*5.5. Secondary Metabolites*

Secondary metabolites also play a critical role in pollen growth and germination, osmotic regulation, the scavenging of reactive oxygen species (ROS), and membrane fluidity, as well as the signaling pathways contributing to pollen viability and fruit set [92]. HT increased the accumulation of soluble phenolics and the activity of phenylalanine by ammonia-lyase, the principal enzyme in the biosynthesis of phenolic compounds, as an acclimated mechanism against HS [93]. A significant increase in total flavonoids including kaempferol was observed in tomato plants under HS conditions during the pollen development stage [94]. In addition, the accumulation of polyamines, such as spermidine and spermine, is required for pollen germination in tomato plants [62,95], while polyamine accumulation in transgenic tomato plants enhances HT tolerance [96]. However, no effect of HT on the level of polyamine was observed in a recent study of tomato plants [94].

#### *5.6. Superoxide Dismutase*

HS affects the overproduction of ROS, destroying essential cellular components and structural elements [97]. Superoxide dismutase (SOD) helps break down harmful oxygen molecules and is considered a defense enzyme against oxidative stress [98]. In tomato plants, DAHS (>40 ◦C) for three hours in both ambient and root zone temperatures caused a decrease in total specific SOD activity in two tomato genotypes, one of which has heat tolerance, but no significant difference in SOD activity was observed between the two cultivars [26]. In our recent study, however, HS with 38/30 ◦C (day/night) for three days resulted in a significant increase in SOD activity in two cultivars, and a significant difference between the two cultivars in terms of SOD activity was also observed [87]. The contrasting results may be due to the small number of genotypes used and the different environmental conditions. Further studies are needed, with a large number of genotypes in the same environment, to clarify the role of SOD activity in the heat tolerance of tomato plants.
