Production of summer vegetables is tremendously influenced by elevated temperature overhead thresh hold level which ultimately carries about menace to food security in hot areas of Indo-Pak. To manage this subject, genetic variability is weighty tool. Four experiments were planned for present research. The first experiment was planned to screen out heat tolerant cucumber genotypes. To evaluate heat tolerance ability of one-month seedlings, twenty-five genotypes grown under normal temperature were studied at elevated temperature (40°C/32°C day/night) for seven days. Selection of cucumber genotypes for heat tolerance and sensitivity was done, based on several growth parameters (shoots and roots lengths of seedlings, mass of fresh and dry seedlings and per plant number of leaves), with some physiological parameters (chlorophyll contents, electrolyte leakage, rate of photosynthesis and transpiration, water use efficiency, stomatal conductance, substomatal/internal CO2 and temperature of leaf surface). Some significant variations among genotypes were observed conferring to their capacity to bear heat stress. Genotypes L3466 and Desi-cucumber were found the most heat tolerant, while Suyo Long and Poinsett were found the most sensitive to heat stress. In the second experiment one-month old seedlings were subjected to heat stress in the same way as in the first experiment. It was confirmed that L3466 and Desi-cucumber have high water potential and antioxidant activates in leaves as compared to Suyo Long and Poinsett under heat stress (40°C/32°C day/night). It was observed that at high temperature, water potential, turgor potential was high, while osmotic potential was low as compared to sensitive genotypes. It was revealed that enzymatic antioxidants (superoxide dismutase, peroxidase and catalase) and osmolytes (proline and glycine betaine) were mostly produced in heat tolerant genotypes as compared to heat sensitive ones, while lipid peroxidation was more pronounced in heat sensitive genotypes. These facts were valuable in deriving conclusion that attributes under consideration depends on nature of genotype, so some genotypes were more tolerant than others. In the third experiment, chitosan foliar spray with different concentrations (0, 50, 100, 150, 200, 250 and 300 ppm) were applied on one-month old seedlings of cucumber at 40°C/32°C day/night temperature grown in similar conditions as in the first and the second experiment. Data regarding growth index (length of shoot and root of seedlings, mass of fresh dry seedlings and per plant number of leaves) and some physiological traits (electrolyte leakage and chlorophyll contents) proved that chitosan at 200 ppm has maximum capability of plants to cope with heat stress. In fourth experiment, selected heat tolerant (L3466 and Desi-cucumber) and sensitive genotypes (Suyo Long and Poinsett) were grown in open field. Seeds were sown in different sowing dates (15th March, 1st April, 15th April) for maximum exposer of plants to environmental temperature. Chitosan was applied three times, the first time after one month of seedling emergence while the second and the third a week interval after the first on each genotype. Chitosan treated plants in all genotypes improved the enzymatic antioxidants and osmolytes in enhancing the ability of plant to tolerant heat stress. Data regarding physiological, biochemical and yield related characteristics were observed. It was revealed that overall 26% yield was increased in chitosan treated plants as compared with non-treated plants in cucumber crop. It also improved the ability of sensitive genotypes for survival in the third sown date when temperature was too high while non-treated plants could not survive. Performance of each genotype was better in the first sowing date, followed by the second and the third. It could be concluded from study that by sowing heat tolerant genotypes, identified in present research, growth period of cucumber can also be extended. Exogenously application of chitosan at 200 ppm have potential further alleviate the drastic effects of high temperature, enhancing yield and growing period can be extended.