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Heat stress (HS) is a serious environmental stress that negatively affects crop growth and productivity across the globe. The recent increase in atmospheric temperature caused by global warming has increased its intensity, which is a serious challenge that needs to be addressed. Plant growth and development involves a series of physiological, metabolic, and biochemical processes that are negatively affected by heat-induced oxidative stress, disorganization of cellular membranes and disturbed plant water relations, nutrient uptake, photosynthetic efficiency, and antioxidant activities. Plant tolerance to abiotic stresses can be substantially increased by the application of bio-stimulants, without posing a threat to the ecosystem. Melatonin (MT) is a multi-functional signaling molecule that has the potential to protect plants from the adverse impacts of HS. MT protects the cellular membranes, maintains the leaf water content, and improves the water use efficiency (WUE) and nutrient homeostasis; thereby, improving plant growth and development under HS. Moreover, MT also improves gene expression, crosstalk of hormones, and osmolytes, and reduces the accumulation of reactive oxygen species (ROS) by triggering the antioxidant defense system, which provides better resistance to HS. High endogenous MT increases genes expression and antioxidant activities to confer HS tolerance. Thus, it is important to understand the detailed mechanisms of both exogenous and endogenous MT, to induce HS tolerance in plants. This review highlights the versatile functions of MT in various plant responses, to improve HS tolerance. Moreover, we also discussed the MT crosstalk with other hormones, antioxidant potential of MT, and success stories of engineering MT to improve HS tolerance in plants. Additionally, we also identified various research gaps that need to be filled in future research using this important signaling molecule. Thus, this review will help the readers to learn more about MT under changing climatic conditions and will provide knowledge to develop heat tolerance in crops.

Severe water stress conditions limit growth and development of floricultural crops which affects flower quality. Hence, development of effective approaches for drought tolerance is crucial to limit recurring water deficit challenges. Foliar application of various plant growth regulators has been evaluated to improve drought tolerance in different floricultural crops; however, reports regarding the role of chitosan (Ci) on seasonal flowers like calendula are still scant. Therefore, we evaluated the role of Ci foliar application on morphological, physiological, biochemical, and anatomical parameters of calendula under water stress conditions. Different doses of Ci (0, 2.5, 5, 7.5, 10 mg L−1) were applied through foliar application to evaluate their impact in enhancing growth and photosynthetic pigments of calendula. The optimized Ci level of 7.5 mg L−1 was further evaluated to study mechanisms of water stress tolerance in calendula. Ci application significantly increased biomass and pigments in calendula. Ci (7.5 mg L−1) resulted in increased photosynthetic rate (72.98%), transpiration rate (62.11%), stomatal conductance (59.54%), sub-stomatal conductance (20.62%), and water use efficiency (84.93%). Furthermore, it improved catalase, guaiacol peroxidase, and superoxide dismutase by 56.70%, 64.94%, and 32.41%, respectively. These results highlighted the significance of Ci in inducing drought tolerance in pot marigold.