UV screening in lodgepole pine (Pinus contorta ssp. latifolia) cotyledons and needles

Conifer needles of high-elevation species have been reported to screen UV r adiation effectively from penetrating the leaf interior. Experiments were d esigned to investigate the epidermal transmittance (ET) of lodgepole pine ( Pinus contorta ssp. latifolia) to ambient UV-A and UV-B radiation in the Ro cky Mountains of southeastern Wyoming. Two exclusion treatments used plasti c filters to regulate the amount of incident UV radiation in natural sunlig ht. One experiment utilized potted seedlings grown in the field at 2488-m e levation, while the other used natural field saplings >0.5 m in height at a nother field site (3567-m elevation). Both experiments consisted of a contr ol and three exclosures covered with different plastic filters: (1) polyeth ylene plastic (control) that transmitted a high percentage of UV-A and UV-B , (2) clear polyester that excluded UV-B wavelengths only, (3) Plexiglas th at excluded most of the UV-A and UV-B wavelengths, and (4) a control treatm ent with no filtering (natural sunlight). The polyethylene plastic transmit ted 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 mea sured with a fiber-optic microprobe. In all treatments, ET was lowest for U V-B wavelengths (300-320 nm) and increased exponentially as wavelength incr eased into the UV-A region (320-360 nm). Results showed that (1) the epider mis of mature primary needles transmitted <2.5% of the incident UV radiatio n; (2) cotyledons of seed-grown seedlings placed in the field had substanti ally less UV screening than primary leaves, especially for UV-A; (3) needle s of both cotyledons and primary needles grown without UV exposure (Plexigl as screening) had substantial declines in both UV-A and UV-B transmittance; and (4) needle length, surface area, biomass, or other growth variables we re not influenced by any of the exclosure treatments. Also, ET was greatest (30% at 360 nm) for cotyledons grown without exposure to UV (Plexiglas), c ompared with ca. 14% when UV-B was excluded and a 12% ET when all wavelengt hs were allowed to pass (polyethylene). These results indicate that the nor mal development of UV screening in lodgepole pine needles is dependent on e xposure to UV during early ontogeny, especially UV-B wavelengths.