Growth and photosynthetic responses of three aeroponically grown lettuce cultivars (Lactuca sativa L.) to different rootzone temperatures and growth irradiances under tropical aerial conditions

Abstract

SummaryThe 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 aeroponic systems under warm aerial temperatures. The rootzone temperatures were controlled at 15°C, 20°C and 25°C. The control rootzone was subjected to diurnal temperatures which ranged from 28°C to 39°C during the day. Plants exposed to different rootzone temperatures were grown under three light regimes in a greenhouse: 100%, 70% and 40% of prevailing solar radiation with maximum midday irradiances of ca. 1800, 1250 and 720 (junol photons m–2 s–1 respectively on sunny days. For all three cultivars grown under different rootzone temperatures, photosynthetic rates decreased as irradiances 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 in maximum photosynthetic rates was observed among those plants with rootzones exposed to the lower temperatures (15°C, 20°C and 25°C) under the same growth irradiance. Both rootzone temperatures 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 growth 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.