Growth and photosynthetic characteristics of lettuce (Lactuca sativa L.) under fluctuating hot ambient temperatures with the manipulation of cool root-zone temperature

Growth and photosynthetic characteristics were studied in lettuce (Lactura sativa L.) cultured in an aeroponic system at two different times of the year. Midday ambient and leaf temperatures recorded in January were significantly lower than those measured in June. When the aerial parts were grown under hot ambient temperature but with their root zones exposed to 20 degrees C, photosynthetic capacity and productivity were, respectively, about 20 % and 30 % higher measured from the leaves grown in January as compared with those planted in June. However, photosynthetic rate and productivity decreased by more than 50% at both periods when the whole plants were grown under hot ambient temperature as compared with those with their shoots maintained at hot ambient temperature but with their root zones exposed to a cool temperature of 20 degrees C. There was no difference in quantum yield of photosynthetic O-2 evolution and PS II efficiency (chlorophyll fluorescence F-v/F-m ratio) measured from leaves of those plants grown at both periods with their roots exposed to 20 degrees C while decreases in these two parameters were observed when the whole plants were grown under hot ambient temperature, indicating photoinhibition.