High temperature is a severe environmental stressor that reduces crop yield. Tomato is one of the vegetable crops whose growth, physiology, and yields are highly affected by high temperature during the summer season in Bangladesh. Bioregulators are vital in controlling plant growth and development. In this study, the external application of 4–Chlorophenoxyacetic acid (4-CPA), Naphthalene Acetic Acid (NAA) and Gibberellic Acid (GA3) were identified as a positive tool in decreasing the stress of the high temperature effect. A pot experiment was arranged under a Completely Randomized Design (CRD) containing three replications. The bioregulator concentrations consisted of control, 20 ppm 4-CPA, 40 ppm 4-CPA, 60 ppm 4-CPA, 10 ppm NAA, 20 ppm NAA, 30 ppm NAA, 20 ppm GA3, 30 ppm GA3 and 40 ppm GA3. The different concentrations of bioregulators had more potentiality to enhance plant height (36.02%), number of branches plant-1 (50%), number of leaves plant-1 (50%), chlorophyll a content (46.95%), chlorophyll b content (27.97%), total chlorophyll content (38.49%), number of flower cluster plant-1 (67.02%), number of flower plant-1 (50%), number of fruits plant-1 (48.72%) and fruit weight plant-1 (64.18%) compared to control. Among the bioregulators, 20 ppm NAA demonstrated the best functional to solved flower and fruit dropping problems of summer tomato. The findings of the study imply that bioregulators can be utilized as a protective agent to increase water use efficiency, osmotic management, and pigment content to reduce the negative effects of high temperature on tomato growth and physiology, resulting in optimum yield.
