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WE THOUGHT WE KNEW HOW TO GROW LETTUCE: STUDIES WITH LIGHT AND TEMPERATURE  J. M. Frantz, G. L. Ritchie, and B. Bugbee.  Crop Physiology Lab.  Dept of Plants, Soils, and Biometeorology, Utah State University, Logan, UT 84341.

      The temperature optimum for lettuce is thought to be 20 to 25 C. Warmer  temperatures are not used because they increase bitterness and tip burn.  Temperatures above 25 C are also thought to reduce growth rates. We examined  growth of lettuce canopies at five constant temperatures: 21, 25, 30, 32.5,  & 35 C with two replicate chambers at each temperature.  Our studies indicate that the temperature optimum for leaf expansion, light  interception, and growth is 30 C, which is much higher than used in previous  studies. The growth rate increased by more than 3-fold from 21 to 25 C and by 40% from 25 to 30 C; but decreased by 20 % from 30 to 32.5 C. Plants  appeared chlorotic at 20 and 25 C, but chlorophyll content increased with  increasing temperature, and was more than 30 times higher at 35 than at 21 C.  Tip burn was not observed at or below 30 C. These results indicate that the  temperature optimum for lettuce growth and appearance is 5 to 10 C above that  used in previous studies.  In a separate study, PPF levels were reduced by 20 to 80% in the middle of  the life cycle to examine adaptation rates. Whole plant photosynthesis and  respiration rates adapted to within 80% of the initial rate after a few days.  These results indicate that lettuce is surprisingly tolerant of the system  failures that are associated with power loss in space flight conditions.