-1’. Treatment with Trichoderma viride (through both soil drenching and foliar application) and Pseudomonas nitroreducens (via soil drenching) proved to be an effective strategy for improving marigold growth and yield under heat stress conditions, thus offering farmers a potential means of sustaining their livelihood in challenging environmental conditions.