📚 A-Level Edexcel Biology: Plant Hormones Exam Essentials | A-Level Edexcel 生物：植物激素 考点精讲
Plant hormones (plant growth regulators) are key to coordinating growth, development and responses to environmental stimuli in plants. Understanding their mechanisms and interactions is essential for mastering the Edexcel A-Level Biology specification. This revision guide breaks down every major plant hormone, supported by experimental evidence and real-world applications, to help you excel in exams.
植物激素（植物生长调节剂）是协调植物生长、发育及对环境刺激响应的关键。掌握其作用机制与相互作用对于攻克 Edexcel A-Level 生物考试至关重要。本考点精讲逐一解析每一种主要植物激素，结合实验证据与实际应用，助你备考无忧。
1. Introduction to Plant Hormones | 植物激素简介
Plant hormones are organic compounds produced in one part of the plant and transported to target cells, where they elicit specific responses at very low concentrations. Unlike animal hormones, they are not secreted by specialised glands and often act locally or synergistically. The five classical groups covered in Edexcel A-Level are auxins, gibberellins, cytokinins, abscisic acid (ABA) and ethene.
植物激素是在植物某部位产生并被运输到靶细胞的有机化合物，在极低浓度下即可引发特定响应。与动物激素不同，它们并非由专门的腺体分泌，常常在局部起作用或通过协同作用影响植物。Edexcel A-Level 涉及的五大经典激素类别为：生长素、赤霉素、细胞分裂素、脱落酸（ABA）和乙烯。
The table below summarises their major roles.
下表总结了它们的主要功能。
| Plant Hormone | Key Functions (主要功能) |
|---|---|
| Auxin (IAA) | Cell elongation (细胞伸长), phototropism (向光性), gravitropism (向地性), apical dominance (顶端优势), lateral root initiation (侧根发生) |
| Gibberellins | Stem elongation (茎伸长), seed germination (种子萌发), mobilisation of food reserves (养分动员) |
| Cytokinins | Cell division (细胞分裂), delay of leaf senescence (延缓叶片衰老), antagonise apical dominance (对抗顶端优势) |
| Abscisic acid (ABA) | Stomatal closure (气孔关闭), seed dormancy (种子休眠), inhibition of growth (抑制生长) |
| Ethene | Fruit ripening (果实成熟), leaf abscission (叶片脱落), triple response in seedlings (幼苗三重反应) |
2. Auxin (IAA): Discovery and Chemical Nature | 生长素（IAA）：发现与化学本质
The principal natural auxin is indole-3-acetic acid (IAA), synthesised from the amino acid tryptophan in shoot tips and young leaves. IAA is transported mainly by polar transport from cell to cell, requiring energy and PIN efflux carriers. This directional movement creates concentration gradients essential for tropisms and developmental patterning.
主要的天然生长素为吲哚-3-乙酸（IAA），由色氨酸在茎尖和幼叶中合成。IAA 主要通过极性运输在细胞间传递，这一过程需要能量和 PIN 输出载体。这种定向运输建立了浓度梯度，对向性运动和发育模式至关重要。
Charles Darwin and his son Francis conducted pioneering experiments on phototropism using grass coleoptiles. They showed that the tip is necessary to perceive light, and that some mobile signal travels from the tip to the bending region. Later, Frits Went extracted this signal into agar blocks and demonstrated that the substance could induce curvature in decapitated coleoptiles, proving the existence of a chemical messenger – auxin.
查尔斯·达尔文与其子弗朗西斯利用禾本科胚芽鞘进行向光性实验，证明尖端是感受光所必需的，并且某种可移动信号会从尖端传递到弯曲区。随后，弗里茨·温特将该信号提取至琼脂块中，证明这种物质能引发去尖胚芽鞘的弯曲，从而证实了化学信使——生长素的存在。
3. Phototropism: Experimental Evidence and Auxin Redistribution | 向光性：实验证据与生长素再分布
In unidirectional light, IAA redistributes from the illuminated side to the shaded side of a shoot. Because IAA promotes cell elongation in shoots, cells on the shaded side elongate more, causing the shoot to bend towards the light. The Cholodny–Went hypothesis explains this lateral transport, which has been supported by measurements of IAA concentration and the localisation of PIN proteins.
在单向光下，IAA 从茎的照光侧重新分布到背光侧。由于 IAA 促进茎中细胞伸长，背光侧细胞伸长更旺盛，导致茎向光弯曲。Cholodny–Went 假说通过 IAA 浓度测量和 PIN 蛋白定位证实了这种横向运输机制。
Key experiments reinforcing this concept include using mica sheets to block signal movement, and using agar receivers placed on decapitated coleoptile tips exposed to unidirectional light. The receiver from the shaded side contained more auxin activity, confirming asymmetric distribution.
强化这一概念的关键实验包括：用云母片阻断信号移动，以及将琼脂接收块置于受单向光照射的胚芽鞘去尖表面；结果显示背光侧的接收块含有更多的生长素活性，证实了不对称分布。
4. Gravitropism: Mechanism of Root and Shoot Response | 向地性：根与茎的响应机制
When a root or shoot is placed horizontally, IAA accumulates on the lower side due to gravity-induced lateral transport. In shoots, higher auxin concentrations stimulate elongation, so the lower side grows faster and the shoot curves upward (negative gravitropism). In roots, auxin at high concentrations inhibits cell elongation; thus the lower side grows less, and the root bends downward (positive gravitropism).
当根或茎水平放置时，IAA 因重力诱导的横向运输而积累于下侧。在茎中，较高浓度的生长素促进伸长，因而下侧生长较快，茎向上弯曲（负向地性）。在根中，高浓度生长素抑制细胞伸长，因此下侧生长更慢，根向下弯曲（正向地性）。
This differential sensitivity explains why the same hormone can produce opposite effects in different organs. Statoliths (amyloplasts) in root cap cells are thought to trigger the redistribution of auxin transporters in response to gravity.
这种差异敏感性解释了同一激素为何在不同器官中产生相反效果。根冠细胞中的平衡石（淀粉体）被认为能在重力刺激下触发生长素转运体的重新分布。
5. Auxin in Apical Dominance | 生长素在顶端优势中的作用
Apical dominance is the phenomenon where the shoot apex suppresses the growth of lateral buds. Auxin produced by the apical bud travels downward and inhibits axillary bud development. If the apex is removed, lateral buds are released and grow, causing the plant to become bushier.
顶端优势是指顶芽抑制侧芽生长的现象。顶芽产生的生长素向下运输，抑制腋芽发育。若去除顶芽，侧芽便会得到释放并生长，植株因而变得更为丛生。
Cytokinins antagonise this effect; application of cytokinin to lateral buds can overcome auxin-induced inhibition. This interplay demonstrates how hormone ratios, rather than absolute concentrations, often determine developmental outcomes.
细胞分裂素能拮抗此效应；对侧芽施用细胞分裂素可克服生长素引起的抑制。这种相互作用表明，激素的相对比例而不仅仅是绝对浓度，往往决定发育结果。
6. Gibberellins: Seed Germination and Stem Growth | 赤霉素：种子萌发与茎生长
Gibberellins are a large family of compounds with GA3 being the most commonly studied. They promote stem elongation by stimulating cell division and cell elongation. Dwarf varieties of plants often lack functional gibberellin biosynthesis; application of gibberellin restores normal height, providing strong evidence for their role.
赤霉素是一个庞大的化合物家族，其中 GA3 研究最为普遍。它们通过刺激细胞分裂与细胞伸长来促进茎的伸长。矮化品种往往缺乏功能性赤霉素合成途径；施用赤霉素可使其恢复正常高度，这是证明赤霉素作用的有力证据。
In seed germination, gibberellins are released from the embryo, diffuse to the aleurone layer, and trigger the synthesis of α-amylase. This enzyme hydrolyses starch stored in the endosperm into sugars, providing energy for the growing embryo.
在种子萌发过程中，赤霉素从胚中释放，扩散至糊粉层，诱导 α-淀粉酶的合成。该酶将胚乳中贮存的淀粉水解为糖，为生长的胚提供能量。
7. Abscisic Acid (ABA): Stomatal Closure and Seed Dormancy | 脱落酸（ABA）：气孔关闭与种子休眠
ABA is often called the stress hormone because it accumulates under drought conditions. It triggers stomatal closure by causing guard cells to lose potassium ions, reducing turgor pressure. This minimises water loss by transpiration, helping the plant survive short-term water deficit.
ABA 常被称为胁迫激素，因为它在干旱条件下积累。ABA 通过促使保卫细胞丧失钾离子，降低膨压，从而诱导气孔关闭。这样可以最大程度减少蒸腾失水，帮助植物抵御短期缺水。
In seeds, ABA maintains dormancy and prevents premature germination. A decrease in ABA levels, often accompanied by an increase in gibberellins, is required to break dormancy. Abscission of leaves and fruits was historically linked to ABA, but ethene plays a more direct role.
在种子中，ABA 维持休眠状态并阻止过早萌发。休眠的打破需要 ABA 水平下降，常伴随赤霉素水平的上升。叶和果实的脱落历史上曾与 ABA 相关联，但乙烯在其中起更直接的作用。
8. Ethene: Fruit Ripening and Leaf Abscission | 乙烯：果实成熟与叶片脱落
Ethene (ethylene) is a simple gaseous hydrocarbon that acts as a hormone. It promotes fruit ripening by stimulating the breakdown of cell walls, conversion of starches to sugars, and production of volatile aroma compounds. Once ripening begins, ethene stimulates further ethene production in a positive feedback loop, which explains why one ripe fruit can trigger ripening in nearby fruits.
乙烯是一种简单的气态碳氢化合物，作为一种激素起作用。它通过刺激细胞壁分解、淀粉转化为糖以及挥发性香气化合物的产生来促进果实成熟。一旦成熟开始，乙烯会通过正反馈环刺激更多乙烯生成，这就解释了为什么一个成熟果实能够引发周围果实成熟。
Ethene also promotes leaf abscission by inducing the formation of an abscission zone at the base of the leaf petiole, where cell wall-degrading enzymes weaken the attachment. In seedlings, ethene induces the triple response: reduced stem elongation, thickening of the stem, and horizontal growth – an adaptive mechanism when encountering obstacles.
乙烯还能通过在叶柄基部诱导离层的形成来促进叶片脱落，离层中的细胞壁降解酶使附着弱化。在幼苗中，乙烯引发三重反应：茎伸长减缓、茎加粗和水平生长——这是遇到障碍物时的一种适应机制。
9. Cytokinins: Cell Division and Delay of Senescence | 细胞分裂素：细胞分裂与延缓衰老
Cytokinins are produced in root tips and transported upward via the xylem. They promote cytokinesis (cell division) in shoots and roots, acting synergistically with auxin. In tissue culture, a high cytokinin-to-auxin ratio stimulates shoot formation, while a low ratio favours root initiation.
细胞分裂素在根尖产生，通过木质部向上运输。它们与生长素协同作用，促进茎和根的胞质分裂（细胞分裂）。在组织培养中，高细胞分裂素/生长素比例刺激芽的形成，低比例则有利于根的发生。
Cytokinins also delay senescence by maintaining protein synthesis and preventing chlorophyll breakdown. Leaves treated with cytokinin stay green longer, which has commercial implications for extending the shelf life of leafy vegetables.
细胞分裂素还能通过维持蛋白质合成和防止叶绿素降解来延缓衰老。经细胞分裂素处理的叶片能保持绿色更久，这对延长叶菜类蔬菜的货架期具有商业意义。
10. Commercial Applications of Plant Hormones | 植物激素的商业应用
Plant hormones and synthetic analogues have wide agricultural and horticultural uses. Synthetic auxins such as 2,4-D act as selective herbicides; they are absorbed more readily by broad-leaved weeds, causing uncontrolled growth and death, while leaving cereal crops relatively unaffected.
植物激素及其人工合成类似物在农业和园艺中广泛应用。2,4-D 等合成生长素作为选择性除草剂，更容易被阔叶杂草吸收，引发无序生长并致死，而对禾谷类作物相对无害。
- Rooting powders contain synthetic auxins like NAA to promote adventitious root formation in cuttings.
- 生根粉 含有 NAA 等合成生长素，可促进插条不定根的形成。
- Gibberellins are used to produce seedless grapes, increase internode length in sugarcane, and improve fruit size in apples.
- 赤霉素 用于生产无籽葡萄、增加甘蔗节间长度以及增大苹果果实体积。
- Ethene is applied to ripen bananas and tomatoes during transport; synthetic ethephon releases ethene conveniently.
- 乙烯 用于在运输过程中催熟香蕉和番茄；合成乙烯利可方便地释放乙烯。
- Cytokinins are sprayed on cut flowers and leafy greens to delay senescence.
- 细胞分裂素 被喷洒于切花和叶菜上以延缓衰老。
11. Experimental Evidence and Key Exam Tips | 实验证据与关键考试技巧
Edexcel exam questions often ask you to interpret data from classic experiments, such as the Darwin and Went experiments, or from graphs showing the effect of hormone concentrations on growth. You should be able to explain the role of auxin in tropisms using the concept of asymmetric distribution and differential sensitivity. Be precise with terminology: ‘polar auxin transport’ and ‘PIN proteins’ score marks.
Edexcel 考试常要求你解释经典实验（如达尔文和温特实验）中的数据，或者分析显示激素浓度对生长影响的图表。你必须能够运用不对称分布和差异敏感性的概念来解释生长素在向性中的作用。使用精确术语如 ‘极性生长素运输’ 和 ‘PIN 蛋白’ 能为你加分。
When comparing hormones, note that gibberellins break seed dormancy, while ABA imposes it; ethene promotes abscission, but auxin can delay it. Practice linking hormone actions to commercial use – this shows application of knowledge.
比较各类激素时，注意赤霉素打破种子休眠，而 ABA 则维持休眠；乙烯促进脱落，而生长素可延缓脱落。练习将激素作用与商业用途联系起来——这体现了知识的应用能力。
For top marks, include details such as the positive feedback loop of ethene and the role of statoliths in gravitropism. Remember: plant hormone responses are often dependent on concentration, tissue type and interaction with other hormones.
要拿到高分，务必提及乙烯的正反馈环以及平衡石在向地性中的作用。牢记：植物激素响应通常依赖于浓度、组织类型以及与其他激素的相互作用。
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