Burrows Geoffrey E, Connor Celia
School of Agricultural and Wine Sciences, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW 2678, Australia.
Plants (Basel). 2020 Dec 21;9(12):1814. doi: 10.3390/plants9121814.
Small diameter branchlets and smooth barked stems and branches of most woody plants have chloroplasts. While the stems of several eucalypt species have been shown to photosynthesise, the distribution of chloroplasts has not been investigated in detail. The distribution of chloroplasts in branchlets (23 species) and larger diameter stems and branches with smooth bark (14 species) was investigated in a wide range of eucalypts (species of , and ) using fresh hand sections and a combination of bright field and fluorescence microscopy. All species had abundant stem chloroplasts. In both small and large diameter stems, the greatest concentration of chloroplasts was in a narrow band (usually 100-300 μm thick) immediately beneath the epidermis or phellem. Deeper chloroplasts were present but at a lower density due to abundant fibres and sclereids. In general, chloroplasts were found at greater depths in small diameter stems, often being present in the secondary xylem rays and the pith. The cells of the chlorenchyma band were small, rounded and densely packed, and unlike leaf mesophyll. A high density of chloroplasts was found just beneath the phellem of large diameter stems. These trees gave no external indication that green tissues were present just below the phellem. In these species, a thick phellem was not present to protect the inner living bark. Along with the chlorenchyma, the outer bark also had a high density of fibres and sclereids. These sclerenchyma cells probably disrupted a greater abundance and a more organised arrangement of the cells containing chloroplasts. This shows a possible trade-off between photosynthesis and the typical bark functions of protection and mechanical strength.
大多数木本植物的小直径小枝以及树皮光滑的茎和枝条都含有叶绿体。虽然已经证明几种桉树种的茎能进行光合作用，但叶绿体的分布尚未得到详细研究。利用新鲜徒手切片以及明场和荧光显微镜相结合的方法，对多种桉树（、和属的物种）的小枝（23种）以及树皮光滑的较大直径茎和枝条（14种）中叶绿体的分布进行了研究。所有物种的茎中都有丰富的叶绿体。在小直径和大直径的茎中，叶绿体最集中的区域是紧挨着表皮或木栓层下方的一条窄带（通常100 - 300μm厚）。更深层也有叶绿体，但由于纤维和石细胞丰富，其密度较低。一般来说，小直径茎中的叶绿体分布更深，常存在于次生木质部射线和髓中。叶肉组织带的细胞小、呈圆形且排列紧密，与叶片叶肉不同。在大直径茎的木栓层下方发现了高密度的叶绿体。这些树从外部没有显示出木栓层下方存在绿色组织。在这些物种中，没有厚木栓层来保护内部的活树皮。除了叶肉组织外，外树皮中也有高密度的纤维和石细胞。这些厚壁组织细胞可能破坏了含有叶绿体的细胞更丰富且更有序的排列。这表明在光合作用与保护和机械强度等典型树皮功能之间可能存在权衡。
