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.
Highlights
The baby-leaf category is important among the leafy vegetables
The average electrical conductivity of nutrient solution (NS) during the crop cycle showed negligible differences between the EF and FL systems, with even higher values in this latter (Supplementary Table 1). These results suggest that in the EF system the partial drying between the intermittent wettings (3-min wetting flux at the base of the trays every 100 min) exacerbated salt accumulation in the substrate where roots mainly developed, so mirroring the effect of the EC3.5 treatment on shoot growth
The effectiveness of the strategies considered in this work to reduce nitrate level in soilless grown baby-leaf romaine lettuce and C. endivia mainly depends on species-specific responses to salinity imposed by the soilless system and nutrient solution, and on the climatic conditions throughout the growing season
Summary
The baby-leaf category is important among the leafy vegetables. It is harvested at an early-vegetative phase (8–12 cm in length), the name, and includes many species, with Lactuca sativa L. and Cichorium endivia L. varieties among the most popular (Nicola and Fontana, 2014). They are mainly used as minimally processed vegetable products (Conesa et al, 2015) and consumed in increasing amounts as they provide an important source of health-promoting compounds such as carotenoids, vitamin C, and polyphenols (DuPont et al, 2000; El-Nakhel et al, 2019). Romaine lettuce has been reported to be less prone to accumulate nitrate than oak-leaf lettuce in fall-winter cycles (Di Gioia et al, 2017)
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