Effect of NaCl and harvest time on antioxidant compounds and morphological cell changes in Lollo Bionda and Lollo Rosso lettuces

It is known that lettuce (Lactuca sativa L. var. crispa L.) phenological stage and environmental changes can cause changes in the plant cell morphology and an important variation in secondary plant metabolites. Antioxidant compounds are beneficial to health and salinity can increase them. The aim of this study was to determine the effect of salt concentration (NaCl 0.05 and 0.1 mol L-1) and harvest time (days 10, 20 and 30) on mass, color parameters, antioxidant compounds and proline concentration as well as the morphological tissue changes on two lettuce cultivars: Lollo Bionda 'Levistro' (green leaves) and Lollo Rosso 'Carmoli' (red leaves). NaCl treatments and time of harvest affected fresh matter (FM), DM and DM%, total phenol content, concentration of anthocyanin and antioxidant capacity of both cultivars. High NaCl concentrations decreased FM (from 16.8 to 8.7 g in 'Levistro' in 3rd harvest) and the intracellular space of the leaf tissue but increased the concentration of proline (from 19.6 to 292.2 mu g 100 g-1 FM in 'Levistro') and antioxidant compounds (from 412.9 to 487.3 mg Trolox eq. 100 g-1 FM in 'Levistro'). In addition, concentrations of antioxidant compounds (from 458.2 to 506.4 mg Trolox eq. 100 g-1 FM in 'Levistro') and cell density were significantly higher in late than early harvested plants. Thus, successive harvesting may lead to higher antioxidant capacities of lettuces leaves. A moderate decline in FM, a higher cell density and proline concentrations may indicate the better adaptability to salinity stress of red 'Carmoli' than the green 'Levistro' lettuces.