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Abstract
Nitrogen is one of the essential elements for plant growth. The most common nitrogen source in hydroponics is nitrate (NO⁻₃).When nitrate uptake rate exceeded its assimilation because of excessive nitrate application, nitrate accumulation in plant tissue occurs. Vegetables, particularly leafy vegetables, accumulated nitrate readily. Some previous researches suggested that the vegetables with high nitrate in the diet will cause some health problems. The objective of this work was to study the means by which nitrate concentration can be depressed in lettuce plants, to achieve this purpose, two experiments were conducted. The first experiment was conducted to study the effects of nitrate-N/ amino acid sources ratios on the growth, NO-₃ accumulation and some macronutrient concentrations of lettuce (lactuca sativa L.) var. Sahara to select the best ratios of amino acids to NO-₃-N for hydroponics culture of lettuce. Eighteen treatments were applied to lettuce plants. The eighteen treatments were a combination of six amino acids (AA), argnine (Arg), alanine (Ala), aspartic acid (Asp), glutamic acid (Glu), glutamine (Gln) and glycine (Gly), and three NO₃-N/AA-N molar ratios: (1) 100: 0, (2) 80:20 (3) 60:40 .All treatments had the same total N concentration at a rate of 12.5 mmolL-1 in nutrient solution. The results of this experiment indicated that increasing the portion of AA-N to replace NO₃-N in nutrient solution decreased plant biomasses and glutamic acid had stronger decrease than the other AAs. Nitrate concentration decreased with decreasing NO₃-N/AA-N ratios and glutamic treatment had stronger effect than other AAs. Replacing of amino acids in nutrient solution increased essential, semi essential and non essential amino acids content of lettuce shoot compared to 100% NO₃-N treatment