Nitrate Levels in Lettuce at Three Times During a Diurnal Period

ABSTRACT With the objective of establishing a pattern of fluctuation in the nitrate content during the course of the day, four lettuce (Lactuca sativa L.) cultivars ('Ierapetras', ‘Vassilikon’, ‘Emperor’, and ‘Parris Island’) were grown hydroponically in perlite with recirculating nutrient solution and harvested on March 18, 1996, at 0700, 1500 and 2300 h. Under the conditions of this study, no significant variation in the nitrate content on a dry weight basis was found during the course of the day in any of the cultivars tested. However, a significant but small reduction (12.4%) in the nitrate content on a fresh weight basis was observed when harvesting ‘Emperor’ in the afternoon (1500 h). Thus, during the course of a day of low global irradiation (405 Wh · m-2) and low temperature (10.4 and 13.4_C min and max temperature, respectively), altering the harvest time did not considerably minimize nitrate level in lettuce.