GROWTH AND PHOTOSYNTHETIC RESPONSES OF 3 AEROPONICALLY GROWN LETTUCE CULTIVARS (LACTUCA-SATIVA L.) TO DIFFERENT ROOTZONE TEMPERATURES AND GROWTH IRRADIANCES UNDER TROPICAL AERIAL CONDITIONS

The effects of three irradiances and four rootzone temperatures on the growth and photosynthesis of three cultivars of Lactuca sativa L. (a temperate plant) 'Armada', 'Olympia', and 'Palma' were studied using a eroponic systems under warm aerial temperatures. The rootzone temperat ures were controlled at 15 degrees C, 20 degrees C and 25 degrees C. T he control rootzone was subjected to diurnal temperatures which ranged from 28 degrees C to 39 degrees C during the day. Plants exposed to d ifferent rootzone temperatures were grown under three light regimes in a greenhouse: 100%, 70% and 40% of prevailing solar radiation with ma ximum midday irradiances of ca. 1800, 1250 and 720 mu mol photons m(-2 ) s(-1) respectively on sunny days. For all three cultivars grown unde r different rootzone temperatures, photosynthetic rates decreased as i rradiances decreased. Regardless of irradiance, maximum photosynthetic rates were much lower in the control plants than in plants grown with rootzones at lower temperatures. However, no significant difference i n maximum photosynthetic rates was observed among those plants with ro otzones exposed to the lower temperatures (15 degrees C, 20 degrees C and 25 degrees C) under the same growth irradiance. Both rootzone temp eratures and growth irradiance affected the growth and development of root and shoot (the formation of compact heads) of all three cultivars . Interaction of rootzone temperature and growth irradiance on the gro wth of shoot and root and photosynthesis are discussed with particular emphasis on the optimum conditions for growing temperate plants such as Lactuca sativa L. in the tropics.