Abstract
Nutritional eustress such as salinity or nutrient stress applied in soilless systems, is a convenient pre-harvest factor efficient in modulating the phytochemical components of horticultural crops, by triggering defensive mechanisms and accumulating plant secondary metabolites in plants tissues. Nevertheless, genetic material (cultivars with different pigmentation) dictates lettuce metabolites and physiological response to extrinsic eustress, with red leaf cultivars being highly nutrient packed notwithstanding the stress. Product quality can be meliorated equally by applying several cuts, a practice proven to increase bioactive compounds accumulation. In this study, we analyzed the effects of four salinity levels (1, 10, 20 and 30 mM NaCl) on green and red pigmented Salad Bowl lettuce (Lactuca sativa L. var. acephala) in two successive harvests cultivated in a floating raft system. The morphological parameters, mineral composition, leaf gas exchanges, bioactive compounds, and antioxidant activity of both cultivars were assessed. The green cultivar exhibited superior crop productivity but was more prone to salinity effect than the red cultivar. Irrespective of cultivar and cut order, the net photosynthesis decreased with increasing salinity in the nutrient solution. The second cut incurred higher dry biomass, greater accumulation of most minerals and higher photosynthetic activity. In red lettuce, 20 mM NaCl proved adequate eustress to increase phytonutrients and beneficial minerals (K, Ca, and Mg) with minimal loss of yield. Mild salinity and sequential harvest have proven effective pre-harvest tools in positively modulating the quality of lettuce. Eustress interaction with genotype was demonstrated as a promising field for future breeding programs targeting select genotypes for agronomic application of eustress to improve the nutraceutical value of vegetable crops.
Highlights
Salinity eustress in combination with successive harvests modulated the physiological and chemical composition of red and green pigmented genotypes of baby lettuce grown in floating raft system
It was clearly shown that mild salinity can induce an increase of phytonutrients and beneficial minerals (K, Ca, and Mg) in red lettuce, enhancing the nutraceutical and nutritional quality of the product
This improvement was incurred at the expense of an acceptable, moderate yield reduction, ranging between 8% and 10% at 10 and 20 mM NaCl, respectively
Summary
Lettuce (Lactuca sativa L.) is one of the main leafy green vegetables cultivated worldwide, whose consumption has steadily increased due to its perception as a health-promoting fresh food [1,2,3].In particular, baby lettuce leaves with their petiole, at an optimum size of 8–12 cm long, have becomeAgronomy 2020, 10, 1358; doi:10.3390/agronomy10091358 www.mdpi.com/journal/agronomyAgronomy 2020, 10, 1358 very popular in recent years as minimally processed or fresh-cut salad vegetables ready to eat [4,5].They represent an innovative produce corresponding to the lifestyle of modern consumers [2].Indicatively, the sector of minimally processed agricultural products in Italy has grown from the early1980s to 2019 by about 400%, with approximately 19.4 million regular consumers in 2017 [6]. Agronomy 2020, 10, 1358 very popular in recent years as minimally processed or fresh-cut salad vegetables ready to eat [4,5] They represent an innovative produce corresponding to the lifestyle of modern consumers [2]. There is growing evidence that these secondary bioactive metabolites, in particular polyphenols, can act as antioxidants counteracting the post-harvest decay of lettuce tissues and the development off-odors, and as nutraceuticals with a preventive role against oxidative stress-related diseases, such as cardiovascular diseases, atherosclerosis, neurodegenerative disorders and cancer [3,13,14,15,16] This evidence has attracted the interest of researchers and producers toward ways of enhancing the content of these compounds [3,13,14,15,16], including genetic modification and novel agronomic techniques [1]. Agronomic techniques such as cropping system, nutrient solution management and elicitation factors (e.g., salinity, high CO2 , light intensity, and quality) are the favored methods employed to improve crop performance, to enhance the nutritional and nutraceutical qualities of products and extend their post-harvest life [14,18,19,20,21]
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