Screening and validation of tomato genotypes under heat stress using Fv/Fm to reveal the physiological mechanism of heat tolerance

• Fv/Fm is an early indicator of heat stress tolerance in tomatoes.
• Physiological responses to heat stress differs in tomato genotypes.
• Pollen germination under high temperature differs in tomato genotypes.
• Stay-green trait, stomata regulation and ROS scavengers play roles in heat tolerance.

Cultivation of tomatoes at high temperatures negatively affects growth and yield. Our aim was to screen tomato genotypes under heat stress for differences in the maximum quantum efficiency of photosystem II (Fv/Fm) and uncover the physiological traits for heat tolerance in a three-step process. Initially, 67 tomato genotypes were ranked according to Fv/Fm. Two genotypes with higher Fv/Fm (heat-tolerant group) and two genotypes with lower Fv/Fm (heat-sensitive group) were selected from the initial screening. Second, the physiological responses of the four genotypes to seven days of heat stress (36/28 °C) were analyzed in detail. Third, pollen germination and fruit set of the four genotypes were investigated at high temperature conditions in the field. The results showed that the heat-tolerant group maintained higher leaf pigment content and higher total phenolic content (TPC) than the heat-sensitive group under heat stress. The heat-tolerant group maintained unaltered stomata and pore area and net photosynthesis rate (PN) but increased stomatal conductance (gs) under heat stress compared with the control. Chloroplasts in the heat-tolerant group maintained a normal shape, whereas the chloroplasts in the heat-sensitive group became swollen with decomposed starch grain after heat stress. The heat-tolerant group exhibited a higher pollen germination rate (%), longer pollen tube length and higher fruit set rate compared with the heat-sensitive group at high temperatures in the field. Thus, we concluded that Fv/Fm is an early indicator of heat stress tolerance and that the responses of tomatoes to heat stress were identical in widely different growing conditions. The stay-green trait, improved ability of stomata regulation and higher contents of reactive oxygen species scavengers seemed to be part of the protective mechanisms in heat-tolerant tomatoes.