The effect of humic and glutamic acids in nutrient solution on the N metabolism in lettuce

BACKGROUND: This paper reports for the first time the influence of partially replacing the nitrogen (N) source of a nutrient solution with amino acids and humic acid on the physiological and antioxidant activities and N metabolism in lettuce. A hydroponic experiment was designed to replace 25 and 50% of the N in nutrient solutions (NSs) with glutamic acid (GA) and humic acid (HA) and evaluate the effects on growth, nitrate (NO 3 ) assimilation, protein content, nitrate reductase (NR) activity and antioxidant changes in lettuce. RESULTS: The results showed that, when a portion of the N was replaced with GA and HA, the fresh and dry weights of lettuce shoots did not change significantly compared with the full NO 3 treatment. The titratable acidity was not affected by adding HA and/or GA to the NS. The nitrite concentration and NR activity decreased with reductions in the N concentration of the NS and improved with the addition of GA and HA. GA enhanced the NO 3 uptake and protein content more than HA. Changes in the superoxide dismutase (SOD) and peroxidase (POD) activities and malondialdehyde content were inconclusive: SOD and POD activities decreased with decreasing N content of the NS, and HA addition improved the SOD and POD activities. CONCLUSION: When HA and GA were substituted for NO 3 in an NS, the acids effectively adjusted the N metabolism and growth in lettuce and decreased the N consumption of the NS. Copyright © 2012 Society of Chemical Industry