Reduction of Nitrate Content in Baby-Leaf Lettuce and Cichorium endivia Through the Soilless Cultivation System, Electrical Conductivity and Management of Nutrient Solution

Abstract

Soilless cultivation systems are efficient tools to control nitrates by managing nutrient solution (NS) salinity and nitrogen availability, however, these nitrate-lowering strategies require appropriate calibration based on species/genotype-specific responses interacting with climate and growing conditions. Three experiments were carried out on lettuce and Cichorium endivia grown in ebb-and-flow (EF) and floating (FL) systems at two levels of NS salinity (EC = 2.5 and 3.5 dS m−1) (EC2.5, EC3.5, respectively) under autumn and early-spring (lettuce) and winter and late-spring conditions (C. endivia). Nitrogen deprivation (NS withdrawal a few days before the harvest) was tested at EC2.5, in the autumn and winter cycles. The EF-system caused an increase in salinity in the substrate where roots mainly develop so it mimicked the effect of the EC3.5 treatment. In the winter-grown lettuce, the EF-system or EC3.5 treatment was effective in reducing the nitrate level without effects on yield, with the EF baby-leaf showing an improved quality (color, dry matter, chlorophylls, carotenoid, vitamin C, phenol). In both seasons, the EF/EC3.5 treatment resulted in a decline in productivity, despite a further reduction in nitrate content and a rise in product quality occurring. This response was strictly linked to the increasing salt-stress loaded by the EC3.5/EF as highlighted by the concurrent Cl− accumulation. In early-spring, the FL/EC3.5 combination may represent a trade-off between yield, nitrate content and product quality. In contrast, in winter-grown endive/escarole the EC3.5, EF and EC3.5/EF reduced the nitrate level with no effect on yield, product quality or Cl− uptake, thus proving them to be more salt-tolerant than lettuce. High temperatures during the late-spring cycle promoted nitrate and Cl− uptake, overcoming the nitrate-controlling effect of salinity charged by the EF system or EC3.5. The nitrate level decreased after 3 day-long (lettuce) or 6 day-long (C. endivia) NS withdrawal. In C. endivia and EF-grown lettuce, it provoked a decrease in yield, but a concurrent improvement in baby-leaf appearance and nutritional quality. More insights are needed to fine-tune the duration of the NS removal taking into account the soilless system used and species-specific characteristics.

Nitrate Uptake and Use Efficiency: Pros and Cons of Chloride Interference in the Vegetable Crops

“… Rouphael et al (2019) obtained a decrease in accumulation of NO 3 – in leaves of green and red-pigmented perilla by applying a 10 mM NaCl eustress, and at the same time, this treatment enhanced polyphenols and therefore the antioxidant activity. Lettuce plants underwent a decrease in the leaf NO 3 – content between 20 and 35 mM NaCl, which determined an increase in polyphenols but also a decrease in the growth and yield proportional to the increase in the salinity ( Carillo et al, 2020 , 2021 ; Conversa et al, 2021 ). However, Conversa et al (2021) found that the endive plants showed a decrease in the antinutrient nitrate without a simultaneous effect on the yield even at 35 mM NaCl (3.5 dS m –1 ), probably due to the higher salt tolerance of this plant.…”

Environmental and Cultivation Factors Affect the Morphology, Architecture and Performance of Root Systems in Soilless Grown Plants

“…Savvas and Gruda [10] and Gruda et al [59] also reported some methods to reduce the nitrate contents in SCS-leafy vegetables by lowering or eliminating the NO 3 -N supply a few days before harvesting. However, nitrate-lowering strategies require appropriate calibration based on species-/genotype-specific responses interacting with climate and growing conditions [60].…”

Impacts of Reduced Nitrate Supply on Nitrogen Metabolism, Photosynthetic Light-Use Efficiency, and Nutritional Values of Edible Mesembryanthemum crystallinum

“…NO 3 concentration in leaf vegetables can be manipulated by reducing or withdrawing NO 3 supply prior to harvesting (He et al, 2011a(He et al, , 2014Conversa et al, 2021). In this study, all M. crystallinum plants were first grown with full N for 11 days before transferring them to 3/4 N, 1/2 N, 1/4 N, and 0 N conditions for another 7 days.…”

“…Reduced NO 3 availability in the growth media results in reduced accumulation of NO 3 in edible portions of crop plants (He et al, 2014;Bian et al, 2020;Conversa et al, 2021). However, in the present study, compared with M. crystallinum plants grown with full N, those transferred from full N to 3/4 N, 1/2 N, and 1/4 N seemed to be constant or slightly increased in shoot NO 3 concentrations during the reduced NO 3 treatments on a DW (Figure 4A) or a FW basis (Figure 4B).…”

“…Shoot NO 3 concentration decreased in M. crystallinum plants transferred from full N to 0 N during 7-day treatment compared with that of full N plants ( Figures 4A,B). Recently, Conversa et al (2021) reported that the NO 3 level decreased after 3 days of treatment for baby-leaf lettuce (L. sativa var. longifolia L.) or 6 days of treatment for Cichorium endivia L. after nutrient solution withdrawal.…”