Abstract
Six cultivars of iceberg lettuce (Latuca sativa L.) were tested for their response to root-zone temperatures of 15 and 20ºC under warm tropical conditions. Control plants were grown without root-zone cooling diurnal root-zone temperatures ranging from 24 to 30ºC. Two cultivars ‘Chieftain Zero’ and ‘Empire’ formed compact heads when the root-zone temperature was cooled to 15ºC for 24 h each day. Root-zone cooling could be delayed until the seedlings reached the 5 true leaf stage, but when the cooling period was reduced to 20 h each day, plants of these two cultivars either formed irregular heads or bolted. Plants of the remaining four cultivars formed irregular heads or bolted even though the root-zone temperature was kept at 15ºC for 24 h each day. The control plants of all cultivars and most of those growth at a root-zone temperature of 20ºC did not form heads and bolted in about 25 days after transplanting.
