Effect of Reduced Nitrogen and Supplemented Amino Acids Nutrient Solution on the Nutritional Quality of Baby Green and Red Lettuce Grown in a Floating System

Pavlos Tsouvaltzis,Anastasios S. Siomos,Danai-Christina Aktsoglou,Nikolaos Barbayiannis,Dimitrios S. Kasampalis

doi:10.3390/agronomy10070922

Pavlos Tsouvaltzis, Anastasios S. Siomos + Show 3 more

Open Access

https://doi.org/10.3390/agronomy10070922

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Abstract

Excessive nitrogen fertilization results in nitrate accumulation in leafy vegetables. Reducing the dose of mineral nitrogen or using alternate fertilizers lowers the nitrate accumulation; however, a critical minimum level of mineral nitrogen is necessary to maintain yield and nutritional quality. The aim of this study was to evaluate the effect of two levels of mineral nitrogen (100% and 50%) and three levels of an amino acid solution (0, 0.3, and 0.9%) in the nutrient solution of two baby lettuce cultivars (green and red) grown in a floating system. Nitrogen reduction did not affect yield (12.9–13.4 and 11.0–11.3 g/plant, respectively) but reduced nitrate accumulation (by 43 and 19%, respectively) in both green and red lettuce, while enhancing phenolic content (by 28%) and antioxidant capacity (by 69%) in green lettuce and soluble solid (by 7%) and total chlorophyll content (by 9%) in red lettuce. Although nitrate accumulation was prevented (< 355 mg/kg FW) and most nutritional components increased in both lettuce types by amino acids supplementation, plant growth was negatively affected, especially in red lettuce, in both concentrations of amino acids (reduction by 9 and 35% in 0.3 and 0.9%, respectively). In both lettuce types, proline content increased by 0.9% amino acids supplementation (by 45%), implying a probable induction of a stress condition. Mineral nutrients were slightly affected by nitrogen reduction, which was probably perceived as an abiotic stress.

Highlights

Modern agricultural management techniques in vegetable cultivation are target towards the intensive controlled production of superior nutritional quality products, along with the highest possible water and nitrogen use efficiency [1,2,3]
The addition of 0.3 or 0.9% amino acids in the nutrient solution resulted in a decrease of the pH to around 4.0 and 3.4, respectively, which was followed by a sigmoidal pattern of increase, reaching the control levels after 10 and 24 days, respectively, regardless of the N concentration; the pH even increased beyond that of the control while reaching the end of the production cycle
In order to elucidate the effect of N and amino acids concentrations in the nutrient solution, data were processed within each lettuce type

Summary

Introduction

Modern agricultural management techniques in vegetable cultivation are target towards the intensive controlled production of superior nutritional quality products, along with the highest possible water and nitrogen use efficiency [1,2,3]. Closed hydroponic systems lead to a great reduction of inputs, such as water and fertilizers and are suitable for the growth of leafy vegetables even without the use of substrates, as in the case of floating tray systems. This soilless system has many advantages, such as the low installation and operation costs, the use of high plant densities, and the absence of any substrate residues on the harvested products [6,7,8]. The lack of interactions among roots, soil, and soil-containing microorganisms [9,10] renders this soilless system suitable to study in detail the actual needs of a plant in vivo in terms of water and nitrogen rates and forms requirements [11], along with the plant’s response to biostimulant supplements, such as amino acids [12,13].

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