📚 Translocation in Phloem | 韧皮部中的物质运输
Translocation is the process by which organic solutes, primarily sucrose, are transported from sources to sinks through the phloem tissue in vascular plants. This active, energy-dependent process ensures that every part of the plant receives the carbohydrates needed for respiration, growth and storage. Unlike transpiration in the xylem, phloem transport operates via a pressure-driven mass flow that can move materials both upwards and downwards. A clear understanding of translocation is essential for explaining how plants distribute the products of photosynthesis, respond to seasonal changes and maintain metabolic balance across different organs.
转运是有机溶质（主要是蔗糖）通过维管植物韧皮部组织从源运输到库的过程。这种主动的、依赖能量的过程确保植物的每个部分都能获得呼吸、生长和储存所需的碳水化合物。与木质部的蒸腾作用不同，韧皮部运输通过压力驱动的集流工作机制进行，物质既可以向上也可以向下移动。清楚地理解转运对于解释植物如何分配光合作用产物、响应季节变化以及维持不同器官间的代谢平衡至关重要。
1. Defining Translocation | 定义转运
Translocation refers specifically to the movement of assimilates — mainly sucrose, amino acids and certain mineral ions — through the sieve tubes of the phloem. It is a selective, active process that links regions of net production (sources) to regions of net consumption or storage (sinks). In A-Level Biology, translocation is contrasted with transpiration, which is the passive, unidirectional movement of water and inorganic ions in the xylem driven by evaporation.
转运特指同化物（主要是蔗糖、氨基酸和某些矿物离子）通过韧皮部筛管的移动。这是一个选择性的、主动的过程，将净生产区域（源）与净消耗或储存区域（库）连接起来。在 A-Level 生物课程中，转运常与蒸腾作用进行对比，后者是木质部中水和无机离子在蒸发驱动下进行的被动单向流动。
Key assimilates transported in the phloem include sucrose, which is the main carbohydrate form, amino acids for protein synthesis, and sometimes plant hormones and RNA molecules. The phloem sap can flow at velocities of about 0.5–1.5 m h⁻¹, which is much faster than simple diffusion could achieve over long distances.
韧皮部中运输的关键同化物包括作为主要碳水化合物形式的蔗糖、用于蛋白质合成的氨基酸，有时还有植物激素和 RNA 分子。韧皮部汁液的流速约为 0.5–1.5 m h⁻¹，远快于简单扩散在长距离内能够达到的速度。
2. Source and Sink Concept | 源与库的概念
The source–sink model is fundamental to understanding translocation. A source is any plant organ that produces more assimilates than it consumes. Mature, photosynthesising leaves are typical sources in the growing season. A sink is any organ that consumes or stores more assimilates than it produces. Examples include developing roots, shoot apical meristems, flowers, fruits and storage organs like potato tubers.
源–库模型是理解转运的基础。源是指产生多于消耗的同化物的任何植物器官。成熟的、进行光合作用的叶片是生长季节的典型源。库是指消耗或储存多于生产的同化物的任何器官，例如正在发育的根、茎端分生组织、花、果实以及马铃薯块茎等储存器官。
The identity of a source or sink is not fixed; it can change with the plant's development and the season. For instance, a young developing leaf acts as a sink, importing sucrose to support growth, but once it matures and begins photosynthesis, it becomes a source. Similarly, a potato tuber is a sink during summer but acts as a source in spring when its stored starch is mobilised for sprouting. This dynamic allocation reflects the plasticity of phloem transport.
源或库的身份不是固定的，它可以随着植物发育和季节而改变。例如，一片幼嫩的发育叶最初是库，输入蔗糖来支持生长，但一旦成熟并开始光合作用，它就变成了源。同样，马铃薯块茎在夏季是库，但在春季其储存的淀粉被动员用于发芽时，它就成了源。这种动态分配反映了韧皮部运输的可塑性。
3. Phloem Tissue Structure | 韧皮部组织结构
The phloem is a complex vascular tissue composed of several cell types, the most important for translocation being sieve tube elements and companion cells. Sieve tube elements are elongated cells arranged end-to-end to form sieve tubes. They are living but have lost their nuclei, ribosomes and a large central vacuole, allowing an unobstructed flow of sap. The end walls between adjacent sieve tube elements are modified into sieve plates, which are perforated by large pores.
韧皮部是一个复合维管组织，由几种细胞类型组成，对转运最重要的是筛管分子和伴胞。筛管分子是长形的细胞，首尾相连形成筛管。它们是活细胞，但失去了细胞核、核糖体和大的中央液泡，从而允许汁液无障碍地流动。相邻筛管分子之间的端壁特化为筛板，上面具有大孔的穿孔。
Companion cells are closely associated with each sieve tube element, connected by numerous plasmodesmata. They possess a dense cytoplasm, a large nucleus and many mitochondria, reflecting their high metabolic activity. Companion cells provide the energy and molecular machinery needed to load assimilates into the sieve tubes and maintain the living state of the enucleate sieve tube elements.
伴胞与每个筛管分子紧密相连，通过大量的胞间连丝连接。它们拥有浓密的细胞质、大的细胞核和大量的线粒体，反映出它们高度的代谢活性。伴胞为装载同化物进入筛管以及维持无核筛管分子的生命状态提供能量和分子机制。
4. Phloem Loading at the Source | 源端的韧皮部装载
Phloem loading is the process by which sucrose and other solutes enter the sieve tube–companion cell complex at the source. In most plants, loading is an active process that moves solutes against a concentration gradient. Sucrose synthesised in the mesophyll cells diffuses or is actively transported into the companion cells, often via plasmodesmata or transporter proteins. The exact mechanism can be apoplastic or symplastic depending on the species.
韧皮部装载是指蔗糖和其他溶质在源端进入筛管–伴胞复合物的过程。在大多数植物中，装载是一个逆浓度梯度的主动过程。在叶肉细胞中合成的蔗糖通过扩散或主动运输进入伴胞，通常经过胞间连丝或转运蛋白。根据物种不同，确切机制可以是质外体途径或共质体途径。
In the apoplastic pathway, sucrose is released into the cell wall space and then actively transported into companion cells by H⁺-sucrose co-transporters. The proton gradient is maintained by ATP-powered proton pumps. In the symplastic pathway, sucrose moves from mesophyll to companion cells entirely through plasmodesmata, entering the sieve tube without crossing a membrane. Both pathways raise the solute concentration inside the phloem, generating a low water potential.
在质外体途径中，蔗糖被释放到细胞间隙，然后通过 H⁺-蔗糖共转运体主动运输进入伴胞。质子梯度由 ATP 驱动的质子泵维持。在共质体途径中，蔗糖从叶肉细胞到伴胞整个过程都经过胞间连丝，无需跨膜即可进入筛管。两种途径都使韧皮部内部的溶质浓度升高，从而产生低水势。
5. The Pressure Flow Hypothesis | 压力流动假说
The pressure flow hypothesis, also known as the Münch hypothesis, is the widely accepted model explaining how solutes move through the phloem. According to this model, the active loading of sucrose at the source decreases the water potential inside the sieve tube. Water then enters from the adjacent xylem by osmosis, causing the hydrostatic pressure to rise. At the sink, sucrose is unloaded, raising the water potential and causing water to leave the phloem, reducing the pressure. The resulting pressure gradient drives a bulk flow of phloem sap from source to sink.
压力流动假说，也称为明希假说，是广泛接受的解释溶质如何通过韧皮部运动的模型。根据该模型，在源端主动装载蔗糖降低了筛管内的水势。随后，水通过渗透作用从相邻的木质部进入，使静水压力升高。在库端，蔗糖被卸载，水势升高，水离开韧皮部，压力降低。由此产生的压力梯度驱动韧皮部汁液从源到库的集流。
The flow is passive in the sense that once the pressure difference is established, sap moves without the need for direct pump action along the sieve tube. However, the entire system depends on active transport at loading and unloading sites, making translocation an overall active process. The hypothesis elegantly explains bidirectional movement because different sieve tubes can connect to different sources and sinks, each operating with its own pressure gradient.
集流是被动的，意思是压力差一旦建立，汁液无需沿着筛管直接泵动即可移动。然而，整个系统依赖于装载和卸载位点的主动运输，使得转运成为一个整体上主动的过程。该假说很好地解释了双向运输，因为不同的筛管可以连接不同的源和库，每一个均在其自身的压力梯度下运作。
6. Water Relations and Pressure Generation | 水分关系与压力产生
The osmotic movement of water between xylem and phloem is crucial. At the source, solute concentration in the phloem may reach 10–30% w/v, generating a very negative solute potential Ψₛ. Since the phloem cell walls are relatively inelastic, the influx of water sharply increases the pressure potential Ψₚ. At the sink, unloading reduces the solute concentration, so water exits the sieve tube, returning to the xylem. This circulation of water is sometimes called the phloem–xylem water cycle and helps maintain the pressure gradient.
木质部与韧皮部之间水分的渗透移动至关重要。在源端，韧皮部溶质浓度可达到 10–30% w/v，产生非常负的溶质势 Ψₛ。由于韧皮部细胞壁相对无弹性，水的流入急剧增加了压力势 Ψₚ。在库端，卸载降低了溶质浓度，因此水离开筛管返回木质部。这种水的循环有时称为韧皮部–木质部水循环，有助于维持压力梯度。
owers Ψₛ, causing Ψ to become negative relative to the xylem. Water moves from high Ψ to low Ψ, increasing Ψₚ until the total water potential inside the sieve tube is in equilibrium with the xylem. The pressure gradient ΔP between source and sink can be several megapascals, enough to drive flow through the sieve pores.
水势的简化表达式为 Ψ = Ψₛ + Ψₚ。韧皮部装载降低了 Ψₛ，使得 Ψ 相对于木质部变得更负。水从高水势向低水势移动，增加 Ψₚ，直至筛管内总水势与木质部达到平衡。源和库之间的压力梯度 ΔP 可达数兆帕，足以驱动流过筛孔。
7. Phloem Unloading at the Sink | 库端的韧皮部卸载
Phloem unloading is the exit of assimilates from the sieve tubes into sink tissues. Unloading can be symplastic, where solutes move through plasmodesmata into receiving cells, or apoplastic, where they cross cell membranes via transporters. In many sinks, such as developing roots, sucrose is actively transported into sink cells, which maintains a low solute concentration in the sieve tube at the sink end, sustaining the pressure gradient.
韧皮部卸载是指同化物从筛管进入库组织的过程。卸载可以是通过共质体途径，溶质经胞间连丝进入受体细胞；或者是质外体途径，溶质经转运体跨膜。在许多库中，例如发育中的根，蔗糖被主动运输进入库细胞，这维持了筛管库端较低的溶质浓度，从而维持了压力梯度。
Once inside the sink cell, sucrose is rapidly metabolised — either converted to glucose and fructose by invertase for respiration, or polymerised into starch for storage. This rapid metabolism prevents the accumulation of sucrose and keeps the concentration gradient favourable for continued unloading. In some fruits and storage organs, specific transporters mediate the uptake of sucrose against large concentration differences.
一旦进入库细胞，蔗糖被迅速代谢——或者被转化酶分解为葡萄糖和果糖用于呼吸，或者聚合成淀粉用于储存。这种快速的代谢防止了蔗糖积累，并使浓度梯度保持有利于继续卸载的状态。在一些果实和储存器官中，特定的转运体介导了逆大浓度差的蔗糖吸收。
8. Evidence Supporting the Pressure Flow Hypothesis | 支持压力流动假说的证据
Several lines of experimental evidence support the pressure flow hypothesis. Aphid stylets have been used to sample phloem sap: when an aphid's stylet is severed, sap continues to flow out, confirming positive pressure inside the sieve tube. The composition of this exudate shows high sucrose concentration at sources and lower concentrations at sinks, aligning with the model's predictions.
多种实验证据支持压力流动假说。蚜虫口针用于采集韧皮部汁液样品：当切断蚜虫的口针时，汁液继续流出，证实了筛管内的正压力。这种渗出液的组成显示，源端蔗糖浓度高，而库端浓度较低，这与模型的预测一致。
Radioactive tracer studies using ¹⁴CO₂ applied to source leaves show movement of ¹⁴C-labelled sucrose through the phloem at rates consistent with mass flow. Furthermore, girdling experiments, where the phloem is removed from a ring of bark, lead to accumulation of sugars above the girdle and starvation below, demonstrating that the phloem is essential for downward translocation. These findings collectively reinforce the Münch model.
使用 ¹⁴CO₂ 施用于源叶的放射性示踪研究显示，¹⁴C 标记的蔗糖以与集流一致的速度通过韧皮部移动。此外，环剥实验（将一圈树皮中的韧皮部去除）导致环剥处上方糖分积累而下方饥饿，证明韧皮部对向下转运至关重要。这些发现共同强化了明希模型。
9. Rates and Regulation of Translocation | 转运的速率与调控
The velocity of phloem transport ranges from 0.2 to 1.5 metres per hour in most herbaceous plants. Rates are influenced by the magnitude of the pressure gradient, the resistance of the sieve pores, and the demand from sink tissues. Plants can regulate translocation by modulating plasmodesmatal aperture, the activity of sucrose transporters, and the conversion of sucrose to starch at sinks. Hormones like auxin may also influence sink strength by promoting cell division and growth.
在大多数草本植物中，韧皮部运输的速度范围为每小时 0.2 到 1.5 米。速率受压力梯度大小、筛孔阻力以及库组织需求的影响。植物可以通过调节胞间连丝孔径、蔗糖转运体活性以及库端蔗糖向淀粉的转化来调控转运。像生长素这样的激素也可能通过促进细胞分裂和生长来影响库强度。
Environmental factors such as light, temperature and water availability affect translocation indirectly by altering photosynthesis rates and sink metabolism. For example, on a warm sunny day, source leaves produce abundant sucrose, increasing the phloem loading rate. At night, translocation slows but can be sustained by the mobilisation of stored starch. This finely tuned regulation prevents the excessive build-up of sugars in leaves and ensures a steady supply to heterotrophic organs.
光照、温度和水分等环境因子通过改变光合速率和库代谢间接影响转运。例如，在温暖晴朗的日子里，源叶产生丰富的蔗糖，提高韧皮部装载速率。夜间，转运减慢，但可通过储存淀粉的动员维持。这种精细的调节防止糖分在叶片中过度积累，并确保向异养器官稳定供应。
10. Comparison with Xylem Transport | 与木质部运输的比较
Translocation in the phloem and transpiration in the xylem are fundamentally different processes, though they are physically adjacent and interconnected. A comparison helps consolidate understanding:
韧皮部的转运与木质部的蒸腾作用是根本不同的过程，尽管它们在结构上相邻且相互连通。比较有助于巩固理解：
| Feature | 特征 | Xylem | 木质部 | Phloem | 韧皮部 |
|---|---|---|
| Main substance transported | 运输主要物质 | Water and mineral ions | 水和矿物离子 | Sucrose, amino acids, hormones | 蔗糖、氨基酸、激素 |
| Direction | 方向 | Upwards (unidirectional) | 向上（单向） | Up and down (bidirectional) | 可上下（双向） |
| Driving force | 驱动力 | Transpiration pull + root pressure | 蒸腾拉力+根压 | Pressure gradient from osmosis | 渗透产生的压力梯度 |
| Energy requirement | 能量需求 | Largely passive | 大部分被动 | Active (loading/unloading) | 主动（装载/卸载） |
| Conducting cells | 传导细胞 | Tracheids and vessel elements (dead) | 管胞和导管分子（死细胞） | Sieve tube elements (living, enucleate) | 筛管分子（活细胞，无核） |
Despite these differences, the close spatial association between phloem and xylem in vascular bundles facilitates the efficient water exchange required for the pressure flow mechanism. This anatomical coupling is a key adaptation that enables large, complex plants to function as integrated physiological units.
尽管存在这些差异，韧皮部和木质部在维管束中的紧密空间关联促进了压力流动机制所需的高效水分交换。这种解剖耦合是一项关键适应，使大型复杂植物能够作为整合的生理单位运作。
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