Dynamic adjustments in leaf epidermal UV transmittance of plants in sun-shade transitions

Leaves exposed to UV radiation during development typically accumulate UV-absorbing compounds in their epidermis which then decreases epidermal UV transmittance. To what degree mature leaves can increase or decrease their UV shielding levels when confronted with novel radiation regimes, as would occur in sun-shade transitions, is poorly understood. In this experiment, plants of Vicia faba were grown under one of four solar radiation treatments (full sun+UV, full sun–UV, shade+UV, or shade–UV) at a high elevation site on Mauna Kea, Hawaii. After 5 days of pre-treatment, plants were transferred, in a factorial fashion, to alternate treatments (post-treatment) and epidermal UV transmittance was monitored in-situ with an UVA-PAM fluorometer. Prior to transfer, mean predawn epidermal UV transmittance was highest in plants grown in the sun+UV treatment (94.5%), lowest in the shade-UV treatment (84.4%) and intermediate in sun-UV (91.3%) and shade+UV (89.4%) treatments. Within four days following transfer, plants grown under full sun–UV, shade+UV, and shade–UV treatments all showed epidermal UV transmittances that were statistically comparable (P>0.05) with the sun+UV controls. Transferring plants to the other treatments had minimal to no effect on UV shielding. Results indicate that, 1) both UV and visible irradiation influences epidermal UV transmittance during leaf development, 2) leaves can increase UV shielding levels following development, but this requires exposure to UV, and 3) mature leaves lack the ability to down-regulate base-level UV shielding in response to shade and/or reduced UV environments. These findings, and others, reveal a more dynamic UV “sun screen” mechanism in plants than previously thought.