It is necessary to identify the heat-tolerant cultivars of bedding plants in order to cope with the challenges of the global warming. The physiological and anatomical responses of four calendula cultivars (Candyman ’ , ‘ Zen ’ , ‘ Indian Prince ’ , and ‘ Pacific Beauty ’ ) to four duration levels (0, 7, 14 and 21 days) of heat stress (in a mean range of 35 – 42°C), were investigated after their establishment under greenhouse conditions. The results indicated that calendula cultivars displayed different responses to various heat duration levels in most parameters. Although the gas exchange of ‘ Candyman ’ was the highest in control treatment, it showed a considerable reduction in all levels of stress durations. In contrast, ‘ Indian Prince ’ maintained gas exchange and chlorophyll content and exhibited the most resistance to heat stress among others, especially for longer duration exposure, which might be resulted from the greater soluble sugar content, higher stomatal density, and thicker leaves. Experiment to assess cell membrane thermostability showed heat stress resulted in significant increase in electrolyte leakage percentage as the incubation temperature of all cultivars was increased. However, the least membrane stability was observed in ‘ Zen ’ , indicating the higher sensitivity of this cultivar to heat-stress conditions in comparison to three other cultivars.

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