Effect of solution temperature on growth and shoot nitrate content of lettuce grown in solution culture

Crisped lettuce plants.(cvs. Marbello' and Bastion) were grown during the period of November - January in nutrient solution culture in an unheated glasshouse in Crete with air temperature fluctuating between 9 to 17 C, under two regimes of solution heating (15 and 20 C minimum solution temperature), compared with an unheated (control) regime (minimum solution temperature around 10 C). The plants were grown on channels (9cm wide and 5cm deep) formed on corrugated asbestos-cement hardboard (a roofing material). Plant population was 18 plants/m. Each experimental Nutrient Film Technique (NFT) unit consisted of two channels 9cm apart and "header" and "catchment" tanks containing in total 90 1 of complete nutrient solution with, manually controlled, pH=6.0 and EC=2.0 mS/cm. There were 3 replications (NFT units) for each treatment. Solution heating to the target minimum temperature was achieved through submersible electrical heaters. Heating the solution resulted in increased shoot fresh weight (SFW) with maximum SFW associated with the 20 C for cv. Bastion' and with 15 C for cv. Marbello'. Shoot water content, number of leaves per plant and water uptake increased with increasing minimum solution temperature. Root dry weight was minimum for the 20degreesC treatment. Tissue nitrate content (TNC) was always higher for cv. Marbello' than for cv. Bastion', while solution heating had no effect on TNC for both of the cultivars tested. The results obtained suggest the application of solution heating for lettuce under the climatic conditions of the experiment.