Okra is a vegetable crop adapted to summer temperatures, but heat stress has been shown to reduce its growth and productivity. We measured physiological traits of 104 okra genotypes in response to high temperature, augmented by the molecular characterization of selected genotypes to identify parents for crossing. Genotypes were exposed to a short heat shock (45 °C, 4 h) in a controlled environment, followed by the assessment of chlorophyll fluorescence (F_v/F_m, F_v′/F_m′) and stomatal conductance (g_s). DNA was isolated from all genotypes using a modified CTAB method with additional PVP and RNase, and the amplification of 8 polymorphic SSR markers was used to generate a dendrogram. This preliminary screening identified 33 polymorphic genotypes with less than 50% genetic similarity and contrasting F_v′/F_m′ and g_s responses. More detailed physiological measurements (F_v/F_mF_v′/F_m′, g_s, photosynthesis (A), efficiency of the open reaction centre (Φ_PSII), and electrolyte leakage (EL)) were conducted after exposure to 45 °C for 6 h and compared to the control (30 °C). EL did not significantly increase in the heat treatment; in contrast, there were significant genotype and treatment effects observed for fluorescence (F_v/F_m, F_v′/F_m′) and photosynthetic parameters (A, Φ_PSII, g_s). In conclusion, cell membranes in okra remained unaffected after short periods of heat stress, whereas the ranking of differences of measured physiological traits (∆) between control and heat-treated plants (∆F_v′/F_m′, ∆F_v′/F_m′, ∆A, ∆Φ_PSII, ∆g_s) was indicative of genotype sensitivity to heat.

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