An experiment was carried out in a green house aiming to evaluate the effect of nitrogen source in lettuce plants (Lactuca sativa L var. Verdinha de Vitória) grown in washed sand (NFT). Nitrogen sources were: urea (U), sodium nitrate (NS), potassium nitrate (NP), ammonium nitrate (NA) and ammonium sulphate (SA). The control plants were grown in field condition (CC), and they had a total biomass (f.m.) of 48.51 g.plant-1 while the plants grown in hydroponics system had 163.67; 244.68; 214.08 and 149.60 g.plant-1 respectively for U, NS, NP and NA. All plants treated with SA died three weeks after treatment starting. The content of mineral residue in dry leaf tissue was higher for CC treatment (209.8 mg.g-1) and lower for U treatment (158.0 mg.g-1). The plants treated with NA, NP, NS, U and CC had respectively 37.45; 44.70; 45.91; 46.97 and 48.42 mg.g-1 of N-total. Phosphorus content ranged between 6.18 (NS) and 9.47 mg.g-1 (NA). Sulphur and potassium content were the only nutrients not affected by treatments. The plants treated with NS presented 25.20 mg.g-1 of Na+, while the other plants did not differ significantly in relation to this nutrient, presenting values from 1.00 to 3.20 mg.g-1. Chlorophyll content were lower in plants grown in field condition, which had 0.30 and 0.65 mg.g-1 respectively for chlorophyll B and chlorophyll A. The content of soluble protein was higher for U treatment (203.8 mg.g-1), while the other treatments did not differ significantly, ranging from 109.4 to 121.6 mg.g-1. Free total amino acids ranged from 2.42 (NA) to 4.33 mg.g-1 (CC). The plants treated with urea only presented traces of nitrate in leaf tissue, while plants treated with ammonium nitrate presented the higher nitrate content in leaf tissue (4.07 mg.g-1).
Index terms: Lactuca sativa, mineral nutrition, nitrate, NFT.