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

To clarify the morphological and physiological response mechanism of lettuce to consecutive heat stress and pave the way to improve lettuce thermotolerance in future, 20 indexes of heat sensitive lettuce 'GRAND RAPIDS TBR' were measured during and after 13 days of heat stress (35/25? day/night temperature) treatment and normal temperature (25/15? day/night temperature) treatment (no drought stress during treatment) in artificial climate chamber. The results showed that: (1) after 13 days of heat stress, the biomass accumulation of lettuce increased, the new leaves became narrower and longer; (2) from the fifth day after heat stress, the leaf number, and stalk length increased rapidly; (3) compared with the control, the chlorophyll a + b, carotenoids, proline and soluble sugar content decreased in the early stage and then increased in the later stage during heat stress; but the malondialdehyde content, the catalase, peroxidase, and super oxide dismutase activities increased in the early stage and then decreased in the later stage; (4) correlation analysis showed that the leaf number and stalk length have a significant positive correlation with chlorophyll a + b (P < 0.01), carotenoids (P < 0.01), soluble sugar (P < 0.05), proline (P < 0.01), hydrogen peroxide (P < 0.01) and a negative correlation with malondialdehyde (P < 0.01) and catalase (P < 0.01). These results showed that lettuce responds to heat stress by increasing the activity of antioxidant enzymes in the early stage and lettuce responds to heat stress by improving the photosynthetic pigment, osmotic adjustment contents and changing plant morphology in the later stage, thus balancing heat stress response and growth, ultimately promoting the increase of its biomass.