Ultraviolet-B and visible light penetration into needles of two species of subalpine conifers during foliar development

The depth of penetration of Ultraviolet-B (UV-B, 300 and 320 nm) and visible (680 nm) light was measured in foliage of Abies lasiocarpa and Picea engelmannii using a fibre-optic microprobe. Measurements were made on foliage at four times during development: needles were sampled from within expanding buds (in bud); within 72 h of emergence from the bud scales (emergent); from elongating branches (elongating); and from foliage that emerged the previous summer (mature). Light attenuation in pre-emergent needles of both species was steep and showed strong wavelength dependence. Short wavelength 300-nm light was attenuated strongly in the developing epidermal layer, but a significant proportion of this potentially damaging UV-B radiation penetrated into the mesophyll. For A. lasiocarpa and P. engelmannii, 99% attenuation of 300-nm light occurred at 51 and 96 μm, respectively, well within the mesophyll. At this stage, however, the bud scales were opaque to light below 400nm. As the epidermal cell walls and cuticle continued to develop and chlorophyll accumulated following emergence from the bud scales, light attenuation, particularly of UV-B radiation, increased. Although no UV-B is transmitted through the epidermis-hypodermis of mature needles, small but measurable quantities of 300- and 320-nm light were measured in the photosynthetic mesophyll of post-emergent and elongating needles. Thus, shortly after emergence from the bud scales in mid-June to mid-July, when incident UV doses are highest, absorption of UV-B radiation by potentially sensitive chromophores in the mesophyll may disrupt physiological and developmental processes in these species. Soluble UV-absorbing pigments accumulated during needle maturation for P. engelmannii but not A. lasiocarpa, suggesting that, for A. lasiocarpa at least, the development of effective UV screening properties in the epidermis may not be related to the induction of soluble flavonoids.