*4.4. Flower and Fruit Development*

Flowering and fruit set may be the most critical index in the evaluation of heat-tolerant tomato cultivars under HS [74]. Notably, a failure in normal pollen development and female fertility results in a drastic reduction in flower number, fruit weight, fruit set, and seediness under MCHS, in comparison with CK conditions [20,24,68]. For example, HS influenced floral abortion, which led to 80% of flower abscission and a resulting decrease in fruit set [9,75]. In addition, tomato plants exposed to MCHS (DAT of 34 ◦C/19 ◦C, day/night) caused 34% of flower abscission and decreased the fruit set by 71% [76]. Heat-tolerant genotypes have been successfully selected according to high fruit set and yield under HT in open fields and greenhouses or growth chambers (36/28 ◦C, day/night) [19,57], implying that these traits can be utilized for selecting heat-tolerant genotypes in tomato plants.
