Nitrate content in lettuce (<i>Lactuca sativa</i> L.) grown on aeroponics with different quantities of nitrogen in the nutrient solution

The influence of different quantities of nitrogen in the nutrient solution on growth, development and nitrate content was studied in aeroponically grown lettuce (<i>Lactuca sativa</i> L.). Three successive experiments were conducted in 1999 from April to September, in an aeroponic system. The lettuce plants, cv. Vanity, were grown in aeroponics using four different amounts of nitrogen in the nutrient solutions. The pH level was maintained between 5.5 and 6.5, and the EC between 1.8 and 2.2 mS/cm. Fresh weight measurements were made on all the material. The differences between the mean fresh shoot weights were statistically significant in all three experiments. In the first experiment, the maximum final fresh weight in the treatment with 8 mM nitrogen averaged 999.0 g. In the second and third experiments the largest amount of nitrogen, 12 mM NO₃-N, significantly (p<0.05) increased the fresh shoot weight of lettuce plants. low levels of nitrate in the nutrient solution (4 mM NO₃-N in the first and 1.2 mM NO₃-N in the second aeroponic experiment) significantly (p<0.05) increased the fresh weight of the final roots compared with the level of nitrate in standard nutrient solution (12 mM NO₃-N). The differences between the means for plant height were statistically significant (p<0.05) in all three experiments. In the first and third experiments the lengths of the primary roots of the lettuce plants were significantly (p<0.05) influenced by different NO₃-N concentrations in the nutrient solution. The highest NO₃^- concentration in the lettuce leaves was recorded in plants grown in nutrient solutions with the highest NO₃-N concentration (17 mM in the first, 12 mM in the second and third experiments). An acceptably low NO₃^- concentration was found in the leaves of lettuce treated containing with nutrient solution 4 mM NO₃^-N in all three experiments.