📚 Plant Hormones: Key Points for IB and OCR Biology | 植物激素：IB与OCR生物考点精讲
Plant hormones, also known as phytohormones, are organic substances that regulate virtually every aspect of plant growth, development, and responses to environmental stimuli. Unlike animal hormones, which are typically produced in specialised glands and transported via the bloodstream, plant hormones are often synthesised in a wide range of tissues and can act locally within the same tissue or be transported through the vascular system to distant target cells. For IB and OCR Biology, the five major classes are auxins, gibberellins, cytokinins, abscisic acid (ABA), and ethylene. Understanding their mechanisms, experimental evidence, and commercial applications is essential for top marks.
植物激素，也称植物荷尔蒙，是调控植物生长、发育和应答环境刺激的有机物质。与动物激素通常由特定腺体分泌并经血液运输不同，植物激素往往在多种组织中合成，可在同一组织内局部起作用，也可通过维管系统运输至远处的靶细胞。对IB和OCR生物学考试而言，五大类植物激素为生长素、赤霉素、细胞分裂素、脱落酸和乙烯。掌握它们的作用机制、实验证据和商业应用是取得高分的关键。
1. Defining Plant Hormones | 植物激素的定义
Plant hormones are chemical messengers that are effective at extremely low concentrations (often micromolar or lower). They do not function in isolation; instead, the final physiological response is usually shaped by the balance between several hormones acting synergistically or antagonistically. For example, the ratio of auxin to cytokinin determines whether a callus tissue differentiates into roots or shoots. The table below summarises the major hormones and their common abbreviations.
植物激素是极低浓度（通常为微摩尔级或更低）下即可生效的化学信使。它们并非孤立作用；最终的生理反应往往由多种激素协同或拮抗的平衡所决定。例如，生长素与细胞分裂素的浓度比决定了愈伤组织分化为根还是芽。下表总结了主要激素及其常用缩写。
| Hormone | Abbreviation | Chemical Example |
|---|---|---|
| Auxin | IAA | Indole-3-acetic acid (C₁₀H₉NO₂) |
| Gibberellin | GA | Gibberellic acid (GA₃) |
| Cytokinin | CK | Zeatin |
| Abscisic acid | ABA | (C₁₅H₂₀O₄) |
| Ethylene | C₂H₄ | Gaseous hydrocarbon |
2. Auxin and Phototropism | 生长素与向光性
Phototropism is the directional growth of plant shoots towards unilateral light, mediated by the uneven distribution of auxin. Charles Darwin and his son Francis first demonstrated that the tip of the coleoptile (in canary grass) is essential for detecting the light signal. Later, Frits Went isolated the chemical and showed that agar blocks containing diffusible substance from coleoptile tips could induce curvature when placed asymmetrically on decapitated coleoptiles. This substance was named auxin (from Greek ‘auxein’, to grow).
向光性是植物地上部分朝向单侧光源的方向性生长，由生长素的不均匀分布所介导。查尔斯·达尔文与其子弗朗西斯首先证明，金丝雀虉草的胚芽鞘尖端是感知光信号的关键部位。随后，弗里茨·温特分离出该化学物质，并证明含有胚芽鞘尖端可扩散物质的琼脂块，若不对称地放置在去顶胚芽鞘上，可诱导弯曲。这一物质被命名为生长素。
In modern understanding, light activates phototropin photoreceptors, which trigger the lateral transport of auxin (mainly IAA) from the illuminated side to the shaded side of the shoot. The higher auxin concentration on the shaded side promotes cell elongation via the ‘acid growth’ hypothesis: auxin stimulates plasma membrane H⁺-ATPase activity, pumping protons (H⁺) into the cell wall. The resulting acidification activates expansin proteins, loosening the cellulose–hemicellulose network so that turgor pressure can drive cell expansion. Consequently, the shoot bends towards the light.
现代理解认为，光激活向光蛋白光受体，触发生长素（主要是IAA）从向光侧向背光侧的横向运输。背光侧较高的生长素浓度通过“酸生长”假说促进细胞伸长：生长素激活质膜H⁺-ATP酶，将质子（H⁺）泵入细胞壁。由此产生的酸化激活扩张蛋白，松解纤维素–半纤维素网络，从而使膨压得以驱动细胞扩展。结果是地上部分向光弯曲。
Light → Phototropin activation → Auxin redistribution → H⁺ efflux → Wall loosening → Cell elongation → Bending
光 → 向光蛋白激活 → 生长素重新分布 → H⁺ 外流 → 细胞壁松弛 → 细胞伸长 → 弯曲
3. Auxin and Gravitropism | 生长素与向地性
Gravitropism (formerly called geotropism) is a growth response to gravity. In roots, the gravity-sensing mechanism involves dense, starch-filled amyloplasts (statoliths) that sediment in root cap cells, triggering a redistribution of auxin. In a horizontally placed root, auxin accumulates on the lower side, but because roots are far more sensitive to auxin than shoots, this high concentration inhibits cell elongation. Cells on the upper side elongate faster, causing the root to curve downward. In shoots, the same auxin accumulation on the lower side stimulates elongation, causing upward bending.
向地性是对重力的生长响应。在根中，感知重力的机制涉及根冠细胞内充满淀粉的致密淀粉体（平衡石）沉降，引发生长素的重新分布。水平放置的根中，生长素积累于下侧，但由于根对生长素的敏感度远高于茎，此高浓度会抑制细胞伸长。上侧细胞伸长更快，导致根向下弯曲。而在茎中，同样的下侧生长素积累刺激伸长，引起向上弯曲。
This differential sensitivity is partly explained by the interaction of auxin with other signals and by root-specific auxin-responsive transcription factors. Exam questions often ask you to predict the result of surgical removal of the root cap or application of auxin-transport inhibitors.
这种敏感性差异部分可由生长素与其他信号的互作以及根特异的生长素响应转录因子来解释。考试题目常要求你预测摘除根冠或施用生长素运输抑制剂的结果。
4. Apical Dominance | 顶端优势
Apical dominance is the phenomenon whereby the shoot apex inhibits the growth of lateral (axillary) buds, maintaining a single main stem. Auxin synthesised in the apical bud is transported downwards and suppresses bud outgrowth. When the apex is removed (decapitation), lateral buds are released and begin to grow. This demonstrates the role of auxin as a correlative inhibitor.
顶端优势是指茎尖抑制侧芽（腋芽）生长，从而维持单一主茎的现象。顶芽合成的生长素向下运输并抑制侧芽萌发。若去除顶芽（打顶），侧芽被释放并开始生长。这证明了生长素作为一种相关性抑制剂的作用。
Cytokinins act antagonistically to auxin in this system: they are produced in roots and promote bud growth. Applying cytokinin to a bud overcomes the inhibitory effect of auxin. The balance between auxin and cytokinin, rather than absolute levels of either hormone, controls branching patterns. This concept is frequently tested in data-analysis questions comparing wild-type and mutant plants with altered hormone synthesis or perception.
在该系统中，细胞分裂素与生长素互为拮抗：它在根部产生并促进芽的生长。将细胞分裂素施加于芽能克服生长素的抑制作用。控制分枝模式的是生长素与细胞分裂素的平衡，而非任一激素的绝对水平。此概念常在涉及比较野生型和激素合成或感知突变体的数据分析题中进行考查。
5. Gibberellins: Stem Elongation and Seed Germination | 赤霉素：茎伸长与种子萌发
Gibberellins (GAs) are a large family of tetracyclic diterpenoid compounds. The most commonly studied is gibberellic acid (GA₃). Their most dramatic effects include promotion of internode elongation in dwarf varieties. Rosette plants treated with GA bolt and elongate rapidly. This response is used as a bioassay for GA activity.
赤霉素是一个庞大的四环二萜类化合物家族。最常被研究的是赤霉酸（GA₃）。其最显著的作用包括促进矮化品种的节间伸长。莲座型植物经赤霉素处理后抽薹并迅速伸长。该响应被用作赤霉素活性的生物测定。
In cereal seed germination, the embryo synthesises GA, which diffuses to the aleurone layer and induces the transcription of genes coding for α-amylase. The enzyme hydrolyses starch stored in the endosperm into sugars, providing energy for the growing embryo. This pathway is a classic example of hormonal signal transduction in plants. The relationship can be summarised:
在谷类种子萌发过程中，胚合成赤霉素，其扩散至糊粉层并诱导编码α-淀粉酶的基因转录。该酶将胚乳中储存的淀粉水解为糖，为胚的生长提供能量。此通路是植物激素信号转导的经典范例。该关系可概括为：
Embryo → GA → Aleurone layer → Transcriptional activation → α-Amylase → Starch → Maltose
胚 → 赤霉素 → 糊粉层 → 转录激活 → α-淀粉酶 → 淀粉 → 麦芽糖
6. Cytokinins and Cell Division | 细胞分裂素与细胞分裂
Cytokinins are adenine derivatives that promote cytokinesis, hence the name. They are synthesised primarily in root tips and transported upwards via the xylem. In tissue culture, a high auxin-to-cytokinin ratio induces root formation, whereas a low ratio favours shoot formation. An intermediate ratio maintains a mass of undifferentiated callus. This principle is crucial for micropropagation.
细胞分裂素是腺嘌呤衍生物，因其促进胞质分裂而得名。它们主要在根尖合成，通过木质部向上运输。在组织培养中，高生长素/细胞分裂素之比诱导根的形成，而低比值则有利于芽的形成。中等比值则维持一团未分化的愈伤组织。此原理对微繁技术至关重要。
Another key function is the delay of leaf senescence. Cytokinins can maintain protein synthesis and chlorophyll retention, keeping leaves green. This makes them commercially valuable for extending the shelf life of cut flowers and leafy vegetables.
另一关键功能是延缓叶片衰老。细胞分裂素能维持蛋白质合成和叶绿素保留，使叶片保持绿色。这使其在延长切花和叶菜类蔬菜的货架期方面具有商业价值。
7. Abscisic Acid and Stress Responses | 脱落酸与胁迫响应
Abscisic acid (ABA) is often termed the ‘stress hormone’ because its levels rise in response to abiotic stresses, particularly drought. It is derived from carotenoids. Unlike its historical name suggests, ABA is not directly responsible for leaf abscission, a role now attributed more to ethylene.
脱落酸常被称为“胁迫激素”，因其水平在响应非生物胁迫（尤其是干旱）时上升。它源自类胡萝卜素。与历史上名称所暗示的不同，ABA并非叶片脱落的直接原因，这一作用如今更多归因于乙烯。
The most critical function of ABA is the regulation of stomatal closure. When a plant experiences water deficit, ABA is synthesised in roots and transported to leaves, or produced directly in guard cells. ABA triggers an efflux of K⁺ and anions from the guard cells, leading to osmotic water loss, turgor reduction, and stomatal pore closure. This reduces transpiration. The signal cascade involves calcium ions (Ca²⁺) as second messengers and protein kinases. ABA also promotes seed dormancy and inhibits precocious germination.
脱落酸最关键的功能是调控气孔关闭。当植物缺水时，脱落酸在根部合成并被运输至叶片，或直接在保卫细胞中产生。脱落酸触发K⁺和阴离子从保卫细胞外流，导致渗透失水、膨压降低和气孔关闭，从而减少蒸腾作用。该信号级联涉及钙离子（Ca²⁺）作为第二信使以及蛋白激酶。脱落酸还能促进种子休眠并抑制过早萌发。
8. Ethylene: The Gaseous Hormone | 乙烯：气体激素
Ethylene (C₂H₄) is unusual among plant hormones because it is a gas at room temperature. This property allows it to diffuse rapidly through the air and affect neighbouring fruits, which explains the saying ‘one rotten apple spoils the barrel’. Ethylene is synthesised from the amino acid methionine via SAM (S-adenosyl methionine) and ACC (1-aminocyclopropane-1-carboxylic acid).
乙烯在植物激素中十分独特，因为它在室温下为气体。这一特性使其能通过空气快速扩散并影响邻近果实，这就解释了“一个烂苹果坏了一桶”的谚语。乙烯由氨基酸蛋氨酸经SAM（S-腺苷甲硫氨酸）和ACC（1-氨基环丙烷-1-羧酸）合成。
Ethylene induces the ‘triple response’ in etiolated seedlings: inhibition of stem elongation, radial swelling of the stem, and horizontal growth (diageotropism). It also promotes ripening in climacteric fruits (e.g., bananas, tomatoes) by triggering the conversion of starch to sugars, cell wall softening, and colour changes. In leaves, ethylene stimulates the formation of an abscission layer, leading to leaf fall.
乙烯在黄化幼苗中诱导“三重反应”：抑制茎的伸长、茎的径向膨大以及水平生长（横向向地性）。它还能通过触发淀粉转化为糖、细胞壁软化和颜色变化，促进跃变型果实（如香蕉、番茄）的成熟。在叶片中，乙烯刺激离层的形成，导致落叶。
9. Commercial Applications of Plant Hormones | 植物激素的商业应用
Understanding plant hormone biology has led to numerous agricultural and horticultural innovations. Synthetic auxins such as 2,4-D (2,4-dichlorophenoxyacetic acid) are widely used as selective herbicides. Dicots are far more sensitive to 2,4-D than monocots, so they can be used to kill broadleaf weeds in cereal fields. NAA (naphthaleneacetic acid) is a rooting powder applied to cuttings to promote adventitious root formation.
对植物激素生物学的理解催生了大量农业和园艺创新。合成生长素如2,4-D（2,4-二氯苯氧乙酸）被广泛用作选择性除草剂。双子叶植物对2,4-D远比单子叶植物敏感，因此可用于谷物田中杀灭阔叶杂草。萘乙酸（NAA）是一种生根粉，用于插条以促进不定根形成。
Gibberellins are used to increase the size of seedless grapes, improve internode length in sugarcane, and speed up malting in beer production by promoting α-amylase activity. Cytokinins are sprayed on cut flowers to delay yellowing. The ethylene-releasing compound ethephon is used to synchronise fruit ripening for harvest, and inhibitors of ethylene action, such as 1-MCP (1-methylcyclopropene), prolong the storage life of fruits and vegetables by blocking ethylene receptors.
赤霉素被用于增大无籽葡萄、改善甘蔗节间长度，以及通过促进α-淀粉酶活性加速啤酒酿造中的麦芽化过程。细胞分裂素被喷洒于切花上以延缓黄化。乙烯释放剂乙烯利被用于同步催熟果实以便收获，而乙烯作用抑制剂，如1-MCP（1-甲基环丙烯），通过阻断乙烯受体来延长果蔬的储藏期。
10. Experimental Approaches and Exam Pitfalls | 实验方法与考试雷区
IB and OCR examiners love to test your understanding of classic experiments. Be prepared to interpret diagrams showing Went’s agar block experiment, decapitation and replacement of coleoptile tips, or the effect of GA on dwarf maize. You should be able to identify variables (independent: presence or position of hormone; dependent: curvature angle or stem length) and justify the use of control groups. In data-response questions, always relate the pattern back to the mode of hormone action.
IB和OCR的考官喜欢考查你对经典实验的理解。做好解读显示温特琼脂块实验、胚芽鞘去顶及替换实验或赤霉素对矮化玉米影响的示意图的准备。你应能识别变量（自变量：激素的有无或位置；因变量：弯曲角度或茎长）并论证对照组的使用。在数据分析题中，要始终将模式与激素作用方式联系起来。
A common mistake is confusing the role of ABA in abscission with that of ethylene; remember that ethylene is the primary promoter of leaf and fruit drop. Another pitfall is forgetting that the same hormone (auxin) has opposite effects in roots and shoots due to different concentration–response curves. Always mention the concept of ‘optimum concentration’ and ‘sensitivity’ when explaining these differential responses.
一个常见的错误是将脱落酸在脱落中的作用与乙烯混淆；请记住乙烯才是叶片和果实脱落的主要促进因子。另一个雷区是忘了同一种激素（生长素）在根和茎中因浓度反应曲线不同而具有相反的作用。解释这些差异响应时，务必提及“最适浓度”和“敏感性”的概念。
11. Summary Table for Revision | 复习汇总表
The following table consolidates the essential functions and specific exam-relevant clues for each hormone. Use it as a quick reference.
下表汇总了每种激素的核心功能以及与考试相关的具体线索，可作为快速参考。
| Hormone (激素) | Major Functions (主要功能) | Exam Clues (考试线索) |
|---|---|---|
| Auxin (IAA) | Cell elongation, phototropism, gravitropism, apical dominance, adventitious root formation | Curvature assays, decapitation, agar block experiments |
| Gibberellin (GA) | Stem elongation, seed germination (α-amylase induction), bolting | Dwarf plants, aleurone layer diagrams, maltose production |
| Cytokinin (CK) | Cell division, shoot differentiation, delay of leaf senescence | Tissue culture ratios, green leaf retention |
| ABA | Stomatal closure, seed dormancy, drought tolerance | Guard cell diagrams, K⁺ efflux, water stress |
| Ethylene | Fruit ripening, triple response, leaf abscission | Climacteric fruit, ethephon, 1-MCP |
12. Final Tips for Success | 成功秘诀
To excel in this topic, integrate knowledge across topics: link gas exchange and stomatal regulation with ABA, link seed structure and germination with GA, and connect plant tropisms with experimental design. Use correct terminology such as ‘diffusible chemical’, ‘polar transport’, and ‘aleuron layer’. Never say a hormone ‘just kills’ – explain the mechanism. Finally, practise drawing and annotating the Went experiment and the signal transduction pathway for GA-induced α-amylase expression.
要在此主题中取得优异成绩，需将各知识点融会贯通：将气体交换和气孔调控与ABA联系起来，将种子结构和萌发与GA联系起来，并将植物向性与实验设计挂钩。使用正确术语，如“可扩散化学物质”、“极性运输”和“糊粉层”。永远不要说激素“只是杀死”——要解释机制。最后，练习绘制并为温特实验以及GA诱导α-淀粉酶表达的信号转导通路做注释。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导