In the floriculture industry, the need for heat tolerant bedding plant cultivars is increasing because of the rising temperature around the world. A pot experiment was carried out to examine the impacts of four heat stress duration (0, 7, 14 and 21 days) on growth and ornamental traits to determine the relative heat tolerance of four cultivars of calendula (Calendula officinalis). Growth and development was quantified by measuring plant height, total leaf area, shoot and root fresh weight, shoot and root dry weight, time to flowering, flower number, average flower size and flower longevity. Plant height, leaf area, shoot and root growth at 35-42°C were significantly lower than at normal temperatures (20-30°C). Time to flowering increased as temperature increased. Flower number, size and longevity of flowers were reduced by heat stress, so that among all durations, plants exposed to heat stress for 21d had the least mean. Long time heat stress caused to reduce the plant height, leaf area, shoot and root growth, SPAD value, flower diameter and flower longevity of all cultivars, however the range percentage reduction in growth and flowering parameters were different among cultivars. Experiment to determine the membrane damage showed an increase in percent electrolyte leakage with exposure of plants to high temperature .The studied cultivars differed in their sensitivity to heat stress. The results indicated that the better cell membrane stability, higher shoot and root growth and later flowering led to greater heat tolerance in ‘Indian Prince’ compared to other cultivars.

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