Phloem Transport and Assimilate Allocation in Plants | A-Level 生物：韧皮部运输与有机物分配

📚 Phloem Transport and Assimilate Allocation in Plants | A-Level 生物：韧皮部运输与有机物分配

In flowering plants, the phloem is the living tissue responsible for the long-distance transport of organic nutrients — chiefly sucrose — from regions of production to regions of utilisation or storage. This process, known as translocation, is fundamental to plant growth, development and crop yield.

在开花植物中，韧皮部是负责远距离运输有机养分（主要是蔗糖）的活组织，其运输方向是从生产区域到利用或储存区域。这一过程称为转运（translocation），是植物生长、发育和作物产量的基础。

1. Structure of Phloem Tissue | 韧皮部组织的结构

Phloem tissue is composed of several cell types, the most important being sieve tube elements and companion cells. Sieve tube elements are elongated living cells joined end-to-end to form continuous columns called sieve tubes. Their end walls are perforated by numerous pores, forming sieve plates that allow cytoplasmic continuity between adjacent elements.

韧皮部组织由多种细胞组成，其中最重要的是筛管分子和伴胞。筛管分子是细长的活细胞，首尾相连形成连续的柱状结构，称为筛管。其端壁上有许多穿孔，形成筛板，允许相邻分子之间的细胞质连通。

Mature sieve tube elements lack a nucleus, ribosomes and most organelles, making them dependent on companion cells for metabolic support. Companion cells are connected to sieve tube elements by numerous plasmodesmata and are rich in mitochondria and ribosomes, reflecting their high metabolic activity.

成熟的筛管分子缺乏细胞核、核糖体和大多数细胞器，因此依赖伴胞提供代谢支持。伴胞通过大量胞间连丝与筛管分子相连，并富含线粒体和核糖体，反映出其高代谢活性。

Sieve tubes — conducting cells for sugars and amino acids | 筛管 — 运输糖类和氨基酸的传导细胞
Companion cells — supply ATP and proteins to sieve tubes | 伴胞 — 为筛管提供ATP和蛋白质
Sieve plates — porous end walls enabling mass flow | 筛板 — 具孔端壁，实现集体流动

2. Sources and Sinks | 源与库

A source is any plant organ that produces or releases assimilates for export, such as mature leaves carrying out photosynthesis. A sink is any organ that imports or consumes assimilates, such as growing roots, developing fruits, seeds, storage organs and young expanding leaves.

源（source）是任何产生或释放同化物以供输出的植物器官，例如进行光合作用的成熟叶片。库（sink）是任何输入或消耗同化物的器官，例如生长中的根、发育中的果实、种子、储藏器官和幼嫩展开的叶片。

The direction of phloem transport is always from source to sink. This direction can change during a plant’s life cycle; for example, a storage organ such as a potato tuber acts as a sink during tuber formation but becomes a source when sprouting occurs in spring.

韧皮部运输的方向始终是从源到库。这一方向在植物生命周期中会发生变化；例如，马铃薯块茎在块茎形成时充当库，但在春季发芽时变为源。

Source → Phloem → Sink

源 → 韧皮部 → 库

3. The Pressure Flow Hypothesis | 压力流动假说

The most widely accepted mechanism for phloem transport is the pressure flow hypothesis, first proposed by Ernst Münch in 1930. This hypothesis states that assimilates move from source to sink along a hydrostatic pressure gradient generated by osmosis.

韧皮部运输最广泛接受的机制是压力流动假说，由Ernst Münch于1930年首次提出。该假说认为，同化物沿着由渗透作用产生的静水压力梯度从源向库移动。

At the source, sucrose is actively loaded into sieve tubes, decreasing the water potential. Water enters the sieve tubes by osmosis from adjacent xylem, increasing the pressure inside. At the sink, sucrose is unloaded and metabolised or stored, causing water to leave the sieve tubes, decreasing the pressure. This pressure difference drives bulk flow of solution through the sieve tubes.

在源端，蔗糖被主动装载到筛管中，降低水势。水分通过渗透作用从邻近的木质部进入筛管，使内部压力增加。在库端，蔗糖被卸载并被代谢或储存，导致水分离开筛管，压力降低。这一压力差驱动溶液在筛管中集体流动。

Source: active sucrose loading → low water potential → water influx → high pressure

Sink: sucrose unloading → water efflux → low pressure

源端：蔗糖主动装载 → 水势降低 → 水分流入 → 高压

库端：蔗糖卸载 → 水分流出 → 低压

4. Loading of Sucrose into Sieve Tubes | 蔗糖向筛管的装载

Sucrose concentration in sieve tubes is often two to three times higher than in mesophyll cells, indicating that loading occurs against a concentration gradient and therefore requires energy. This is achieved through an apoplastic loading pathway involving active transport.

筛管中的蔗糖浓度通常是叶肉细胞的2至3倍，表明装载是逆浓度梯度进行的，因此需要能量。这是通过涉及主动运输的质外体装载途径实现的。

In the apoplastic pathway, sucrose is secreted into the cell wall space by mesophyll cells, then actively transported into companion cells by a sucrose-H⁺ co-transporter. This transporter harnesses the energy from the proton gradient established by H⁺-ATPase pumps on the plasma membrane.

在质外体途径中，蔗糖由叶肉细胞分泌到细胞壁间隙，然后通过蔗糖-H⁺共转运体被主动运入伴胞。该转运体利用质膜上H⁺-ATPase泵建立的质子梯度所储存的能量。

Sucrose is produced in mesophyll cells during photosynthesis | 蔗糖在叶肉细胞光合作用中产生
Sucrose diffuses through plasmodesmata (symplastic) or is secreted into apoplast | 蔗糖经胞间连丝扩散（共质体途径）或分泌到质外体
H⁺-ATPase pumps H⁺ out of companion cells, creating an electrochemical gradient | H⁺-ATPase将H⁺泵出伴胞，建立电化学梯度
Sucrose-H⁺ co-transporter moves sucrose into companion cells against its gradient | 蔗糖-H⁺共转运体逆梯度将蔗糖运入伴胞
Sucrose diffuses from companion cells into sieve tubes through plasmodesmata | 蔗糖经胞间连丝从伴胞扩散进入筛管

5. Unloading at Sink Tissues | 库组织中的卸载

At sink tissues, sucrose is unloaded from sieve tubes by one of several mechanisms depending on the type of sink. In storage sinks such as sugar beet roots, sucrose may be transported into cells against a concentration gradient, requiring active transport. In growth sinks such as root tips, sucrose may simply diffuse down its concentration gradient into surrounding cells.

在库组织中，蔗糖根据库的类型通过不同机制从筛管卸载。在甜菜根等储藏库中，蔗糖可能逆浓度梯度被运入细胞，需要主动运输。在根尖等生长库中，蔗糖可能仅顺着浓度梯度扩散到周围细胞中。

Once inside sink cells, sucrose is rapidly converted into other compounds. It may be hydrolysed to glucose and fructose by invertase, or broken down to provide carbon skeletons for biosynthesis. This rapid conversion maintains a low sucrose concentration in sink cells, ensuring a continuous concentration gradient that favours unloading.

进入库细胞后，蔗糖被迅速转化为其他化合物。它可能被转化酶水解为葡萄糖和果糖，或被分解以为生物合成提供碳骨架。这种快速转化使库细胞中蔗糖浓度保持较低，确保持续的浓度梯度有利于卸载。

In storage organs such as potato tubers, sucrose is converted to starch — an osmotically inactive polymer. This removal of soluble sugars prevents water from accumulating in the storage cells, which would otherwise disrupt the pressure gradient essential for mass flow.

在马铃薯块茎等储藏器官中，蔗糖被转化为淀粉——一种无渗透活性的聚合物。这种可溶性糖的移除防止水分在储藏细胞中积累，否则会破坏集体流动所必需的压力梯度。

6. 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 stylet experiments provide direct evidence: when the body of an aphid feeding on phloem sap is cut away, the severed stylet continues to exude phloem sap for hours or even days, demonstrating that sieve tubes are under positive pressure.

多项实验证据支持压力流动假说。蚜虫口针实验提供了直接证据：当取食韧皮部汁液的蚜虫身体被切掉时，被切断的口针会持续数小时甚至数天渗出韧皮部汁液，证明筛管处于正压状态。

Analysis of the exuded sap reveals a high sucrose concentration (often 10–30% w/v), a pH of approximately 8.0, and a positive hydrostatic pressure of 0.5–1.0 MPa. Furthermore, the direction of flow in the phloem is always from source to sink, consistent with along a pressure gradient.

对渗出汁液的分析显示，蔗糖浓度高（通常为10–30% w/v），pH约为8.0，静水压力为0.5–1.0 MPa（正值）。此外，韧皮部中流动的方向始终是从源到库，与沿压力梯度的流动一致。

Additional evidence comes from ringing experiments and the use of radioactively labelled carbon (¹⁴CO₂). When a ring of bark (containing phloem) is removed from a stem, sugars accumulate above the ring and the tissues below eventually die — confirming that phloem is the pathway for sugar transport.

环剥实验和使用放射性标记碳（¹⁴CO₂）提供了补充证据。当从茎上剥去一圈树皮（含韧皮部）时，糖类在环剥上方积累，环剥下方的组织最终死亡——证实韧皮部是糖运输的通道。

7. Limitations and Criticisms | 局限性与质疑

Despite strong support, the pressure flow hypothesis faces certain criticisms. One issue is the resistance of sieve plates: the pores are lined with callose and may be blocked by P-proteins, which could create significant resistance to flow. Calculations suggest that the pressure gradients observed may not always be sufficient to overcome this resistance.

尽管有强有力的支持，压力流动假说仍面临一些质疑。一个问题是筛板的阻力：筛孔内壁有胼胝质衬里，并可能被P-蛋白堵塞，这可能产生显著的流动阻力。计算表明，观察到的压力梯度可能并不总能克服这种阻力。

Another criticism relates to the speed of translocation. Measured rates of phloem transport range from 0.2 to 1.5 m h⁻¹, which is faster than can be accounted for by simple diffusion but consistent with bulk flow. Some researchers argue that cytoplasmic streaming within sieve elements may also contribute to transport, though this remains a minor contributor at best.

另一项质疑涉及转运速度。测得的韧皮部运输速率为0.2至1.5 m h⁻¹，这比简单扩散所能解释的速度快，但与集体流动一致。一些研究者认为筛管分子内的胞质环流也可能有助于运输，尽管这最多只是一个次要贡献因素。

Some plant species load sucrose symplastically rather than apoplastically, which means the pressure gradient may be generated differently in these plants. This observation highlights that a single universal mechanism may not apply to all species and that both apoplastic and symplastic loading occur in nature.

一些植物物种通过共质体途径而非质外体途径装载蔗糖，这意味着这些植物中压力梯度的产生方式可能不同。这一观察强调，单一普适机制可能不适用于所有物种，自然界中质外体装载和共质体装载同时存在。

8. Assimilate Allocation and Partitioning | 同化物的分配与划分

Assimilate allocation refers to the distribution of photosynthates among different organs, while partitioning refers to the differential distribution among competing sinks. Sinks compete for assimilates, and the outcome depends on several factors including sink strength, proximity to the source, and developmental stage.

同化物分配（allocation）是指光合产物在不同器官间的分布，而划分（partitioning）是指同化物在相互竞争的库之间的差异性分配。库之间竞争同化物，结果取决于多个因素，包括库强度、与源的接近程度以及发育阶段。

Sink strength is defined as the product of sink size and sink activity. Sink size refers to the total mass of the sink organ, while sink activity refers to the rate of assimilate uptake per unit mass. A rapidly growing fruit with high metabolic activity has a high sink strength and therefore attracts more assimilates.

库强度（sink strength）定义为库大小与库活性的乘积。库大小指库器官的总质量，库活性指单位质量的同化物吸收速率。快速生长的果实代谢活性高，库强度大，因此能吸引更多的同化物。

Sink Strength | 库强度

= Sink Size × Sink Activity | = 库大小 × 库活性

Preferential allocation occurs: developing fruits and seeds are the strongest sinks in most flowering plants, followed by shoot apices, root apices and storage organs. This hierarchy explains why a heavy fruit load can reduce vegetative growth — the vegetative sinks receive fewer assimilates when reproductive sinks dominate.

分配存在优先性：在大多数开花植物中，发育中的果实和种子是最强的库，其次是茎尖、根尖和储藏器官。这种层级关系解释了为什么大量结果会减少营养生长——当生殖库占主导时，营养库获得的同化物减少。

9. Factors Affecting Phloem Transport Rate | 影响韧皮部运输速率的因素

The rate of translocation is influenced by both source and sink factors. At the source, photosynthetic rate determines the availability of sucrose: factors such as light intensity, CO₂ concentration and temperature all affect photosynthesis and therefore assimilate supply.

转运速率受源因素和库因素共同影响。在源端，光合速率决定蔗糖的可利用性：光照强度、CO₂浓度和温度等因素都影响光合作用，从而影响同化物供应。

At the sink, metabolic demand is the key driver. High respiratory activity, rapid cell division or active storage all increase sugar utilisation, maintaining a steeper concentration gradient and faster transport. Temperature also affects translocation directly by influencing enzyme activity, membrane fluidity and the rate of active transport.

在库端，代谢需求是关键驱动因素。高呼吸活性、快速细胞分裂或活跃的储藏都会增加糖的利用，维持更陡的浓度梯度和更快的运输速度。温度也通过影响酶活性、膜流动性和主动运输速率直接影响转运。

Selective transport is demonstrated by the fact that some substances are translocated preferentially. For example, sucrose is transported rather than glucose or fructose, and certain amino acids and hormones show selective transport patterns. This selectivity indicates the involvement of specific carrier proteins in loading and unloading.

选择性运输可从某些物质被优先转运这一事实得到证明。例如，运输的是蔗糖而非葡萄糖或果糖，某些氨基酸和激素也表现出选择性运输模式。这种选择性表明装载和卸载过程涉及特异性载体蛋白。

10. Agricultural and Horticultural Applications | 在农业和园艺中的应用

Understanding phloem transport has practical applications in agriculture and horticulture. Ringing (girdling) is used commercially to increase fruit size: by removing a ring of bark from a branch, assimilates from leaves are trapped above the ring and directed into developing fruits rather than being transported to roots.

理解韧皮部运输在农业和园艺中有实际应用。环剥（girdling）被商业上用于增大果实：通过从枝条上剥去一圈树皮，叶片中的同化物被阻隔在环剥上方并定向进入发育中的果实，而不是被运输到根部。

Grafting requires careful alignment of phloem tissues between the scion and rootstock so that assimilates can move freely across the graft union. Poor phloem continuity at the graft junction leads to sugar accumulation above the graft and eventual death of the rootstock.

嫁接要求接穗和砧木之间的韧皮部组织精确对齐，以便同化物能自由穿过嫁接愈合处。嫁接接口韧皮部连通性差会导致糖在嫁接处上方积累，最终导致砧木死亡。

Translocation studies also inform weed control strategies. Systemic herbicides are designed to be transported through the phloem from leaves (where they are applied) to roots and other underground organs, ensuring that the entire plant is killed rather than just the sprayed foliage.

转运研究也为除草策略提供依据。内吸性除草剂被设计为通过韧皮部从施药的叶片运输到根和其他地下器官，确保整株植物被杀死，而不仅仅是喷洒过的叶片。

Understanding source-sink relationships can also guide crop management decisions such as defoliation timing, fruit thinning and the application of growth regulators to optimise yield and quality.

理解源-库关系还可指导作物管理决策，如去叶时机、疏果和生长调节剂的应用，以优化产量和品质。

11. Key Exam Points | 考试要点总结

For CIE A-Level examinations, students should be able to describe the structure of phloem tissue, explain the pressure flow mechanism step by step, and evaluate the evidence for and against this hypothesis.

对于CIE A-Level考试，学生应能够描述韧皮部组织的结构，逐步解释压力流动机制，并评价支持与反对该假说的证据。

Know the differences between sieve tubes and companion cells | 掌握筛管与伴胞的区别
Be able to define source and sink with examples | 能够用实例定义源和库
Explain the role of H⁺-ATPase and sucrose-H⁺ co-transporters in phloem loading | 解释H⁺-ATPase和蔗糖-H⁺共转运体在韧皮部装载中的作用
Understand why sucrose is the main transport sugar (non-reducing, less metabolically active, water-soluble) | 理解为什么蔗糖是主要运输糖（非还原性、代谢活性低、水溶性好）
Use aphid stylet and ringing experiments as evidence | 使用蚜虫口针和环剥实验作为证据
Apply source-sink concepts to agricultural scenarios | 将源-库概念应用于农业情境

Sucrose is non-reducing, so it does not react with other cellular components during transport, and it is less readily metabolised than glucose, making it ideal for long-distance transport.

蔗糖是非还原性糖，因此运输过程中不会与其他细胞成分反应，且比葡萄糖更不易被代谢，是远距离运输的理想物质。

12. Common Misconceptions | 常见误区

Students often confuse phloem transport with xylem transport. Xylem transports water and mineral ions upwards from roots, driven by transpiration pull and root pressure, and consists of dead, lignified cells. Phloem transports organic assimilates in all directions from source to sink, driven by pressure gradients, and consists of living cells.

学生常将韧皮部运输与木质部运输混淆。木质部运输水和矿质离子，从根部向上，由蒸腾拉力和根压驱动，由死的木质化细胞组成。韧皮部运输有机同化物，从源向库在各个方向运输，由压力梯度驱动，由活细胞组成。

Another common error is stating that phloem transport is “up and down” without reference to source-sink relationships. Transport direction is determined not by gravity but by the relative positions of sources and sinks in the plant body.

另一个常见错误是笼统地说韧皮部运输”向上和向下”，而没有提及源-库关系。运输方向不是由重力决定的，而是由植物体中源和库的相对位置决定的。

Finally, students should remember that translocation requires energy not for bulk flow itself but for the active loading and unloading of sucrose. The movement of solution through sieve tubes is driven passively by the pressure gradient once loading has established it.

最后，学生应记住转运需要能量不是因为集体流动本身，而是因为蔗糖的主动装载和卸载。一旦装载建立了压力梯度，溶液通过筛管的移动是被动驱动的。

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Phloem Transport and Assimilate Allocation in Plants | A-Level 生物：韧皮部运输与有机物分配