UV Screening in Lodgepole Pine (Pinus contortassp.latifolia) Cotyledons and Needles

Abstract

Conifer needles of high‐elevation species have been reported to screen UV radiation effectively from penetrating the leaf interior. Experiments were designed to investigate the epidermal transmittance (ET) of lodgepole pine (Pinus contorta ssp. latifolia) to ambient UV‐A and UV‐B radiation in the Rocky Mountains of southeastern Wyoming. Two exclusion treatments used plastic filters to regulate the amount of incident UV radiation in natural sunlight. One experiment utilized potted seedlings grown in the field at 2488‐m elevation, while the other used natural field saplings >0.5 m in height at another field site (3567‐m elevation). Both experiments consisted of a control and three exclosures covered with different plastic filters: (1) polyethylene plastic (control) that transmitted a high percentage of UV‐A and UV‐B, (2) clear polyester that excluded UV‐B wavelengths only, (3) Plexiglas that excluded most of the UV‐A and UV‐B wavelengths, and (4) a control treatment with no filtering (natural sunlight). The polyethylene plastic transmitted ca. 83% of total ambient levels of UV‐A and UV‐B, the polyester ca. 50% of total UV, but <2% of the UV‐B, while the Plexiglas filters transmitted 0% of total UV. The ET of UV radiation through leaf epidermal peels was measured with a fiber‐optic microprobe. In all treatments, ET was lowest for UV‐B wavelengths (300–320 nm) and increased exponentially as wavelength increased into the UV‐A region (320–360 nm). Results showed that (1) the epidermis of mature primary needles transmitted <2.5% of the incident UV radiation; (2) cotyledons of seed‐grown seedlings placed in the field had substantially less UV screening than primary leaves, especially for UV‐A; (3) needles of both cotyledons and primary needles grown without UV exposure (Plexiglas screening) had substantial declines in both UV‐A and UV‐B transmittance; and (4) needle length, surface area, biomass, or other growth variables were not influenced by any of the exclosure treatments. Also, ET was greatest (30% at 360 nm) for cotyledons grown without exposure to UV (Plexiglas), compared with ca. 14% when UV‐B was excluded and a 12% ET when all wavelengths were allowed to pass (polyethylene). These results indicate that the normal development of UV screening in lodgepole pine needles is dependent on exposure to UV during early ontogeny, especially UV‐B wavelengths.