The quality of red lettuce is based on the content of anthocyanin pigments, and the content of these pigments increases when the plant receives ultraviolet radiation. Lettuce crops are increasingly being grown in greenhouses to provide better quality fresh lettuce; however, both quality and productivity are affected by the type of cover used. The effects of UV-transparent and UV-blocking plastic films on the growth and quality of three lettuce cultivars (‘Casabella’ and ‘Vera’, which are both green leaf, and ‘Lollo Rosso’, which is red leaf) were investigated. The crop was cultivated at an altitude of 2576 m a.s.l. to naturally expose it to high ultraviolet radiation. The height and diameter of plants, fresh and dry weight, leaf area index, chlorophyll fluorescence, and leaf colour were determined throughout the crop cycle. Growth characteristics were not significantly influenced by the plastic film in all cultivars, probably due to the high altitude and the high solar radiation received. The UV-transparent film could increase the red colour in ‘Lollo Rosso’, and a higher commercial quality was achieved. No effect of the plastic film on the yield component was observed in the ‘Casabella’ and ‘Vera’ cultivars. The findings of this study provide guidance on what cultivar and plastic film must be used in high-altitude areas where the solar and UV radiation is high.
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