Influence of light modification on postharvest butter lettuce quality: differences between external and internal leaves

The aim of the study was to investigate the influence of coloured shade-nets (pearl, blue, red and black with 50% shade index) in comparison to open field (non-shaded plants) on quality traits of different butter lettuce (Lactuca sativa L. var. capitata) cultivar 'Tizian' leaves (external and internal) from summer production at harvest and during storage. Leaves from non-shaded plants were characterized by higher content of soluble sugars and acids in comparison to shaded plants. At harvest, significantly higher content of chlorophyll, total phenols and micro elements was recorded in external than in internal leaves. Internal leaves had higher content of soluble sugars (glucose, fructose and sucrose) and higher organic acids (malic and succinic) content than external (light exposed) leaves, but citric acid was present only in external leaves. Total phenol content in leaves exposed to the light was by 55-85% higher than in shaded lettuce. Total sugar content and total acid content in lettuce leaves increased during storage, more intensively in non-shaded than in shaded leaves. Chlorophyll content changes in lettuce occurred predominantly in outer leaves of lettuce, while in inner leaves no significant changes were noted.