Abstract
Lettuce has a predisposition to nitrate accumulation and has led many researchers to develop research to reduce this accumulation. The objective of this study was to evaluate the effect of nitrate concentration (NO3-) and nitrate/ammonium ratio (NO3-:NH4+) on growth, production and accumulation of these nitrogen compounds in lettuce grown in a hydroponic system in southern Amazon. There were performed two experiments with Iceberg lettuce type, in a floating-hydroponic system under protected cultivation. In the first experiment, five nitrate concentrations (2.5, 5.0, 7.5, 10.0 and 15.0 mmol L-1) were evaluated. In the second experiment, the best NO3- : NH4+ ratio was evaluated with five proportions 0:100, 25:75, 50:50, 75:25 and 100:0 in the nutrient solution. At 35 days after transplanting, the evaluations were performed. The nutrient solution containing nitrogen (N) as nitrate source in the concentration of 15.0 mmol L-1 contributed to higher lettuce productivity, being the concentration recommended for hydroponic grown system in southern Amazon. It is recommended to use about 23% of the N in nutrient solution in ammonium form, where higher N-ammonium concentrations are not indicated for Icebeg lettuce type because it reduces the productivity of this green leaf vegetable, causing toxicity symptoms. Key words: Nitrogen, vegetable, nutrient solution, food security.
Highlights
The consumption of vegetables has increased in recent years; Nielsen (2016) showed that 60% of the population of North America has been trying to change eating habits by the consumption of healthy foods.Lettuce is the foremost green leafy vegetable consumed in the world, standing out for being a good source of vitamins and minerals (Stagnari et al, 2015)
This study aimed to evaluate the effect of nitrate concentration and the nitrate/ammonium ratio on growth, production, nitrate reductase enzyme activity and accumulation of nitrogen compounds in lettuce grown hydroponically in southern Amazon
The increase of nitrogen concentration in the form of NO3- in the nutrient solution contributed to the linear increase in the number of leaves, stem diameter, leaf length and leaf dry mass of lettuce
Summary
The consumption of vegetables has increased in recent years; Nielsen (2016) showed that 60% of the population of North America has been trying to change eating habits by the consumption of healthy foods.Lettuce is the foremost green leafy vegetable consumed in the world, standing out for being a good source of vitamins and minerals (Stagnari et al, 2015). On the other hand, mishandled hydroponic crops may cause excessive nitrate accumulation in plants, especially in leaf vegetables such as lettuce, which has a predisposition to accumulate nitrate (Byrne et al, 2002). Due to nitrogen (N) being the most required nutrient for the most plants, the fertilizers recommendation in higher doses has been increased, which raises concerns in two ways: first by the contamination of waters and springs, which is likely to increase with increasing population and second, by increasing the nitrate content (NO3-) in vegetables which is considered one of the main sources of human nitrate intake (Santamaria, 2006; Di Gioia et al, 2013; Pandorf et al, 2020). Some estimates indicate that the vegetables correspond to the group of food that mostly contributes to nitrate ingestion by humans, accounting for about 72 and 94% of the daily intake (Turazi et al, 2006; Takahashi et al, 2007)
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