**4. The Response of Vegetative and Reproductive Growth to Heat Stress in Tomato Plants**

#### *4.1. Leaf Growth, Plant Height, and Stem Diameter*

Many studies have been conducted to determine heat tolerance in tomato plants using diverse vegetative growth parameters, including leaf growth, PH, and stem diameter (SD) under HS. Leaf-growth parameters, such as the fresh and dry leaf weight and leaf area, have been assessed under different HS regimes. Abdelmageed et al. [53] reported that these parameters in three heat-tolerant tomato cultivars were less and smaller at 37/27 ◦C (day/night) and pre-heat shock than in 26/20 ◦C (day/night) with no pre-heat shock. In the other study, however, leaf number and area in three tomato cultivars were not significantly different between the control conditions (CK) of 26/20 ◦C (day/night) and heat stress conditions of 32/26 ◦C (day/night) [54]. Moreover, Zheng et al. [55] have investigated the leaf area of tomato plants under two HS conditions of 38/18 ◦C and 41/18 ◦C (day/night) and three relative humidity conditions (50%, 70%, and 90%). The leaf area was significantly reduced in two HT regimes with 50% relative humidity (RH), in comparison with a CK of 28/18 ◦C (day/night) in 50% RH, but it was similar and/or larger in two different HT regimes combined with higher RHs, suggesting an alteration of the HS response by RH. In our recently published study with 38 tomato accessions [56], the leaf length and width were not significantly affected under HT greenhouse conditions with CK and MCHS (19.7/35 ◦C and 20.2/38.8 ◦C of the average daily minimum/maximum temperatures, respectively).

The contrasting responses to HS among studies were also observed in PH and SD. In the HS regime of 36/28 ◦C (day/night) and a CK of 26/18 ◦C (day/night), there were no apparent differences in PH and SD between a tolerant and a susceptible genotype [39]. Zheng et al. [55] also assessed PH and SD in the aforementioned HS and CK conditions; the difference in PH under HS was remarkable, whereas that in SD was not significant when compared to CK. In our previous study, the PH and SD in most of the 38 tomato accessions increased regardless of fruit types [56]. Bhattarai et al. [57] have recently reported that the PH and SD among 18 cultivars were dramatically decreased in a constant HS regime maintaining 36/28 ◦C (day/night) in growth chambers, in comparison with those in greenhouses set to 26/20 ◦C (day/night). These contrasting results indicate that vegetative growth parameters cannot be general indicators for heat tolerance and may not be appropriate for indirect selection in tomato breeding programs for heat tolerance.

#### *4.2. Pollen Development*

In tomato plants, HS causes negative effects not only on pollen development and viability but also on ovule development, embryogenesis, and viability [20,58]. The fact that HT affects the fruit set or number more than flower number implies that HT has a greater effect on the process of fertilization. Indeed, reduced fertility is a common problem associated with HT during meiosis and fertilization periods in tomato plants [59]. When pollinated with pollens matured in HT, female plants grown in optimal temperature conditions did not produce fruits, whereas female plants grown in HT that were pollinated with pollens matured in optimal temperature conditions could bear fruits [28].

Favorable temperatures for pollen germination and pollen tube length are between 15 and 22 ◦C in vitro [60] and 25 ◦C in vivo [61]. Temperatures above 30 and 35 ◦C reduce the pollen germination rate and pollen tube growth [61,62]. Since the pollen development stage is very sensitive to HS and is critical for determining fruit set and yield in HS [9,41,63], most of the studies of HT tolerance in tomato plants have been focused on the pollens. HT significantly reduces pollen viability [8,28,37,64,65], germination [56,61] and number [37]. Frank et al. [66] reported that pollen viability is significantly diminished in flowers of three to seven days before the anthesis stage under DAHS (43–45 ◦C, 2 h), whereas the pollen germination rate and the number of pollen grains were not significantly reduced. The pollen's germinability was influenced by a higher DAHS (50 ◦C, 2 h) in flowers of 2, 7, and 9 days before anthesis in the tomato cultivar "MicroTom" [21]. In addition, the long period of MCHS (32/26 ◦C, day/night) for young tomato plants or 1–2 weeks before anthesis had a negative effect on pollen development [29]. Pressman et al. [67] showed that the total number of pollen grains, pollen germinability, and viability are noticeably decreased in MCHS (32/26 ◦C, day/night) in comparison with CK (28/22 ◦C, day/night). The number of pollen grains and the percentage of viable pollen grains are also lower in MCHS (32/26 ◦C, day/night) than those in CK (28/22 ◦C, day/night) [29].

Remarkably, the response of pollen traits is genotype-dependent. In our previous study, the pollen germination of 23 tomato accessions and the pollen tube length of 28 tomato accessions were significantly reduced under MCHS conditions among a total of 38 accessions [56]. Other studies also reported genotype-dependent HT tolerance in pollen germination [68] and viability [37]. Positive correlations were observed between fruit set and pollen viability [37] or pollen germination and tube length [56,68], although the correlation of the latter two was not significant. HT treatment applied to pollen donor plants before and during pollen release caused significant reductions in seed number and fruit set in comparison with HT treatment that was applied to the developing ovule after pollination [28]. These results suggest that the effects of HT are most significant during pollen maturation, rather than during pollen germination and tube growth or fertilization [69].
