📚 GCSE WJEC Biology: Plant Hormones Revision | GCSE WJEC 生物：植物激素 考点精讲
Plant hormones, or phytohormones, are chemical messengers that coordinate plant growth, development, and responses to environmental stimuli. Unlike animal hormones, they are produced in unspecialised tissues and often travel short distances through diffusion or active transport, rather than via a dedicated circulatory system. For the WJEC GCSE Biology specification, a solid understanding of auxin, gibberellins, abscisic acid, and ethene is essential, along with their roles in phototropism, gravitropism, and commercial applications.
植物激素是协调植物生长、发育及对环境刺激响应的化学信使。与动物激素不同，它们由非特化组织产生，通常通过扩散或主动运输进行短距离移动，而非经由专门的循环系统。对于 WJEC GCSE 生物学规范，充分理解生长素、赤霉素、脱落酸和乙烯及其在向光性、向地性和商业应用中的作用至关重要。
1. Introduction to Plant Hormones | 植物激素概述
Plant hormones are chemical signals that regulate processes such as cell division, elongation, differentiation, seed germination, flowering, and fruit ripening. They are produced in minute quantities, yet have profound effects. The main groups studied at GCSE are auxins, gibberellins, abscisic acid (ABA), and ethene. These hormones often interact, either synergistically or antagonistically, to produce the required response.
植物激素是调节细胞分裂、伸长、分化、种子萌发、开花和果实成熟等过程的化学信号。它们以微量产生，却具有深远影响。GCSE 阶段学习的主要类别为生长素、赤霉素、脱落酸（ABA）和乙烯。这些激素常以协同或拮抗的方式相互作用，以产生所需的反应。
2. Auxin and Phototropism | 生长素与向光性
Phototropism is a directional growth response to light. Shoots grow towards light (positive phototropism), while roots generally grow away from it (negative phototropism). This response is driven by the unequal distribution of auxin (indole-3-acetic acid, IAA) in the shoot tip. When light shines on one side of the tip, more auxin accumulates on the shaded side. In shoots, a higher auxin concentration stimulates greater cell elongation, so the shaded side grows faster, bending the shoot towards the light.
向光性是对光的方向性生长反应。茎向着光生长（正向光性），而根通常背光生长（负向光性）。这种反应是由茎尖中生长素（吲哚-3-乙酸，IAA）的不均匀分布驱动的。当光照射在茎尖的一侧时，更多的生长素积聚在背光侧。在茎中，较高的生长素浓度刺激更大的细胞伸长，因此背光侧生长更快，导致茎向光弯曲。
Classic experimental evidence: if the tip of a coleoptile (young shoot) is removed, the shoot no longer bends towards light, showing that the tip is essential for detecting light and producing auxin. When a detached tip is placed on an agar block, auxin diffuses into the block. If this agar block is then placed asymmetrically on a decapitated shoot, the shoot bends even in total darkness, confirming that the auxin concentration gradient alone can cause bending.
经典实验证据：如果切去胚芽鞘（幼茎）的尖端，茎便不再向光弯曲，这表明尖端是检测光和产生生长素所必需的。当将切下的尖端放在琼脂块上时，生长素扩散到琼脂块中。然后将该琼脂块不对称地放在去顶的茎上，即使在完全黑暗的条件下茎也会弯曲，这证实了仅生长素浓度梯度就足以导致弯曲。
3. Auxin and Gravitropism | 生长素与向地性
Gravitropism (geotropism) is the growth response to gravity. Roots exhibit positive gravitropism (growing downwards), while shoots exhibit negative gravitropism (growing upwards). When a seedling is placed horizontally, auxin redistributes to the lower side under the influence of gravity. In roots, high auxin concentration inhibits cell elongation, so the upper side elongates more, causing the root to bend downwards. In shoots, high auxin promotes cell elongation, so the lower side elongates more, and the shoot bends upwards.
向地性是对重力的生长反应。根表现出正向地性（向下生长），而茎表现出负向地性（向上生长）。当幼苗水平放置时，在重力影响下，生长素重新分布到较低的一侧。在根中，高浓度生长素抑制细胞伸长，因此上侧伸长更多，导致根向下弯曲。在茎中，高浓度生长素促进细胞伸长，因此下侧伸长更多，茎向上弯曲。
Experiments using a clinostat – a device that slowly rotates the plant to nullify the directional influence of gravity – can demonstrate that gravitropism depends on the redistribution of auxin. Rotation prevents auxin from accumulating on one side, so the seedling continues to grow straight rather than bending.
使用回转器的实验——一种缓慢旋转植物以抵消重力方向性影响的装置——可以证明向地性依赖于生长素的重新分布。旋转阻止生长素在一侧积累，因此幼苗继续笔直生长，而非弯曲。
4. How Auxin Causes Cell Elongation | 生长素如何引起细胞伸长
Auxin promotes cell elongation by increasing the flexibility of the cell wall. It stimulates cells to pump hydrogen ions (H⁺) into the cell wall, which lowers the pH and activates enzymes that break some of the bonds between cellulose microfibrils. This loosens the wall, allowing the cell to expand more easily when water enters by osmosis and turgor pressure increases.
生长素通过增加细胞壁的弹性来促进细胞伸长。它刺激细胞将氢离子（H⁺）泵入细胞壁，降低 pH 值并激活酶，这些酶会打断纤维素微纤丝之间的一些键。这使细胞壁松弛，当水分通过渗透作用进入且膨压增加时，细胞便更容易扩张。
5. Gibberellins: Seed Germination and Stem Elongation | 赤霉素：种子萌发与茎伸长
Gibberellins are a group of hormones that promote stem elongation, seed germination, and fruit growth. In germinating seeds, the embryo produces gibberellins that diffuse to the aleurone layer, which is a protein-rich layer surrounding the endosperm. Gibberellins trigger the aleurone cells to synthesise and secrete digestive enzymes, such as amylase. Amylase hydrolyses stored starch in the endosperm into sugars, providing energy for the growing seedling.
赤霉素是一类促进茎伸长、种子萌发和果实生长的激素。在萌发的种子中，胚产生赤霉素并扩散到糊粉层，即包围胚乳的富含蛋白质的层。赤霉素触发糊粉层细胞合成并分泌消化酶，如淀粉酶。淀粉酶将胚乳中储存的淀粉水解为糖，为生长的幼苗提供能量。
Commercially, gibberellins are sprayed on sugarcane to increase internode length and sugar yield, and on seedless grapes to increase berry size and reduce bunch compactness. In brewing, gibberellins speed up the malting process by accelerating starch breakdown.
在商业上，赤霉素被喷洒在甘蔗上以增加节间长度和糖产量，并喷洒在无籽葡萄上以增大浆果尺寸并降低果穗紧密度。在酿造中，赤霉素通过加速淀粉分解来加快麦芽制作过程。
6. Abscisic Acid (ABA): Stress Hormone | 脱落酸 (ABA)：胁迫激素
Abscisic acid (ABA) acts primarily as a growth inhibitor. It promotes seed dormancy, ensuring seeds do not germinate until conditions are favourable. During the onset of winter, ABA accumulates in buds, inducing bud dormancy. In response to drought, ABA levels rise rapidly in leaves, causing stomatal closure. ABA binds to receptor proteins in guard cells, triggering a signalling cascade that results in the loss of potassium ions and water from the guard cells, making them flaccid and closing the stomatal pore. This reduces transpiration and conserves water.
脱落酸主要起生长抑制剂的作用。它促进种子休眠，确保种子在条件适宜之前不萌发。在初冬，脱落酸在芽中积累，诱导芽休眠。在应对干旱时，叶片中的脱落酸水平迅速升高，导致气孔关闭。脱落酸与保卫细胞中的受体蛋白结合，触发信号级联反应，导致钾离子和水从保卫细胞中流失，使其变得松软并关闭气孔。这减少了蒸腾作用并保存水分。
7. Ethene and Fruit Ripening | 乙烯与果实成熟
Ethene (ethylene, C₂H₄) is a gaseous plant hormone that plays a central role in fruit ripening. It triggers the breakdown of pectin in cell walls, the conversion of starch into sugars, the synthesis of colour pigments, and the production of volatile aroma compounds. Ethene acts as a positive regulator: ripening fruit releases ethene, which in turn accelerates ripening in neighbouring fruit. This autocatalytic response is used commercially to ensure even ripening of bananas, tomatoes, and citrus fruits during storage and transport.
乙烯是一种气态植物激素，在果实成熟中起着核心作用。它触发细胞壁中果胶的分解、淀粉向糖的转化、色素的合成以及挥发性香气化合物的产生。乙烯起着正向调节作用：成熟水果释放乙烯，进而加速邻近水果的成熟。这种自催化反应被商业利用，以确保香蕉、番茄和柑橘类水果在储存和运输过程中均匀成熟。
8. Commercial Uses of Plant Hormones | 植物激素的商业应用
Synthetic auxins, such as 2,4-D, are widely used as selective herbicides. They produce uncontrolled, rapid growth in broad-leaved dicot weeds, depleting their energy reserves and causing death. Monocot crops such as wheat and grass are much less sensitive to these concentrations, so the herbicide kills weeds without harming the crop.
合成生长素，如 2,4-D，被广泛用作选择性除草剂。它们在阔叶双子叶杂草中引发不受控制的快速生长，耗尽其能量储备并导致死亡。小麦和草等单子叶作物对这些浓度的敏感度低得多，因此除草剂能杀死杂草而不危害作物。
Auxin-based rooting powders are applied to the cut ends of stem cuttings to stimulate adventitious root formation, greatly improving the success rate of vegetative propagation. Gibberellins are sprayed on citrus trees to delay senescence and keep fruit on the tree longer, allowing extended harvest. Conversely, inhibitors of ethene action, such as 1-MCP, are used to delay fruit ripening during long-distance transport.
基于生长素的生根粉被施于茎插条的切口端，以刺激不定根的形成，从而大大提高营养繁殖的成功率。赤霉素被喷洒在柑橘树上以延缓衰老并使果实更久地留在树上，以延长采收期。相反，乙烯作用抑制剂，如 1-MCP，被用于在长途运输中延缓果实成熟。
9. Investigative Experiments: Phototropism and Gravitropism | 探究实验：向光性与向地性
Standard GCSE practical: place canisters of young cress or mustard seedlings in a box with a single lateral opening. After 2–3 days, shoots visibly bend towards the light source. As a control, cover the shoot tips of a separate batch with aluminium foil; these will not bend, confirming that the tip is the site of light perception and auxin redistribution.
标准 GCSE 实践：将装有幼嫩水芹或芥菜苗的罐子放在有一个侧面开口的盒子中。2–3 天后，茎明显地向光源弯曲。作为对照，用铝箔覆盖另一批苗的茎尖；这些苗不会弯曲，从而证实尖端是光感知和生长素重新分布的位点。
To study gravitropism, seedlings can be grown in Petri dishes with the radicle initially pointing sideways. A clinostat can be used as a control: rotation cancels the gravitational stimulus, and the root grows randomly. A stationary dish shows downward curvature of the root, demonstrating positive gravitropism.
为研究向地性，可以在培养皿中培养幼苗，使胚根最初指向侧面。回转器可作为对照：旋转抵消了重力刺激，根便随机生长。而静止的培养皿显示根向下弯曲，表现出正向地性。
10. Common Exam Pitfalls and Key Concepts | 常见考试误区与关键概念
It is crucial to remember that in shoots, auxin promotes cell elongation, while in roots it inhibits elongation. A frequent mistake is to assume that auxin is destroyed by light; instead, light causes auxin to move to the shaded side. Also, do not confuse phototropism with gravitropism: the former is driven by light direction, the latter by gravity.
关键要记住，在茎中，生长素促进细胞伸长，而在根中则抑制伸长。一个常见错误是认为生长素会被光破坏；实际上，光导致生长素移动到背光侧。此外，不要混淆向光性与向地性：前者由光方向驱动，后者由重力驱动。
When describing hormone action, always link the uneven distribution to differential growth, and relate this to the resulting curvature. Commercial questions often ask for specific examples: synthetic auxins as weedkillers, rooting powder, gibberellins for fruit size, and ethene for ripening. Be ready to interpret data from agar-block or clinostat experiments.
在描述激素作用时，始终将不均匀分布与差异生长联系起来，并将其与所产生的弯曲相关联。商业问题常要求给出具体例子：合成生长素作为除草剂、生根粉、赤霉素用于增大果实，以及乙烯用于催熟。要准备好解释琼脂块或回转器实验的数据。
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