Department of Biology, University of Minnesota, Duluth, MN, USA.
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA.
Nat Plants. 2017 Dec;3(12):965-972. doi: 10.1038/s41477-017-0064-y. Epub 2017 Dec 4.
Trees present a critical challenge to long-distance transport because as a tree grows in height and the transport pathway increases in length, the hydraulic resistance of the vascular tissue should increase. This has led many to question whether trees can rely on a passive transport mechanism to move carbohydrates from their leaves to their roots. Although species that actively load sugars into their phloem, such as vines and herbs, can increase the driving force for transport as they elongate, it is possible that many trees cannot generate high turgor pressures because they do not use transporters to load sugar into the phloem. Here, we examine how trees can maintain efficient carbohydrate transport as they grow taller by analysing sieve tube anatomy, including sieve plate geometry, using recently developed preparation and imaging techniques, and by measuring the turgor pressures in the leaves of a tall tree in situ. Across nine deciduous species, we find that hydraulic resistance in the phloem scales inversely with plant height because of a shift in sieve element structure along the length of individual trees. This scaling relationship seems robust across multiple species despite large differences in plate anatomy. The importance of this scaling becomes clear when phloem transport is modelled using turgor pressures measured in the leaves of a mature red oak tree. These pressures are of sufficient magnitude to drive phloem transport only in concert with structural changes in the phloem that reduce transport resistance. As a result, the key to the long-standing mystery of how trees maintain phloem transport as they increase in size lies in the structure of the phloem and its ability to change hydraulic properties with plant height.
树木对长距离运输构成了重大挑战，因为随着树木的高度增加，运输途径的长度增加，血管组织的水力阻力应该会增加。这使得许多人质疑树木是否可以依靠被动运输机制将碳水化合物从叶子输送到根部。尽管像藤本植物和草本植物这样主动将糖装入韧皮部的物种可以随着伸长增加运输的驱动力，但许多树木可能无法产生高膨压，因为它们不使用转运蛋白将糖装入韧皮部。在这里，我们通过分析使用最近开发的准备和成像技术获得的筛管解剖结构，包括筛板几何形状，并通过原位测量高大树木叶片中的膨压，来研究树木如何在生长过程中保持高效的碳水化合物运输。在 9 个落叶物种中，我们发现由于单个树木中筛管结构的变化，韧皮部中的水力阻力与植物高度呈反比。尽管在板解剖结构上存在很大差异，但这种比例关系在多个物种中似乎是稳健的。当使用在成熟的红橡木叶片中测量的膨压对韧皮部运输进行建模时，这种比例关系的重要性变得明显。这些压力的大小足以与韧皮部中降低运输阻力的结构变化共同驱动韧皮部运输。因此，树木在增大尺寸时如何维持韧皮部运输这一长期谜团的关键在于韧皮部的结构及其随植物高度改变水力特性的能力。
