Texas tomato production is vulnerable to extreme heat in the spring-summer cropping period, which is exacerbated by the lack of superior genetic materials that can perform well in such environments. There is a dire need for selecting superior varieties that can adapt to warm environments and exhibit high yield stability under heat stress conditions. This research aimed at identifying heat-tolerant varieties under heat-stress conditions in controlled and open-field environments and was carried out in three stages. For the first experiment, 43 varieties were screened based on yield responses in natural open-field environment. From those, 18 varieties were chosen and exposed to control (greenhouse: 26/20 °C) and constant heat-stress (growth-chamber: 34/24 °C) conditions for three months. Measurements were done for chlorophyll fluorescence, chlorophyll content (SPAD), plant height, stem diameter and heat injury index (HII). The last experiment was conducted in an open field with a pool of varieties selected from the first and second experiments. Leaf gas exchange, leaf temperature, chlorophyll fluorescence, SPAD value, electrolyte leakage, heat injury index and yield were assessed. From the combined studies, we concluded that heat-tolerant genotypes selected by using chlorophyll fluorescence and HII in controlled heat-stress conditions also exhibited heat-tolerance in open-field environments. Electrolyte leakage and HII best distinguished tomato varieties in open-field environments as plants with low electrolyte leakage and HII had higher total yield. 'Heat Master,' 'New Girl,' 'HM-1823,' 'Rally,' 'Valley Girl,' 'Celebrity,' and 'Tribeca' were identified as high heat-tolerant varieties. Through trait correlation analysis we provide a better understanding of which traits could be useful for screening and breeding other heat-tolerant tomato varieties.

The increasing frequency and duration of high-temperature extremes have adversely impacted tomato production globally. An improved understanding of the physio-biochemical traits under such climatic changes will enable the selection, development, and adoption of resilient tomato varieties. Thus, this study investigated a suite of fifteen tomato plant traits that could be involved in the physio-biochemical mechanisms of thermotolerance to facilitate the selection of heat-tolerant tomato varieties. Ten tomato varieties selected from a previously published study were exposed to three temperature conditions (26/18 °C as control, 38/28 °C for 7 d as heat stress, and 40 °C for 7 h as heat-shock treatment). The results demonstrated that a composite variable of membrane injury metrics, including electrolyte leakage, malondialdehyde content, and heat injury index, was the most successful variable for discriminating varietal thermotolerance under either heat stress or heat shock conditions among all the evaluated traits. Principal components of other plant physiological traits performed poorly in distinguishing thermotolerance among the varieties and essentially did not match the results from the membrane injury composite variable. We identified ‘Celebrity,’ ‘Heat Master,’ and ‘Valley Girl’ as the most thermotolerant varieties under both heat treatment conditions, whereas ‘Arkansas Traveler’, ‘Pruden's Purple’, and ‘Tasti-Lee’ were found to be the most thermosensitive. These results echoed results from a previous open-field screening trial of 43 tomato varieties. Results from this study suggest that measurements of membrane injury are more effective than leaf physiological traits and that heat shock treatments may be as effective as longer term, moderate heat stress treatments for screening tomato thermotolerance, implying that future controlled environment screenings may reduce the number of variables measured and the length of their trials.