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.

中文翻译：

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维持高大树木中碳水化合物的运输。

树木对长距离运输提出了严峻的挑战，因为随着树木的高度增长和运输路径长度的增加，维管组织的水力阻力将增加。这使许多人怀疑树木是否可以依靠被动运输机制将碳水化合物从叶子移到根部。尽管主动将糖装载到韧皮部中的树种（例如藤本和草本植物）在伸长时会增加运输的驱动力，但是许多树木可能不会产生高膨胀压力，因为它们没有使用转运蛋白将糖装载到韧皮部中。在这里，我们使用最近开发的制备和成像技术，通过分析筛管解剖结构（包括筛板的几何形状）来研究树木长高时如何保持有效的碳水化合物运输，并通过现场测量一棵高大的树木的叶子中的膨胀压力。在九个落叶树种中，我们发现韧皮部的水力阻力与植物高度成反比，这是因为筛分元素结构沿单个树木的长度方向发生了变化。尽管在板解剖学上有很大差异，但这种缩放关系似乎在多个物种之间都很稳健。当使用在成熟的赤栎树的叶子中测得的膨胀压力对韧皮部运输进行建模时，这种缩放的重要性变得显而易见。这些压力足以驱动韧皮部运输，仅与韧皮部中降低运输阻力的结构变化协同作用。因此，