A-Level AQA Biology: Plant Hormones Revision | A-Level AQA 生物：植物激素考点精讲

📚 A-Level AQA Biology: Plant Hormones Revision | A-Level AQA 生物：植物激素考点精讲

Plant hormones, also known as plant growth factors, are chemical messengers that coordinate growth and development in plants. Unlike animal hormones, they are not produced in discrete glands but in various tissues, and they often act locally rather than being transported over long distances. In AQA A-Level Biology, you must understand the roles of auxins, gibberellins, ethene and abscisic acid, as well as the mechanisms of tropisms, apical dominance, seed germination and commercial applications.

植物激素，也称为植物生长因子，是协调植物生长与发育的化学信使。与动物激素不同，它们并非由独立的腺体产生，而是产生于多种组织，且通常在局部起作用，不进行长距离运输。在AQA A-Level生物课程中，你必须掌握生长素、赤霉素、乙烯和脱落酸的作用，以及向性运动、顶端优势、种子萌发和商业应用的机制。

1. What Are Plant Hormones? | 什么是植物激素？

Plant hormones are organic compounds that regulate plant growth and development at very low concentrations. They are not secreted by specialised glands; instead, they are produced in regions like shoot tips, root tips and developing leaves. Key plant hormones include auxins (e.g. IAA), gibberellins, ethene and abscisic acid (ABA). Unlike animal hormones, plant growth factors can exert different effects in different tissues, and they often interact synergistically or antagonistically.

植物激素是在极低浓度下调控植物生长与发育的有机化合物。它们不由特化的腺体分泌，而是产生于茎尖、根尖和发育中的叶片等区域。主要的植物激素包括生长素（如 IAA）、赤霉素、乙烯和脱落酸（ABA）。与动物激素不同，植物生长因子可在不同组织中产生不同效应，且常以协同或拮抗的方式相互作用。

It is important to avoid calling them ‘plant hormones’ too loosely — AQA prefers the term ‘plant growth factors’ because they do not always travel in a circulatory system and may act very close to their site of synthesis. Nevertheless, the terms are often used interchangeably in exam contexts.

需要注意，AQA 考试更倾向于使用“植物生长因子”一词，因为它们并非始终通过循环系统运输，而可能在其合成位点附近就近作用。尽管如此，在考试语境中这两个术语常混用。

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2. Auxin (IAA) and Cell Elongation | 生长素 (IAA) 与细胞伸长

Indole-3-acetic acid (IAA) is the most important natural auxin in plants. It is synthesised in shoot and root apical meristems and young leaves. IAA promotes cell elongation by increasing the plasticity of the cell wall. According to the acid growth hypothesis, IAA stimulates proton pumps to move H⁺ ions into the cell wall space, lowering the pH and activating expansin proteins that loosen the cellulose microfibrils. Water then enters the cell by osmosis, causing the cell to elongate.

吲哚-3-乙酸 (IAA) 是植物中最重要的天然生长素，在茎尖、根尖分生组织和幼叶中合成。IAA 通过提高细胞壁的可塑性来促进细胞伸长。根据酸生长假说，IAA 刺激质子泵将 H⁺ 离子泵入细胞壁间隙，降低 pH 值，激活扩张蛋白，使纤维素微纤丝松散。随后水分通过渗透进入细胞，引起细胞伸长。

In roots, IAA at very low concentrations promotes growth, but higher concentrations actually inhibit cell elongation. This concentration-dependent effect is crucial for gravitropism (see Section 4).

在根中，极低浓度的 IAA 促进生长，但较高浓度反而抑制细胞伸长。这种浓度依赖性效应对于向地性至关重要（见第4节）。

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3. Phototropism | 向光性

Phototropism is the directional growth of a plant shoot towards light. When a shoot is exposed to unilateral light, IAA synthesised in the tip is redistributed to the shaded side. The higher IAA concentration on the shaded side stimulates faster cell elongation there, causing the shoot to bend towards the light. The photoreceptor phototropin detects blue light and triggers this lateral transport of auxin.

向光性是植物茎向光发生的定向生长。当茎受到单侧光照时，尖端合成的 IAA 会重新分布至背光侧。背光侧 IAA 浓度较高，刺激该侧细胞更快伸长，导致茎向光弯曲。光受体向光蛋白检测蓝光，触发这种生长素的侧向运输。

Classic experiments by Darwin and Went using grass coleoptiles demonstrated that the tip of the shoot produces a chemical messenger (now known as auxin) that moves to the zone of elongation. AQA often asks students to describe or interpret these classical experiments.

达尔文和温特利用禾本科胚芽鞘进行的经典实验证明，茎尖产生某种化学信使（现已知为生长素），可传递至伸长区。AQA 常要求考生描述或解读这些经典实验。

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4. Gravitropism | 向地性

Gravitropism is the directional growth of roots (positive gravitropism) and shoots (negative gravitropism) in response to gravity. When a root is placed horizontally, IAA is transported to the lower side under the influence of statoliths (starch grains that sink to the bottom of root cap cells). In roots, the high IAA concentration on the lower side inhibits cell elongation, while the upper side continues to elongate, causing the root to bend downwards.

向地性是根（正向地性）和茎（负向地性）对重力产生的定向生长。当根水平放置时，IAA 在平衡石（沉至根冠细胞底部的淀粉粒）的影响下被运输到下侧。在根中，下侧的高浓度 IAA 抑制细胞伸长，而上侧继续伸长，导致根向下弯曲。

In shoots, high IAA concentration promotes elongation, so the lower side of a horizontal shoot elongates more, causing it to bend upwards. Therefore, the differential effect of IAA on cell elongation in roots and shoots explains opposite tropic responses.

在茎中，高浓度 IAA 促进伸长，因此水平放置的茎下侧伸长更多，导致其向上弯曲。因此，IAA 对根和茎细胞伸长的差异效应解释了相反的向性反应。

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5. Apical Dominance | 顶端优势

Apical dominance is the phenomenon where the growing shoot tip (apical bud) suppresses the growth of lateral buds further down the stem. Auxin produced in the apical meristem travels down the stem and inhibits the outgrowth of side shoots. If the apical bud is removed (decapitation), lateral buds are released from inhibition and begin to grow, producing a bushier plant.

顶端优势是指旺盛生长的茎尖（顶芽）抑制其下方侧芽生长的现象。顶端分生组织产生的生长素沿茎向下运输，抑制侧枝的萌发。若摘除顶芽，侧芽便解除抑制并开始生长，使植株变得更加丛生。

The mechanism is thought to involve interaction with cytokinins (another group of plant growth substances) and strigolactones. AQA may ask you to explain how pinching out the tips of plants in horticulture encourages branching.

这一机制被认为涉及细胞分裂素（另一类植物生长物质）和独脚金内酯的相互作用。AQA 可能会要求你解释园艺中摘心如何促进分枝。

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6. Gibberellins and Seed Germination | 赤霉素与种子萌发

Gibberellins are a group of plant hormones that promote stem elongation, fruit growth and, most importantly for AQA, seed germination. In barley seeds, for example, the embryo secretes gibberellin upon absorbing water. Gibberellin diffuses to the aleurone layer, where it triggers the synthesis of α-amylase. This enzyme hydrolyses starch stored in the endosperm into maltose and glucose, providing respiratory substrate for the growing embryo.

赤霉素是一类植物激素，可促进茎的伸长、果实发育，对 AQA 考试而言至关重要的是促进种子萌发。以大麦种子为例，胚吸水后分泌赤霉素，赤霉素扩散至糊粉层，触发 α-淀粉酶的合成。该酶将胚乳中储存的淀粉水解为麦芽糖和葡萄糖，为生长的胚提供呼吸底物。

This pathway can be demonstrated experimentally using de-embryonated seeds, where the addition of gibberellin induces amylase activity even in the absence of the embryo. The gibberellin signal transduction pathway involves the degradation of DELLA repressor proteins, allowing gene transcription to proceed.

这一途径可通过去除胚的种子从实验上证实：即使没有胚，添加赤霉素也能诱导淀粉酶活性。赤霉素信号转导途径涉及 DELLA 阻遏蛋白的降解，从而启动基因转录。

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7. Ethene and Fruit Ripening | 乙烯与果实成熟

Ethene (C₂H₄) is a gaseous plant hormone involved in fruit ripening, leaf abscission and stress responses. It promotes the breakdown of cell walls, conversion of starch to sugar and changes in colour during ripening. Ethene is often used commercially to synchronise the ripening of fruits like bananas and tomatoes. A climacteric fruit (e.g. apple, banana) shows a burst of respiration and ethene production at the onset of ripening.

乙烯 (C₂H₄) 是一种气态植物激素，参与果实成熟、叶片脱落及应激反应。它促进细胞壁降解、淀粉向糖的转化以及成熟过程中的色泽变化。乙烯在商业上常被用来使香蕉、番茄等果实同步成熟。跃变型果实（如苹果、香蕉）在成熟开始时会出现呼吸速率和乙烯产量的骤增。

Interestingly, ethene is also produced in response to mechanical stress and can induce the ‘triple response’ in seedlings: reduced stem elongation, thickening of the stem and horizontal growth. This is an adaptive trait to overcome physical obstacles in the soil.

有趣的是，乙烯也会在机械胁迫下产生，并能诱导幼苗的“三重反应”：茎伸长减缓、茎加粗以及水平生长。这是克服土壤物理障碍的一种适应性状。

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8. Abscisic Acid (ABA) and Stress | 脱落酸 (ABA) 与胁迫

Abscisic acid is often called the ‘stress hormone’. It is involved in seed dormancy and the response to drought. During water stress, ABA accumulates in leaves and causes stomatal closure by triggering the efflux of K⁺ ions from guard cells, reducing turgor pressure and thus closing the stomatal pore. This reduces water loss by transpiration.

脱落酸常被称为“胁迫激素”，参与种子休眠和干旱响应。在水分胁迫下，ABA 在叶片中积累，通过促使 K⁺ 离子从保卫细胞外流而关闭气孔，降低膨压，进而闭合气孔孔隙，从而减少蒸腾失水。

ABA also maintains seed dormancy by inhibiting the action of gibberellins. The balance between ABA and gibberellin determines whether a seed germinates. AQA may ask about the roles of ABA in coping with environmental challenges.

ABA 还通过抑制赤霉素的作用来维持种子休眠。ABA 与赤霉素之间的平衡决定了种子是否萌发。AQA 可能会考查 ABA 在应对环境挑战中的作用。

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9. Commercial Applications of Plant Hormones | 植物激素的商业应用

The knowledge of plant hormones has led to numerous agricultural and horticultural applications. Auxin-based rooting powders promote the formation of adventitious roots on stem cuttings. Selective weedkillers (e.g. 2,4-D) are synthetic auxins that cause broad-leaved dicotyledonous weeds to grow abnormally and die, while monocotyledonous cereal crops are less affected.

对植物激素的了解带来了众多农业和园艺应用。生长素类生根粉可促进茎插条上不定根的形成。选择性除草剂（如 2,4-D）是人工合成的生长素，可使阔叶双子叶杂草生长异常死亡，而对单子叶谷类作物的影响较小。

Gibberellins are used to produce seedless grapes and to delay ripening in citrus fruits, maintaining their marketability. They are also used in the malting industry to speed up the production of malt for brewing. Ethene is applied to pre-climacteric fruits to initiate uniform ripening. Hormone-based products must be carefully timed and dosed to maximise yield and quality while minimising environmental impact.

赤霉素被用于生产无籽葡萄，以及延缓柑橘类果实的成熟以保持其商品性。在麦芽制造工业中，赤霉素也用于加速酿造用麦芽的生产。乙烯可用于跃变期前的果实，以启动整齐一致的成熟。激素基产品必须精确安排使用时点和剂量，以最大化产量和品质，同时最小化环境影响。

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10. Comparison of Plant and Animal Hormones | 植物与动物激素的比较

Although both plant and animal hormones are chemical messengers, there are fundamental differences that AQA expects you to be able to articulate:

尽管植物和动物激素都是化学信使，但二者存在根本性差异，AQA 期望你能够阐明：

Feature | 特征	Plant Hormones | 植物激素	Animal Hormones | 动物激素
Production site | 产生部位	Scattered tissues (e.g. meristems, leaves) | 散在组织中（如分生组织、叶片）	Specialised endocrine glands | 特化的内分泌腺体
Transport | 运输	Diffusion, xylem/phloem, short-distance | 扩散、木质部/韧皮部、短距离	Bloodstream, over long distances | 经血液长距离运输
Target response | 靶标反应	Variable; same hormone can have different effects in different tissues | 可变；同一激素在不同组织可有不同效应	Specific response in specific target cells | 在特定靶细胞产生特异反应
Chemical nature | 化学本质	Diverse: small organic acids (IAA), gases (C₂H₄), terpenoids (gibberellins) | 多样：小分子有机酸 (IAA)、气体 (C₂H₄)、萜类（赤霉素）	Proteins/peptides, steroids, amino acid derivatives | 蛋白质/肽类、固醇类、氨基酸衍生物
Example of mechanism | 机制举例	IAA activates proton pumps, loosens cell walls | IAA 激活质子泵、松弛细胞壁	Adrenaline binds to G-protein-coupled receptor, activates cAMP cascade | 肾上腺素与G蛋白偶联受体结合，激活cAMP级联

Understanding these differences helps clarify why plant growth factors are not always termed ‘hormones’ in the strict zoological sense. Nonetheless, they serve analogous coordinating functions in plants.

理解这些差异有助于明确为何植物生长因子在严格的动物学意义上并不总是被称为“激素”。然而，它们在植物中发挥着类似的协调功能。

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Published by TutorHao | Biology Revision Series | aleveler.com

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