Johnson Daniel M, Brodersen Craig R, Reed Mary, Domec Jean-Christophe, Jackson Robert B
Nicholas School of the Environment, Duke University, Durham, NC 27708, USA.
Ann Bot. 2014 Mar;113(4):617-27. doi: 10.1093/aob/mct294. Epub 2013 Dec 20.
Despite the importance of vessels in angiosperm roots for plant water transport, there is little research on the microanatomy of woody plant roots. Vessels in roots can be interconnected networks or nearly solitary, with few vessel-vessel connections. Species with few connections are common in arid habitats, presumably to isolate embolisms. In this study, measurements were made of root vessel pit sizes, vessel air-seeding pressures, pit membrane thicknesses and the degree of vessel interconnectedness in deep (approx. 20 m) and shallow (<10 cm) roots of two co-occurring species, Sideroxylon lanuginosum and Quercus fusiformis.
Scanning electron microscopy was used to image pit dimensions and to measure the distance between connected vessels. The number of connected vessels in larger samples was determined by using high-resolution computed tomography and three-dimensional (3-D) image analysis. Individual vessel air-seeding pressures were measured using a microcapillary method. The thickness of pit membranes was measured using transmission electron microscopy.
Vessel pit size varied across both species and rooting depths. Deep Q. fusiformis roots had the largest pits overall (>500 µm) and more large pits than either shallow Q. fusiformis roots or S. lanuginosum roots. Vessel air-seeding pressures were approximately four times greater in Q. fusiformis than in S. lanuginosum and 1·3-1·9 times greater in shallow roots than in deep roots. Sideroxylon lanuginosum had 34-44 % of its vessels interconnected, whereas Q. fusiformis only had 1-6 % of its vessels connected. Vessel air-seeding pressures were unrelated to pit membrane thickness but showed a positive relationship with vessel interconnectedness.
These data support the hypothesis that species with more vessel-vessel integration are often less resistant to embolism than species with isolated vessels. This study also highlights the usefulness of tomography for vessel network analysis and the important role of 3-D xylem organization in plant hydraulic function.
尽管被子植物根中的导管对于植物水分运输至关重要，但对木本植物根的微观解剖学研究却很少。根中的导管可以是相互连接的网络，也可以几乎是孤立的，导管与导管之间的连接很少。连接少的物种在干旱生境中很常见，大概是为了隔离栓塞。在本研究中，对两种共生植物——绒毛铁线子和梭形栎深根（约20米）和浅根（<10厘米）的根导管纹孔大小、导管空气进入压力、纹孔膜厚度以及导管相互连接程度进行了测量。
使用扫描电子显微镜对纹孔尺寸进行成像，并测量相连导管之间的距离。通过高分辨率计算机断层扫描和三维（3-D）图像分析确定较大样本中相连导管的数量。使用微毛细管法测量单个导管的空气进入压力。使用透射电子显微镜测量纹孔膜的厚度。
导管纹孔大小在两个物种和不同生根深度之间存在差异。梭形栎深根的纹孔总体上最大（>500微米），比梭形栎浅根或绒毛铁线子根的大纹孔更多。梭形栎的导管空气进入压力大约是绒毛铁线子的四倍，浅根的导管空气进入压力比深根大1.3 - 1.9倍。绒毛铁线子有34 - 44%的导管相互连接，而梭形栎只有1 - 6%的导管相互连接。导管空气进入压力与纹孔膜厚度无关，但与导管相互连接程度呈正相关。
这些数据支持了这样的假设，即导管间整合程度较高的物种通常比导管孤立的物种对栓塞的抵抗力更弱。本研究还强调了断层扫描在导管网络分析中的有用性以及三维木质部组织在植物水分运输功能中的重要作用。
