Development-dependent UV-B responses in red oak leaf lettuce (Lactuca sativa L.): physiological mechanisms and significance for hardening.

摘要

Plants protect themselves against potentially harmful UV-B radiation mainly by epidermal accumulation of UV-B shielding phenolic compounds and repair of cellular damage. The capacity and necessity for these protection mechanisms alter with the stage of plant and leaf development, which might be crucial for horticultural hardening techniques aiming at a UV-B pre-acclimation of greenhouse grown seedlings before transplantation to the field. In order to examine the development-dependence of UV-B responses and to estimate the efficiency of different hardening strategies, lettuce (Lactuca sativa L., var. crispa L. 'Bughatti') was raised in greenhouses covered with a UV-B transmitting and a UV-B excluding covering material and transferred from one treatment to the other at the age of 20 days. Plant fresh mass and flavonoid content were frequently determined. Distribution of flavonoids within the plants was assessed by extraction of different leafage classes. Continuous exposure to near ambient UV-B induced a strong enhancement in cyanidin and quercetin content and a severe growth reduction, whereas late UV-B treatment merely led to a slight enhancement of quercetin in the absence of any growth response. At the leaf level, continuous UV-B exposure caused a comparable quercetin induction at all three stages of leaf development, whereas late treatment caused a much stronger response in young compared to intermediate and old leaves. These findings indicate that UV-B effects on growth and flavonoid content and pattern of lettuce plants are highly dependent on the leaf and plant developmental stage. We conclude that UV-B exposure throughout seedling development is a more efficient hardening strategy than late short-term exposure.