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.