📚 IB Edexcel Biology: Plant Hormones Key Points Review | IB Edexcel 生物：植物激素 考点精讲
Plant hormones are chemical signals produced in one part of a plant that act at very low concentrations in target tissues to regulate growth and development. Unlike animals, plants do not have a dedicated endocrine system; instead, hormones can be transported via xylem, phloem, or by cell-to-cell diffusion. The five major classes are auxins, gibberellins, cytokinins, abscisic acid, and ethylene. Understanding their individual roles and interactions is essential for both IB and Edexcel Biology specifications, particularly topics such as phototropism, gravitropism, seed germination, and commercial uses.
植物激素是在植物某一部位产生、以极低浓度作用于靶组织来调控生长与发育的化学信号。与动物不同，植物没有专门的内分泌系统，激素可通过木质部、韧皮部或细胞间扩散进行运输。五大类激素分别是生长素、赤霉素、细胞分裂素、脱落酸和乙烯。理解它们各自的作用及相互作用是 IB 与 Edexcel 生物大纲中的重要内容，特别是向光性、向地性、种子萌发和商业应用等专题。
1. Introduction to Plant Hormones | 植物激素简介
Plant hormones, also called phytohormones, influence every aspect of plant life, from embryogenesis to senescence. They are effective at concentrations as low as 10⁻⁶ mol dm⁻³ and often work in combinations, where the ratio of two hormones determines the physiological response rather than the absolute amount of a single hormone.
植物激素又称植物生长调节物质，影响着从胚胎发生到衰老的各个方面。它们在低至 10⁻⁶ mol dm⁻³ 的浓度下即可生效，并且常常以组合方式工作，两种激素的比例往往比单一激素的绝对量更能决定生理反应。
Unlike animal hormones that are synthesised in specialised glands, plant hormones are produced in a variety of tissues – often in growing regions such as shoot and root tips, developing leaves, and seeds. Their transport can be strictly directional, as in the polar transport of auxin from shoot apex to base, or non‑polar in the xylem or phloem.
不同于在专门腺体中合成的动物激素，植物激素可由多种组织产生——通常是在生长区域如茎尖、根尖、发育中的叶片和种子中。它们的运输可以是严格方向性的，如生长素从顶端向基部的极性运输；也可以通过木质部或韧皮部进行非极性运输。
A simple framework to remember the primary roles: auxin primarily controls cell elongation, gibberellins promote stem elongation and seed germination, cytokinins stimulate cell division, abscisic acid inhibits growth under stress, and ethylene regulates fruit ripening and senescence.
一个简单的记忆框架是：生长素主要控制细胞伸长，赤霉素促进茎伸长和种子萌发，细胞分裂素刺激细胞分裂，脱落酸在胁迫条件下抑制生长，乙烯调控果实成熟与衰老。
2. Auxins and Cell Elongation | 生长素与细胞伸长
The most common naturally occurring auxin is indole‑3‑acetic acid (IAA). It is synthesised in shoot tips, young leaves, and developing seeds. Auxin promotes cell elongation by stimulating cell wall loosening, a process explained by the acid growth hypothesis.
最常见的天然生长素是吲哚‑3‑乙酸（IAA），在茎尖、幼叶和发育种子中合成。生长素通过刺激细胞壁松弛来促进细胞伸长，这一过程可用酸生长假说来解释。
According to the acid growth hypothesis, auxin activates proton pumps (H⁺‑ATPases) in the plasma membrane, pumping H⁺ ions into the cell wall. The lowered pH activates expansin proteins that disrupt hydrogen bonds between cellulose microfibrils, allowing the cell wall to become more plastic. Water uptake then drives cell expansion as turgor pressure pushes the wall outwards.
根据酸生长假说，生长素激活细胞质膜上的质子泵（H⁺‑ATPase），将 H⁺ 离子泵入细胞壁。降低的 pH 激活扩张蛋白，打破纤维素微纤丝之间的氢键，使细胞壁变得更具可塑性。随后水分吸收在膨压作用下推动细胞壁向外扩展，实现细胞伸长。
In coleoptiles and young shoots, auxin stimulates elongation over a narrow concentration range. However, in roots, the same concentrations that promote shoot growth actually inhibit elongation. This illustrates the importance of tissue sensitivity to auxin concentration.
在胚芽鞘和幼茎中，生长素在较窄的浓度范围内刺激伸长。而在根中，促进茎生长的相同浓度反而抑制根的伸长，这说明组织对生长素浓度的敏感性差异至关重要。
3. Auxin in Phototropism | 生长素在向光性中的作用
Phototropism is the directional growth of a plant in response to unidirectional light. Shoots exhibit positive phototropism, growing towards light; roots are either negatively phototropic or unaffected. Classic experiments by Darwin, Boysen‑Jensen, and Went established the role of a mobile signal – later identified as auxin – in this response.
向光性是植物对单向光照产生的定向生长反应。茎表现出正向光性，朝光生长；根部则通常为负向光性或不受影响。达尔文、博伊森‑延森和温特的经典实验确定了可移动信号（后被认定为生长素）在这一反应中的作用。
Darwin and his son Francis demonstrated that the tip of the coleoptile is essential for perceiving light, but the bending occurred below the tip. Boysen‑Jensen inserted a thin sheet of mica or gelatin to show that a water‑soluble chemical diffused from the illuminated tip to the shaded side. Fritz Went collected this chemical in agar blocks and showed that placing an agar block asymmetrically on a decapitated coleoptile caused bending away from the side with the block.
达尔文父子证实胚芽鞘尖端是感受光的关键部位，但弯曲发生在尖端下方。博伊森‑延森插入云母片或明胶片，证明水溶性化学物质从受光尖端向背光侧扩散。温特用琼脂块收集该物质，并将其不对称地放在去顶胚芽鞘上，结果显示胚芽鞘朝远离琼脂块的一侧弯曲。
The current model states that light causes a lateral redistribution of auxin within the tip, moving more auxin to the shaded side. The higher auxin concentration on the shaded side promotes faster cell elongation, causing the shoot to bend towards the light.
当前模型认为，光照引起尖端内生长素的侧向再分布，更多生长素移动到背光侧。背光侧较高的生长素浓度促进更快的细胞伸长，导致茎向光弯曲。
4. Auxin in Gravitropism | 生长素在向地性中的作用
Gravitropism (geotropism) is the orientation of plant growth in response to gravity. Roots show positive gravitropism – growing downward – and shoots show negative gravitropism, growing upward. Perception of gravity occurs in specialised cells containing statoliths, dense amyloplasts that sediment under gravity.
向地性是指植物根据重力方向定向生长。根表现出正向地性（向下生长），茎为负向地性（向上生长）。感受重力的部位是含有平衡石的特定细胞，平衡石即受重力沉降的致密淀粉体。
In a horizontally placed root, statoliths settle on the lower side of root cap cells. This triggers a redistribution of auxin towards the lower side. Because root cells are much more sensitive to auxin, the higher concentration on the lower side inhibits elongation there, while the upper side continues to grow normally. This differential growth bends the root downwards.
在水平放置的根中，平衡石沉降在根冠细胞的下侧，引发生长素向下侧重新分布。由于根细胞对生长素更为敏感，下侧较高的生长素浓度抑制了该部位的伸长，而上侧继续正常生长，这种差异性生长使根部向下弯曲。
In shoots, the higher auxin concentration on the lower side of a horizontally placed stem stimulates cell elongation, causing the stem to bend upwards. This opposite response between root and shoot reflects the different auxin‑response thresholds in these tissues.
在茎中，水平放置时下侧较高的生长素浓度刺激细胞伸长，导致茎向上弯曲。根与茎这种相反的反应反映了不同组织对生长素响应阈值的差异。
5. Apical Dominance | 顶端优势
Apical dominance is the phenomenon where the main shoot tip inhibits the growth of lateral buds. This ensures vertical growth and limits branching. The classical explanation centres on auxin produced in the shoot apex and transported downwards, which directly or indirectly suppresses lateral bud outgrowth.
顶端优势是指主茎顶芽抑制侧芽生长的现象，确保植物垂直生长并限制分枝。经典解释集中在茎尖产生的生长素向下运输，直接或间接抑制侧芽萌发。
However, the mechanism is more complex. Current evidence suggests that auxin from the apex does not directly inhibit buds but instead maintains high levels of strigolactones and restricts the supply of cytokinins to the lateral buds. Removing the shoot tip (decapitation) causes cytokinins to flow to the buds, triggering branching.
然而，这一机制更为复杂。目前的证据表明，顶芽产生的生长素并非直接抑制侧芽，而是通过维持高水平的独脚金内酯并限制细胞分裂素向侧芽供应。摘除顶芽（打顶）会使细胞分裂素流向侧芽，进而引发分枝。
This interaction is a classic example of hormone crosstalk. By manipulating the auxin‑cytokinin ratio, horticulturists can control the shape and productivity of crops and ornamental plants.
这种相互作用是激素交叉调控的典型例子。通过调控生长素与细胞分裂素的比例，园艺学家可以控制作物和观赏植物的株型与产量。
6. Gibberellins and Stem Elongation | 赤霉素与茎伸长
Gibberellins (GAs) are a large family of tetracyclic diterpenoid compounds. The most common bioactive form is gibberellic acid (GA₃). Gibberellins stimulate both cell division and cell elongation in stems, leading to dramatic increases in stem height. This is especially evident in dwarf varieties of plants, which are often mutants defective in gibberellin biosynthesis or signalling.
赤霉素是一大类四环二萜化合物，最常见的活性形式是赤霉酸（GA₃）。赤霉素通过促进茎的细胞分裂和伸长，显著增加植株高度。这在矮生植物品种中尤为明显，这些矮化突变体通常在赤霉素合成或信号转导方面存在缺陷。
When applied to dwarf pea or maize plants, GA restores normal height. Unlike auxin, GA can act on whole internodes rather than just the zone of elongation. It enhances the activity of enzymes that loosen the cell wall and also promotes the expression of genes involved in cell cycle progression.
将赤霉素施加到矮生豌豆或玉米上可恢复其正常株高。与生长素不同，赤霉素能作用于整个节间而不仅仅是伸长区。它增强松弛细胞壁的酶活性，并促进与细胞周期进程相关基因的表达。
In many species, gibberellins also contribute to the transition from juvenile to adult growth phases, and their interaction with photoperiod and vernalisation is critical for flowering in long‑day plants.
在许多物种中，赤霉素还参与从幼年期到成年期的转变，并且与光周期和春化作用相互作用，对长日照植物的开花至关重要。
7. Gibberellins in Seed Germination | 赤霉素在种子萌发中的作用
A classic role of gibberellins is the mobilisation of food reserves during seed germination. In cereal grains such as barley, the embryo produces GA upon imbibition. GA diffuses to the aleurone layer, a thin layer of living cells surrounding the starchy endosperm, and triggers the synthesis and secretion of α‑amylase.
赤霉素的一个经典作用是种子萌发期间营养储备的动员。在大麦等谷物中，吸胀后胚会产生赤霉素。赤霉素扩散到糊粉层——围绕淀粉胚乳的薄层活细胞——并触发 α‑淀粉酶的合成与分泌。
α‑amylase hydrolyses starch into maltose, which is then further broken down into glucose and transported to the growing embryo to fuel respiration and growth. This hormonal control mechanism is exploited in the malting industry for beer production, where gibberellin application accelerates starch breakdown.
α‑淀粉酶将淀粉水解为麦芽糖，麦芽糖进一步分解为葡萄糖并运往生长中的胚，为呼吸与生长提供能量。这一激素调控机制被制麦芽业用于啤酒生产，通过施用赤霉素加速淀粉分解。
Experiments can demonstrate this using de‑embryonated barley half‑seeds incubated with GA; starch digestion is visible as a clear zone around the seed tissue, whereas no digestion occurs without GA. This supports the messenger role of gibberellin.
可用去胚大麦半粒种子与赤霉素一起培养来验证这一作用；淀粉消化在种子组织周围呈现透明区，而无赤霉素时不发生消化，证实了赤霉素的信使功能。
8. Cytokinins and Cell Division | 细胞分裂素与细胞分裂
Cytokinins are adenine derivatives that primarily promote cytokinesis (cell division) in plant shoots and roots. They are synthesised in actively growing tissues, particularly root tips, and are transported upwards via the xylem. The most abundant natural cytokinin is zeatin.
细胞分裂素是腺嘌呤衍生物，主要促进植物茎和根的胞质分裂。它们在根尖等活跃生长的组织中合成，并通过木质部向上运输。最丰富的天然细胞分裂素是玉米素。
In tissue culture, a high cytokinin‑to‑auxin ratio promotes shoot formation, while a high auxin‑to‑cytokinin ratio favours root formation. At intermediate ratios, callus (undifferentiated) tissue proliferates. This principle is the basis of micropropagation.
在组织培养中，高细胞分裂素/生长素比促进芽形成，而高生长素/细胞分裂素比促进根形成。在中间比例时，愈伤组织（未分化组织）增殖。这一原理是微繁殖的基础。
Cytokinins also delay leaf senescence by preventing the breakdown of chlorophyll and proteins, hence their use in extending the shelf life of leafy vegetables. They act in part by promoting nutrient sink activity, directing sugars and amino acids to cytokinin‑rich areas.
细胞分裂素还能通过阻止叶绿素和蛋白质的分解来延缓叶片衰老，因此可用于延长绿叶蔬菜的货架期。其作用机制部分是通过促进营养物质库活性，将糖和氨基酸导向富含细胞分裂素的区域。
9. Abscisic Acid (ABA) and Stress Response | 脱落酸与胁迫反应
Abscisic acid (ABA) is often called the ‘stress hormone’ because it accumulates under drought, salinity, and cold conditions. It is an isoprenoid compound synthesised from carotenoids, mainly in chloroplasts and roots. ABA’s most prominent role is in closing stomata during water deficit.
脱落酸（ABA）常被称为“胁迫激素”，因为它在干旱、盐碱和寒冷条件下积累。它是由类胡萝卜素合成的异戊二烯化合物，主要在叶绿体和根中产生。ABA 最显著的作用是在水分亏缺时关闭气孔。
When roots sense drying soil, ABA is transported to guard cells, where it triggers the opening of Ca²⁺ channels. The influx of calcium causes the opening of anion channels and the loss of K⁺ and malate²⁻ from guard cells, reducing turgor pressure and closing the pore. This reduces transpiration and conserves water.
当根感知到土壤干燥时，ABA 被运送到保卫细胞，触发 Ca²⁺ 通道开放。钙的内流引起阴离子通道开放，保卫细胞失去 K⁺ 和苹果酸根离子，膨压下降，气孔关闭，从而降低蒸腾作用、保存水分。
ABA also promotes seed dormancy and inhibits premature germination. High ABA levels in developing seeds maintain dormancy, whereas a drop in the ABA/gibberellin ratio releases dormancy and allows germination.
ABA 还能促进种子休眠并抑制过早萌发。发育种子中高水平的 ABA 维持休眠，而 ABA/赤霉素比例下降则解除休眠、允许萌发。
10. Ethylene and Fruit Ripening | 乙烯与果实成熟
Ethylene (C₂H₄) is unique among plant hormones as it is a gas at ambient temperatures. It is produced from the amino acid methionine via ACC (1‑aminocyclopropane‑1‑carboxylic acid). Ethylene regulates numerous developmental processes, most famously the ripening of climacteric fruits such as bananas, tomatoes, and apples.
乙烯（C₂H₄）在植物激素中独树一帜，因为它在环境温度下为气体。它由甲硫氨酸通过 ACC（1‑氨基环丙烷‑1‑羧酸）途径生成。乙烯调控诸多发育过程，最知名的是跃变型果实（如香蕉、番茄和苹果）的成熟。
During ripening, ethylene triggers the breakdown of cell walls by cellulases and pectinases, conversion of starches to sugars, and degradation of chlorophyll. A surge in ethylene production coordinates the ripening of an entire fruit, and interestingly, ethylene can induce its own synthesis in an autocatalytic feedback loop.
成熟过程中，乙烯激发纤维素酶和果胶酶降解细胞壁、淀粉转化为糖、叶绿素降解。乙烯产量激增协调了整个果实的成熟进程，有趣的是，乙烯还能通过自催化反馈环诱导自身合成。
Ethylene is also responsible for the ‘triple response’ in etiolated seedlings: reduction of stem elongation, swelling of the stem (radial expansion), and horizontal bending of the shoot. This response helps seedlings push through obstacles in the soil.
乙烯还负责黄化幼苗的“三重反应”：茎伸长减缓、茎增粗（径向扩展）和茎横向弯曲。该反应有助于幼苗穿破土壤中的障碍物。
11. Commercial Uses of Plant Hormones | 植物激素的商业应用
Understanding plant hormones allows for numerous agricultural and horticultural applications. Synthetic auxins such as 2,4‑D are used as selective herbicides against broad‑leaved dicot weeds, as they cause uncontrolled cell elongation and death in broad‑leaved plants while leaving monocot cereals unharmed.
了解植物激素促进了众多农业和园艺应用。如 2,4‑D 等人工合成的生长素用作针对阔叶双子叶杂草的选择性除草剂，它们导致阔叶植物细胞伸长失控而死亡，却不伤及单子叶禾谷类作物。
Rooting powders contain synthetic auxins like NAA or IBA to promote adventitious root formation on stem cuttings. Gibberellins are used to enlarge seedless grape berries, to increase internode length in sugar cane, and to synchronise flowering in fruit trees.
生根粉含有 NAA 或 IBA 等合成生长素，促进插条不定根的形成。赤霉素用于增大无籽葡萄果粒、增加甘蔗节间长度以及使果树花期同步。
Ethylene‑releasing compounds such as ethephon are sprayed to promote uniform ripening of fruits or to induce fruit drop (abscission) in cotton and cherry trees for mechanical harvesting. Conversely, inhibitors of ethylene perception, such as 1‑MCP, prolong the storage life of cut flowers and fruits.
乙烯释放剂如乙烯利被喷洒以促进果实均匀成熟或诱导棉花、樱桃树的果实脱落以便机械化采收。相反，乙烯感知抑制剂如 1‑MCP 可延长切花和水果的保鲜期。
Cytokinins are applied to leafy vegetables to delay yellowing, while ABA analogues can be investigated as drought‑tolerance agents. The hormone industry is founded on precise knowledge of concentration‑response curves and tissue specificity.
细胞分裂素用于延迟绿叶蔬菜的黄化，而脱落酸类似物正被研究作为抗旱剂。整个激素产业都基于对浓度‑响应曲线和组织特异性的精确认识。
12. Hormone Interactions and Exam Tips | 激素相互作用与考试技巧
In both IB and Edexcel examinations, it is critical to recognise that plant responses are rarely controlled by a single hormone. Instead, crosstalk and ratio effects dominate. For instance, the auxin/cytokinin ratio determines shoot vs root regeneration, the ABA/gibberellin ratio controls seed dormancy, and the auxin/ethylene balance can influence stem elongation and fruit drop.
在 IB 和 Edexcel 考试中，关键在于认识到植物的反应很少由单一激素控制，而是交叉调控与比例效应占主导。例如，生长素/细胞分裂素比例决定芽与根的再生，脱落酸/赤霉素比例控制种子休眠，生长素/乙烯的平衡影响茎伸长和落果。
A common exam question asks candidates to analyse experimental data on phototropism or gravitropism, such as agar block experiments or decapitation and auxin application. Be prepared to describe, explain, and evaluate evidence from classical Went and Boysen‑Jensen experiments. Diagrams showing coleoptile curvature are frequently used.
常见的考试题目要求分析向光性或向地性的实验数据，例如琼脂块实验或去顶与施加生长素的实验。考生应准备好描述、解释和评价来自经典温特和博伊森‑延森实验的证据。展示胚芽鞘弯曲的示意图经常出现。
Likewise, data analysis may involve seed germination experiments measuring the effect of GA on α‑amylase production, with negative controls lacking GA. Always refer to the aleurone layer as the target tissue. When discussing commercial uses, link the hormone’s natural role to the application, e.g. ethylene naturally ripens fruit → used to synchronise ripening.
同样，数据分析可能涉及测量赤霉素对 α‑淀粉酶产生影响的种子萌发实验，其中缺少赤霉素的为阴性对照。务必指明糊粉层是靶组织。讨论商业用途时，要将激素的天然角色与应用联系起来，比如乙烯天然使果实成熟→用于同步催熟。
Remember the following key comparisons: auxin causes cell elongation, gibberellin causes both cell elongation and division in stems; cytokinin promotes cell division; ABA is generally inhibitory; ethylene is a gaseous ripening and stress hormone. These simple distinctions, combined with specific examples, form a solid foundation for high‑mark answers.
记住以下关键对比：生长素引起细胞伸长，赤霉素在茎中既促进细胞伸长又促进分裂；细胞分裂素促进细胞分裂；脱落酸一般起抑制作用；乙烯是气体状的成熟与胁迫激素。这些简洁的区别，结合具体实例，将构成高分答案的坚实基础。
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