Morphological and Physiological Response Mechanism of Lettuce (Lactuca Sativa L.) to Consecutive Heat Stress

•After heat stress, the biomass of lettuce increased and the lettuce leaf became narrower and longer.•The chlorophyll, carotenoids, proline, and soluble sugar content decreased first and then increased due to heat stress, but the malondialdehyde content, catalase, peroxidase, and super oxide dismutase activities increased first and then decreased.•Leaf number and stalk length show a significant positive correlation with chlorophyll, carotenoids, soluble sugar, proline, hydrogen peroxide, but have a negative correlation with malondialdehyde and catalase.•Lettuce tends to balance heat stress response and growth through changes in their morphological and psychological indexes, ultimately contributing to biomass increase.