UV-B Transmittance of Greenhouse Covering Materials Affects Growth and Flavonoid Content of Lettuce Seedlings

In Europe, lettuce (Lactuca sativa L., Asteraceae) is commonly raised in greenhouses and transplanted to the field at the age of two to four weeks in order to prolong the growing season. The sudden exposure to outdoor conditions including altered temperature, radiation levels and rainfall events is extremely stressful for non-acclimated seedlings. Particularly the increase in ultraviolet-B radiation is considered a serious threat. A new approach to pre-acclimate seedlings to ambient ultraviolet-B radiation is the use of ultraviolet-B transparent covering materials. In order to estimate the benefit of UV-B pre-acclimation, lettuce plants were raised in greenhouses covered with three different materials varying in ultraviolet-B transmittance and transplanted to the field at the age of three weeks. Ultraviolet-B exposure during the greenhouse period led to a reduction in growth (leaf length, leaf area and leaf number) and an increase in flavonoid content. Transplantation to the field induced a strong enhancement in flavonoid content and a severe growth reduction overriding differences between UV-B treatment groups within a few days. At the time of harvest plant fresh weight was therefore independent from previous ultraviolet-B treatment. Effects of UV-B acclimation on plant performance immediately after transplantation require more detailed examination.