Department of Life Sciences, University of Trieste, Via L. Giorgieri 10, Trieste 34127, Italy.
Department of Biology, University of Padova, Via Ugo Bassi 58B, Padova 35121, Italy.
Tree Physiol. 2023 Jun 7;43(6):893-908. doi: 10.1093/treephys/tpad013.
Leaves are the most important photosynthetic organs in most woody plants, but chloroplasts are also found in organs optimized for other functions. However, the actual photosynthetic efficiency of these chloroplasts is still unclear. We analyzed bark and wood chloroplasts of Fraxinus ornus L. saplings. Optical and spectroscopic methods were applied to stem samples and compared with leaves. A sharp light gradient was detected along the stem radial direction, with blue light mainly absorbed by the outer bark, and far-red-enriched light reaching the underlying xylem and pith. Chlorophylls were evident in the xylem rays and the pith and showed an increasing concentration gradient toward the bark. The stem photosynthetic apparatus showed features typical of acclimation to a low-light environment, such as larger grana stacks, lower chlorophyll a/b and photosystem I/II ratios compared with leaves. Despite likely receiving very few photons, wood chloroplasts were photosynthetically active and fully capable of generating a light-dependent electron transport. Our data provide a comprehensive scenario of the functional features of bark and wood chloroplasts in a woody species and suggest that stem photosynthesis is coherently optimized to the prevailing micro-environmental conditions at the bark and wood level.
叶子是大多数木本植物最重要的光合作用器官，但也有一些器官中的叶绿体被优化用于其他功能。然而，这些叶绿体的实际光合效率仍然不清楚。我们分析了欧洲白蜡树幼苗的树皮和木质部叶绿体。应用光学和光谱方法对茎样进行了分析，并与叶片进行了比较。在茎的径向方向上检测到一个明显的光梯度，蓝色光主要被外树皮吸收，远红光富集光到达下面的木质部和髓部。木质部射线和髓部中存在叶绿素，并且向树皮方向呈现出浓度梯度增加的趋势。茎的光合作用器官表现出适应低光环境的特征，例如与叶片相比，更大的基粒堆叠、更低的叶绿素 a/b 和光系统 I/II 比值。尽管可能接收很少的光子，但木质部叶绿体仍然具有光合作用活性，并完全能够产生依赖于光的电子传递。我们的数据提供了木质部和树皮叶绿体在木本植物中的功能特征的综合情况，并表明茎的光合作用与树皮和木质部水平上的流行微环境条件一致地进行了优化。
