**3. Optimum Temperature Range and Heat Stress Threshold in Tomato Plants**

Some studies have shown that a night temperature of 13 ◦C maintains a good fruit set, [45] while night temperatures ranging from 15–20 ◦C influence the increment of marketable yields of tomatoes [46]. However, the daily average temperature (DAT) is more crucial than the day or night temperature or the difference between day and night temperatures during the reproductive processes in tomato plants [20]. For example, the increments or differentials of day and night temperature did not show constant patterns of flower number and fruit weight, but a DAT of 29 ◦C considerably diminished the fruit number and weight, and the seed number per fruit in comparison with those in a DAT of 25 ◦C. Moreover, when the average night temperature was 19.2 ◦C, which is just below the upper critical point (20 ◦C), and the DAT was 26.8 ◦C led to a significant decrease in the fruit set of the tomato plants [30,45]. It has been demonstrated that a DAT of 25–30 ◦C is markedly optimal for the net assimilation rate [47]. DATs of 21–24 ◦C [48], 22–25 ◦C [49], and 22–26 ◦C [24] are optimal for fruit set and yield. An average temperature of approximately 21.3 ◦C, with an average day and night temperature of 27.3 and 15.1 ◦C [30], and 26.3 and 15.6 ◦C [50], are also beneficial for the fruit set and yield of tomato plants, respectively.

The threshold temperature of crops is defined as a value of DAT where a decline in crop growth begins and can be termed as the lower/base and upper threshold temperatures of plant development [4]. This has been determined via the environmental regulation of laboratory and field conditions. Lower and upper threshold temperatures in plant development represent points below and above those at which plant growth and development ceases, respectively [8]. The lower threshold temperatures vary depending on plant species, such as spinach (2 ◦C), pea (4.4 ◦C), pumpkin (13 ◦C), and tomato (15 ◦C) [51]. However, 0 ◦C is often considered to be a predicted lower threshold temperature for cool-season plants [52]. Notably, the upper threshold temperatures are critical for tropical and subtropical crops, which are important limiting factors for determining crop yields. The upper threshold temperatures also vary among plant species, such as wheat (26 ◦C), tomato (30 ◦C), corn (38 ◦C) and cotton (45 ◦C), and even among genotypes within the same species [8]. The determination of an exact upper threshold temperature is also difficult since the physiological responses of plants to different environmental stimuli, as well as to habitats, vary greatly [4,8]. For example, when the ambient temperature was over 35 ◦C as an upper threshold temperature, the seed germination rate, vegetative growth, flowering time, and fruit set and ripening were significantly inhibited in tomato plants [8]. On the other hand, an upper threshold temperature of only 30 ◦C can damage tomato plants at the period of seedling emergence [4,26], suggesting that the evaluation of threshold temperatures for tomato plants must be performed in each growing stage to apply these threshold temperatures for screening heat-tolerant genotypes.
