Impact of Light Variation on Development of Photoprotection, Antioxidants, and Nutritional Value in Lactuca sativa L.

Lettuce plants were grown at low (I-L), middle (ML), and high light (HL) conditions to examine the relationship between photoacclimatory plasticity, light energy utilization, and antioxidant capacity. With the increase in light intensity from LL to ML, the energy flux via Delta pH- and xanthophylls-regulated thermal dissipation, fluorescence and constitutive thermal dissipation, and electron transport for photorespiratory carbon oxidation all increased significantly. However, plants at HL exhibited reduced electron transport for photosynthetic carbon reduction and decreased maximal photochemical efficiency of photosytem II (PSII) as compared to that at ML. Increasing light level significantly increased the alternative electron transport, O-2(center dot-) production rate, and H2O2 and malondialdehyde (MDA) contents followed by increased ferric reducing antioxidant power (FRAP) and activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR). Moreover, plants exposed to HL showed higher nutritional value as indicated by the high contents of ascorbate, glutathione, carotenoids, and alpha-tocopherol. It was concluded that absorption of excess photon energy at high light was associated with increased antioxidant capacity and that produce quality could. be improved by short-term exposure to suboptimum irradiance.