Department of Experimental Biology, Faculty of Science, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic.
Faculty of Environmental Sciences and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, NO-1432, Ås, Norway.
Plant Physiol Biochem. 2018 Nov;132:89-94. doi: 10.1016/j.plaphy.2018.08.033. Epub 2018 Aug 27.
Lichen photobionts in situ have an extremely UV-B tolerant photosystem II efficiency (Fv/Fm). We have quantified the UV-B-screening offered by the mycobiont and the photobiont separately. The foliose lichens Nephroma arcticum and Umbilicaria spodochroa with 1: intact or 2: removed cortices were exposed to 0.7 Wm UV-B for 4 h. Intact thalli experienced no reduction in Fv/Fm, whereas cortex removal lowered Fv/Fm in exposed photobiont layers by 22% for U. spodochroa and by 14% for N. arcticum. We also gave this UV-B dose to algal cultures of Coccomyxa and Trebouxia, the photobiont genera of N. arcticum and U. spodochroa, respectively. UV-B caused a 56% reduction in Fv/Fm for Coccomyxa, and as much as 98% in Trebouxia. The fluorescence excitation ratio (FER) technique comparing the fluorescence from UV-B or UV-A-excitation light with blue green excitation light using a Xe-PAM fluorometer showed that these photobiont genera did not screen any UV-B or UV-A The FER technique with a Multiplex fluorometer estimated the UV-A screening of isolated algae to be 13-16%, whereas intact lichens screened 92-95% of the UV-A. In conclusion, the cortex of N. arcticum and U. spodochroa transmitted no UV-B and little UV-A to the photobiont layer beneath. Thereby, the upper lichen cortex forms an efficient fungal solar radiation screen providing a high UV-B tolerance for studied photobionts in situ. By contrast, isolated photobionts have no UV-B screening and thus depend on their fungal partners in nature.
地衣共生藻在体内具有极强的耐 UV-B 的光系统 II 效率（Fv/Fm）。我们已经分别量化了共生真菌和共生藻提供的 UV-B 屏蔽作用。叶状地衣北极衣和 Umbilicaria spodochroa 有 1：完整或 2：去除皮层，暴露在 0.7 Wm 的 UV-B 下 4 小时。完整的地衣体 Fv/Fm 没有降低，而皮层去除使暴露的共生藻层的 Fv/Fm 降低了 22%的 U. spodochroa 和 14%的 N. arcticum。我们还将这种 UV-B 剂量给予北极衣和 Umbilicaria spodochroa 的共生藻 Coccomyxa 和 Trebouxia 的藻类培养物。UV-B 导致 Coccomyxa 的 Fv/Fm 降低了 56%，而 Trebouxia 则高达 98%。使用 Xe-PAM 荧光计比较 UV-B 或 UV-A 激发光与蓝绿光激发光的荧光的荧光激发比（FER）技术表明，这些共生藻属没有屏蔽任何 UV-B 或 UV-A。使用 Multiplex 荧光计的 FER 技术估计分离藻类的 UV-A 屏蔽作用为 13-16%，而完整的地衣则屏蔽了 92-95%的 UV-A。总之，北极衣和 Umbilicaria spodochroa 的皮层对共生藻层没有传递任何 UV-B 和少量的 UV-A。因此，上层地衣皮层形成了一种有效的真菌太阳辐射屏蔽物，为原位研究的共生藻提供了高的 UV-B 耐受性。相比之下，分离的共生藻没有 UV-B 屏蔽作用，因此在自然界中依赖于它们的真菌伙伴。
