Department of Horticulture, Cornell University, Ithaca, New York 14853, USA.
Plant Physiol. 2011 Nov;157(3):1518-27. doi: 10.1104/pp.111.184820. Epub 2011 Aug 26.
Most herbaceous plants employ thermodynamically active mechanisms of phloem loading, whereas in many trees, the mechanism is passive, by diffusion. Considering the different water transport characteristics of herbs and trees, we hypothesized that water relations play a role in the adoption of phloem loading strategies. We measured whole-plant hydraulic conductance (K(p)), osmolality, concentrations of polar metabolites, and key inorganic ions in recently mature leaves of 45 dicotyledonous species at midafternoon. Trees, and the few herbs that load passively, have low K(p), high osmolality, and high concentrations of transport sugars and total polar metabolites. In contrast, herbs that actively load sucrose alone have high K(p), low osmolality, and low concentrations of sugars and total polar metabolites. Solute levels are higher in sugar alcohol-transporting species, both herbs and trees, allowing them to operate at lower leaf water potentials. Polar metabolites are largely responsible for leaf osmolality above a baseline level (approximately 300 mm) contributed by ions. The results suggest that trees must offset low K(p) with high concentrations of foliar transport sugars, providing the motivating force for sugar diffusion and rendering active phloem loading unnecessary. In contrast, the high K(p) of most herbaceous plants allows them to lower sugar concentrations in leaves. This reduces inventory costs and significantly increases growth potential but necessitates active phloem loading. Viewed from this perspective, the elevation of hydraulic conductance marks a major milestone in the evolution of the herbaceous habit, not only by facilitating water transport but also by maximizing carbon use efficiency and growth.
大多数草本植物采用热力学活跃的韧皮部装载机制，而在许多树木中，机制是被动的，通过扩散。考虑到草本植物和树木不同的水分运输特性，我们假设水分关系在韧皮部装载策略的采用中起作用。我们在午后测量了 45 种双子叶植物最近成熟叶片的整株植物水力传导率（K(p)）、渗透压、极性代谢物浓度和关键无机离子。树木和少数被动装载的草本植物具有低 K(p)、高渗透压和高运输糖和总极性代谢物浓度。相比之下，单独主动装载蔗糖的草本植物具有高 K(p)、低渗透压和低糖和总极性代谢物浓度。在糖醇运输物种中，无论是草本植物还是树木，溶质水平都较高，这使它们能够在较低的叶片水势下运作。极性代谢物在离子贡献的基础渗透压（约 300mm）之上主要负责叶片渗透压。结果表明，树木必须通过高浓度的叶片运输糖来抵消低 K(p)，为糖扩散提供动力，并使主动韧皮部装载变得不必要。相比之下，大多数草本植物的高 K(p)允许它们降低叶片中的糖浓度。这降低了库存成本，显著增加了生长潜力，但需要主动韧皮部装载。从这个角度来看，水力传导率的提高标志着草本习性进化的一个重要里程碑，不仅通过促进水分运输，而且通过最大化碳利用效率和生长来实现。
