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Objective. 'lo agronomically and physiologically evaluate the development of lettuce cultivars under protected conditions. Materials and methods. Under a completely randomized block design, the growth, agronomic and physiological characteristics of five lettuce cultivars were evaluated in two protected agriculture systems and in open field conditions. Results. The cultivars White Boston and Romana Parris Island Coss in house mesh and open field had the highest values in physiological variables (Photosynthesis close to 25.00/¿mol CO2 m2/s, transpiration greater than 7.00 mmol water m2/s and stomatai conductance greater than 1.50 mmol water mmol agua m2/s), which caused the highest values in fresh biomass area and root (95.90 and 3.55 g). The photosynthesis variable in plants sown in the open field reached values of22.29/¿mol CO2 m2/s, compared with house mesh and greenhouse with 17.14 and 14.16/¿mol CO2 m2/s. Conclusions. The ability of these lettuce cultivars to tolerate heat stress was due to morphological and physiological mechanisms, including long-lived leaf, stomatai control, and high photosynthetic potential. [ABSTRACT FROM AUTHOR]