**5. Physiological and Biochemical Responses of HS Tolerance in Tomato**

HS directly influences the alteration of photosynthetic parameters, including the net photosynthetic rate (*Pn*), CO2 assimilation, transpiration rate (*Tr*), stomata conductance (*Ci*), Photosystem II (PSII, *Fv/Fm*), and chlorophyll contents [33,77], which are closely related to delayed plant growth and development. The ability to adjust the accumulation of primary and secondary metabolites and proteins is also important for plants to display heat tolerance [5,78]; heat-tolerant tomato genotypes with a high fruit set and pollen viability might have this ability. In the plants, primary metabolites such as soluble sugars, glycine betaine, and proline accumulate in response to HS [8]. The production of these osmolytes under HS may increase the protein stability and membrane bilayer structure [79].

#### *5.1. Photosynthesis*

Photosynthetic apparatus under HS causes the severe malfunction of chloroplasts, which are dedicated to the generation of ATP and metabolites in plants [4,80]. Thus, good performance of the photosynthetic apparatus under HS would be seen in the capability of the plant to overcome stress conditions in response to HS [81]. In particular, since HS prohibits successful chlorophyll biosynthesis, chlorophyll content (chl*a* and chl*b*) could be utilized as a reliable evaluation index for identifying heat-tolerant plants [26]. In tomato plants, the chlorophyll *a*/*b* ratio decreased and the chlorophyll/carotenoid ratio increased in a heat-tolerant tomato cultivar under DAHS (45 ◦C, 2 h) compared to CK (25/20 ◦C, day/night), whereas *Pn*, the CO2 assimilation rate, and PSII (*Fv/Fm*) were reduced in heat-susceptible cultivars [26]. In addition, PSII was sensitively stimulated by HS and *Fv/Fm*; the ratio representing the maximum quantum efficiency of PSII is often utilized to measure the normal or better performance of chloroplasts under HS [82].
