Role of important physiological traits and development of heat tolerance index in a large set of diverse cucumber germplasm

Cucumber is an important vegetable crop that suffers from significant yield loss because of sub-optimal temperatures during the growing season. High temperature affects the plant's health and reduces the quality and quantity of the final harvest. Huge diversity in terms of different economically important traits, including wide temperature adaptation, is recorded in indigenous cucumber germplasm because of its Indian origin. It is necessary to identify the key traits and genotypes with the contrasting response from a large set of germplasm associated with heat stress response for understanding the physio-biochemical and molecular network associated with heat tolerance. A set of 123 germplasm was evaluated in a growth chamber with temperature stress treatment (40 °C/35 °C) for two subsequent seasons. Besides, 10 selected genotypes based on their response in the seedling stage were grown under natural field conditions with high temperatures to validate the physio-biochemical response in the seedling stage and yield parameters in the reproductive stage. Among the different parameters, slow degradation of chlorophyll, higher anti-oxidant enzyme activity, higher membrane stability index, and higher canopy temperature depression were identified as key traits explaining the heat stress response in cucumbers. Besides, the photosynthetic activities of the tolerant genotypes at the reproductive stage were also higher under field conditions, resulting in higher economic yield. Heat tolerance index was developed for 123 genotypes for seven physiological traits recorded in the present study. The optimised screening technique in the seedling stage and their validation for yield response under natural field facilitated the evaluation of a large number of genotypes for use in breeding for heat stress tolerance in cucumbers. Besides, the identified germplasm, WBC-13, DGC-103 and DARL-106 with effective heat stress tolerance will be instrumental in understanding the molecular basis of heat tolerance and designing climate-smart cucumber cultivars.