Assessment of romaine lettuce cultivars grown in a vertical hydroponic system at two levels of LED light intensity

Romaine lettuce (Lactuca sativa var. longifolia) was cultivated in a hydroponic system, in the free-standing container equipped with two-shelf racks. The luminous intensity of the upper shelves was 200 µmol m−2 s−1, and that of the bottom was 160 µmol m−2 s−1. The plants were exposed to light for 16 h each day. Seven cultivars from two groups were cultivated in this study. There were four cultivars from the midi subtype (‘Casual,’ ‘Pivotal,’ ‘Delcaso,’ and ‘Airton’) and three from the mini subtype (‘Xiomara,’ ‘Coventry,’ and ‘Elizium’). The growth period of lettuce in the hydroponic chamber was 30 days. Immediately after harvest, lettuce was evaluated for plant morphological parameters and for the content of macro- and microelements and metabolites with health-promoting properties. Additionally, sensory evaluation was performed and leaf color parameters were measured. The genotype of romaine lettuce had a major impact on its extrinsic and intrinsic properties. The mini cultivars produced lower fresh mass but contained higher amounts of nitrogen, phosphorus, potassium, copper, and boron than varieties from the midi group. The contents of bioactive compounds were higher for midi romaine lettuce than for the mini type. In terms of nitrate, there were significant differences, and lower nitrate levels occurred in the mini group, except in ‘Coventry.’ The results obtained in this study did not allow us to single out any cultivar from the midi group as the best or the worst in terms of chemical composition. Among the mini cultivars, ‘Elizium’ shown the tendency to accumulate a relatively small amount of nitrate and a relatively large amount of chlorophyll, carotenoid, polyphenol and flavonoid. ‘Coventry’ with the lowest chlorophyll, carotenoid, and flavonoid contents was found to be the poorest in bioactive compounds. The sensory evaluation indicated the good quality of all mini and two midi cultivars, while ‘Delcaso’ (midi cultivar) received the worst marks, because of poor smell and crispness and a bitter aftertaste. The higher light intensity (200 µmol m−2 s−1) used for plant illumination did not unequivocally increase the content of health-promoting components or improve other characteristics of romaine lettuce compared to the light 160 µmol m−2 s−1.