Effects of different nitrogen levels on lettuce growth and nitrate accumulation in iceberg lettuce (Lactuca sativa var. capitata L.) grown hydroponically under greenhouse conditions

The influence of different greenhouse conditions and decreasing nitrogen level in nutrient solution on growth and on nitrate accumulation and its distribution in lettuce plants was studied. Three successive experiments were conducted on aeroponic systems, from May to August 1999. The lettuce plants cv. 'Vanity' were grown in hydroponics using 13 and 5 mM NO3-N in nutrient solution. Differences among averages of fresh shoot weight measurements were statistically significant in all three aeroponic experiments. The largest fresh shoot weights were obtained in the experiment lasted from May to June and the lettuce heads from that growing period were compact enough for marketable yield. In the 2(nd) and the 3(rd) experiments, elongation of stalk and puffy heads were observed in all lettuce plants irrespective of NO3-N concentration in nutrient solution. In all three experiment the low NO3-N level in nutrient solution statistically diminished nitrate content in the shoots. The highest nitrate concentration was recorded in outer leaves whereas the lowest concentration was recorded in inner leaves of lettuce plants grown in both treatment with 13 and 5 mM NO3-N in nutrient solution.