*4.3. Ovule Development*

HS also causes the abnormal development of female reproductive organs [20] or reduced female fertility [37] in tomato plants, although the female gametophyte is generally considered to have more heat tolerance than its male counterpart. It is noticeable that HS influences ovule development under open-field and greenhouse conditions, leading to the malfunction of both male and female reproductive organs at the same time [21,69]. Ovule development is significantly affected by MCHS. The fruit set, number and weight, and seediness were significantly reduced when DAT during the pollination inclined from 25–26 ◦C to 28–29 ◦C, even though pollens for pollination were collected under optimal temperatures of 26/22 ◦C (day/night) [20]. Similarly, Xu et al. [37] reported a reduction in female fertility and seediness in fruit under MCHS (32/26 ◦C, day/night) when handpollinating using pollens from both MCHS and optimal temperature samples (25/19 ◦C, day/night). In addition, the interactions of pollen and pistil are essential for pollen tube growth from the stigma to the ovules; therefore, a pistil exposed to HS would be dedicated to the considerable regulation of pollen performance in vivo [42,70–72]. These studies suggest the importance of the synergistic effect of male and female organs to achieve the

resulting level of fertility under HS [73]. However, it is not likely that the evaluation of female fertility by screening a large number of germplasms is to be recommended, due to the labor-intensive evaluation process, including hand-pollination and counting seeds in fruits [63].
