Changes in leaf expansion and epidermal screening effectiveness in Liquidambar styraciflua and Pinus taeda in response to UV‐B radiation

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Abstract
Absorption or screening of ultraviolet‐B (UV‐B) radiation by the epidermis may be an important protective method by which plants avoid damage upon exposure to potentially harmful UV‐B radiation. In the present study we examined the relationships among epidermal screening effectiveness, concentration of UV‐absorbing compounds, epidermal anatomy and growth responses in seedlings of loblolly pine (Pinus taeda L.) and sweetgum (Liquidambar styraciflua L.). Seedlings of each species were grown in a greenhouse at the University of Maryland under either no UV‐B radiation or daily supplemental UV‐B radiation levels of 4, 8 or 11 kJ m−2 of biologically effective UV‐B (UV‐BBE) radiation. Loblolly pine seedlings were subsequently grown in the field under either ambient or supplemental levels of UV‐B radiation. At the conclusion of the growing season, measurements of epidermal UV‐B screening effectiveness were made with a fiber‐optic microprobe. In loblolly pine, less than 0.5% of incident UV‐B radiation was transmitted through the epidermis of fascicle needles and about 1% was transmitted in primary needles. In contrast, epidermal transmittance in sweetgum ranged from about 20% in leaves not preconditioned to UV‐B exposure, to about 10% in leaves grown under UV‐B radiation. The concentration of UV‐absorbing compounds was unaffected by UV‐B exposure, but generally increased with leaf age. Increases in epidermal thickness were observed in response to UV‐B treatment in loblolly pine, and this accounted for over half of the variability in UV‐B screening effectiveness. In spite of the low levels of UV‐B penetration into the mesophyll, delays in leaf development (both species) and final needle size (loblolly pine) were observed. Seedling biomass was reduced by supplemental UV‐B radiation in loblolly pine. We hypothesize that the UV‐induced growth reductions were manifested by changes in either epidermal anatomy or epidermal secondary chemistry that might negatively impact cell elongation.

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