Response of lettuce (Lactuca sativa L.) growth to reciprocal root-zone temperature (RZT) transfer at different growth stages

Responses of Butterhead lettuce (Lactuca sativa L.) to the reciprocal RZT transfer were studied using aeroponic systems under tropical fluctuating aerial temperatures. Lettuce plants were transferred reciprocally between optimum 20degreesC-RZT and hot ambient-RZT (A-RZT) (25degreesC to 39degreesC) at several growth stages (6, 11, 16, 21 and 26 d after transplanting (DAT). Changes in total number of leaves, total leaf area, fresh and dry weights of shoot, were linear correlated with the number of days that plants were grown at 20degreesC-RZT for both reciprocal RZT transferred plants. These indicate that the longer Butterhead lettuce plants were grown at 20degreesC-RZT, the higher the productivity. However, plants grown at A-RZT at early stage (i.e. before 11 DAT) and then transferred to 20degreesC-RZT in the later growth stage (A-->20degreesC-RZT plants) achieved a higher shoot productivity similar to that of the 20degreesC-RZT plants. Changes in root morphology during the RZT transfer period clearly showed that the development of root system in the two reciprocal RZT transferred plants showed different trends. For plants transferred from 20degreesC-RZT to hot A-RZT (20degreesC-->A-RZT plants), there were linear relationships between root total length, number of root tips, root surface area and the number of days plants spent at 20degreesC-RZT, respectively. However, the correlation was quadratic for plants transferred from A-RZT to 20degreesC-RZT (A-->20degreesC-RZT). All 20degreesC-->A-RZT plants, regardless their growth stage, had significantly larger average root diameters than those of plants maintained at 20degreesC-RZT. Average root diameter in all A-20degreesC-RTZ plants was much smaller than that of plants grown at A-RZT but similar to those of plants maintained at 20degreesC-RZT. The relationships between the changes in root variables and shoot productivities are discussed.