📚 Plant Hormones: Key Points for A-Level CIE Biology | 植物激素：A-Level CIE 生物考点精讲
Plant hormones (phytohormones) are chemical signals that coordinate growth, development, and responses to environmental changes in plants. In the CIE A-Level Biology syllabus, a detailed understanding of five major classes of plant hormones — auxins, gibberellins, cytokinins, abscisic acid, and ethene — is required. This article summarises all the essential concepts, experimental evidence, mechanisms of action, and commercial applications you need to master for your examinations.
植物激素（植物激素）是协调植物生长、发育以及响应环境变化的化学信号。在 CIE A-Level 生物课程大纲中，要求详细掌握五大类植物激素：生长素、赤霉素、细胞分裂素、脱落酸和乙烯。本文总结了考试需要掌握的所有核心概念、实验证据、作用机制和商业应用。
1. Introduction to Plant Hormones | 植物激素概述
Unlike animals, plants do not have a nervous system. Instead, they use chemical messengers called plant hormones to regulate their activities. Plant hormones are produced in small quantities in one region of the plant and are often transported to target tissues, where they can stimulate or inhibit specific physiological responses. They may act locally or at a distance, and a single hormone can have multiple effects depending on its concentration, the site of action, and the presence of other hormones.
与动物不同，植物没有神经系统。它们使用称为植物激素的化学信使来调节自身活动。植物激素在植物某一区域少量产生，通常运输到靶组织，在那里刺激或抑制特定的生理反应。它们可能局部作用或通过长距离运输起作用，并且一种激素可能根据其浓度、作用位点以及其他激素的存在而产生多种效应。
The five major groups covered in CIE A-Level are auxin (e.g., indole-3-acetic acid, IAA), gibberellins (e.g., gibberellic acid, GA₃), cytokinins (e.g., zeatin), abscisic acid (ABA), and ethene (ethylene). Their roles range from cell elongation and division to seed germination, stomatal closure, and fruit ripening.
CIE A-Level 涵盖的五大类激素包括：生长素（如吲哚-3-乙酸，IAA）、赤霉素（如赤霉酸，GA₃）、细胞分裂素（如玉米素）、脱落酸（ABA）和乙烯（乙烯）。它们的作用涵盖细胞伸长与分裂、种子萌发、气孔关闭和果实成熟等各个方面。
2. Auxin: Discovery and Went’s Experiment | 生长素的发现与温特实验
The existence of auxin was first suggested by Charles Darwin and his son Francis through experiments with grass coleoptiles. They showed that when the tip of a coleoptile was removed, the seedling no longer bent towards unilateral light, but if the tip was replaced, the bending response was restored. They concluded that some chemical influence was transmitted from the tip to the growing region below.
生长素的存在最早由查尔斯·达尔文和他的儿子弗朗西斯通过草胚芽鞘实验提出。他们证明，当胚芽鞘尖端被切除后，幼苗不再向单侧光弯曲，但如果将尖端放回，弯曲反应则恢复。他们推断，某种化学影响从尖端传递到下方的生长区。
Later, Frits Went devised an elegant experiment that confirmed the chemical nature of the signal. He cut off coleoptile tips and placed them on agar blocks, allowing the chemical to diffuse into the agar. When an agar block was placed asymmetrically on a decapitated coleoptile, the coleoptile bent away from the side where the block was placed, even in the dark. Went named this growth-promoting substance ‘auxin’.
后来，弗里茨·温特设计了一项精妙的实验，证实了信号的化学本质。他切下胚芽鞘尖端，放置在琼脂块上，使化学物质扩散到琼脂中。当琼脂块不对称地置于去顶胚芽鞘上时，胚芽鞘向琼脂块放置侧的对面弯曲，即便在黑暗中也如此。温特将这种促进生长的物质命名为“生长素”。
3. Auxin: Polar Transport and the Chemiosmotic Model | 生长素的极性运输与化学渗透模型
Auxin (mainly IAA) is synthesised in shoot and root apical meristems and young leaves. It is transported unidirectionally from the apex towards the base — this is known as polar transport. Polar transport requires energy and is independent of gravity. In roots, auxin also moves towards the root tip and then upwards in the outer tissues, but the classical polar pathway is from shoot tip to base.
生长素（主要是 IAA）在茎尖、根尖分生组织和幼叶中合成。它以极性的方式从顶端向基部单向运输——这被称为极性运输。极性运输需要能量，且不受重力影响。在根中，生长素也会向根尖移动，然后在外层组织中向上，但经典的极性途径是从茎尖到基部。
The chemiosmotic model explains how auxin moves through cells. The cell wall has an acidic pH (about 5.5), where a fraction of IAA (a weak acid) remains protonated (IAAH) and can diffuse across the plasma membrane into the neutral cytoplasm (pH ~7). Inside the cell, IAAH dissociates into IAA⁻, which cannot diffuse back out. Therefore, auxin must exit the cell via specific efflux carrier proteins called PIN proteins, which are located asymmetrically at the basal end of the cell. This arrangement drives unidirectional transport.
化学渗透模型解释了生长素如何在细胞间移动。细胞壁具有酸性 pH 值（约 5.5），部分 IAA（弱酸）保持质子化形式（IAAH），可以扩散穿过质膜进入中性细胞质（pH 约 7）。在细胞内，IAAH 解离成 IAA⁻，无法自由扩散出去。因此，生长素必须通过特定的输出载体蛋白——PIN 蛋白离开细胞，这些蛋白不对称地分布在细胞的基端。这一安排驱动了单向运输。
The activity of PIN proteins can be regulated by environmental signals, leading to changes in auxin distribution, which is crucial for tropic responses.
PIN 蛋白的活性可受到环境信号的调节，导致生长素分布发生变化，这对于向性反应至关重要。
4. Auxin in Phototropism and Geotropism | 生长素与向光性、向地性
In phototropism, unilateral light causes a redistribution of auxin to the shaded side of a shoot. Auxin stimulates cell elongation by increasing the plasticity of the cell wall; therefore, the shaded side elongates more than the illuminated side, and the shoot bends towards the light. The redistribution is thought to occur because light induces PIN proteins to relocate, transporting auxin laterally towards the shaded side.
在向光性中，单侧光导致生长素向茎的背光侧重新分布。生长素通过增加细胞壁的可塑性来促进细胞伸长；因此，背光侧比向光侧伸长快，茎向光弯曲。这种重新分布被认为是由于光诱导 PIN 蛋白重新定位，将生长素横向运输至背光侧。
In roots, auxin also mediates gravitropism (geotropism). Amyloplasts (statoliths) in root cap cells settle under gravity, triggering a redistribution of auxin to the lower side of the root. However, root cells respond differently from shoot cells: high auxin concentration inhibits cell elongation in roots. Thus, the lower side grows more slowly than the upper side, causing the root to bend downwards.
在根中，生长素也介导向地性。根冠细胞中的淀粉体（平衡石）在重力作用下沉降，触发生长素重新分布到根的下侧。然而，根细胞与茎细胞反应不同：高浓度生长素抑制根细胞伸长。因此，下侧生长比上侧慢，导致根向下弯曲。
5. Auxin: Apical Dominance and Other Effects | 生长素的顶端优势及其他作用
Apical dominance is the phenomenon whereby the shoot apex inhibits the growth of lateral buds. Auxin produced in the apical bud is transported downwards and suppresses the outgrowth of axillary buds further down the stem. If the leading shoot tip is removed (decapitation), lateral buds are released from inhibition and begin to grow. This response can be mimicked in decapitated plants by applying auxin to the cut stump, confirming the dominant role of auxin.
顶端优势是指茎顶端抑制侧芽生长的现象。顶芽产生的生长素向下运输，抑制下方腋芽的萌发。如果去除主茎尖端（打顶），侧芽便会解除抑制，开始生长。在去顶植物上，若将生长素施于切口处，可以模拟顶端优势的效果，证实生长素的主导作用。
Other auxin effects include promotion of fruit development (parthenocarpy can be induced by auxin application to unpollinated flowers), stimulation of adventitious root formation in cuttings, and use as selective herbicides (e.g., 2,4-D) that disrupt normal growth in dicotyledonous weeds.
生长素的其他作用包括促进果实发育（对未授粉的花朵施用生长素可诱导单性结实）、刺激插条不定根的形成，以及作为选择性除草剂（如 2,4-D）破坏双子叶杂草的正常生长。
6. Gibberellins: Discovery and Stem Elongation | 赤霉素的发现与茎伸长
Gibberellins were first discovered in Japan in connection with a fungal disease of rice that caused excessive stem elongation (‘foolish seedling’ disease). The fungus Gibberella fujikuroi produced a substance that promoted growth; this was later identified as gibberellic acid (GA). Plants themselves also produce gibberellins, which are involved in stem elongation, seed germination, and flowering.
赤霉素最早在日本被发现，与引起水稻过度茎伸长的真菌病害（“恶苗病”）有关。真菌 Gibberella fujikuroi 产生一种促进生长的物质，后来被鉴定为赤霉酸（GA）。植物自身也能合成赤霉素，参与茎伸长、种子萌发和开花等过程。
Gibberellins promote stem elongation by stimulating both cell division and cell elongation. They overcome the dwarf phenotype in certain mutant plants (e.g., dwarf peas) when applied externally. The mechanism involves the degradation of DELLA proteins, which are repressors of growth; once DELLA proteins are removed, transcription factors that promote growth become active.
赤霉素通过刺激细胞分裂和细胞伸长来促进茎伸长。当外源施加时，它们可以克服某些突变植物（如矮生豌豆）的矮化表型。其机制涉及 DELLA 蛋白的降解，这些蛋白是生长的抑制因子；一旦 DELLA 蛋白被移除，促进生长的转录因子便被激活。
7. Gibberellins in Seed Germination: α-Amylase Induction | 赤霉素在种子萌发中的作用：诱导 α-淀粉酶
One of the classic experiments for the CIE syllabus involves the role of gibberellins in mobilising food reserves during seed germination. In cereal grains such as barley, the embryo releases gibberellins upon imbibition. The GA diffuses to the aleurone layer, a thin layer of living cells surrounding the endosperm. Here, it triggers the transcription of the gene encoding α-amylase, an enzyme that hydrolyses starch stored in the endosperm into soluble sugars (maltose and glucose).
CIE 大纲中一项经典实验涉及赤霉素在种子萌发过程中动员营养储存的作用。在大麦等谷物种子中，吸胀后胚释放赤霉素。GA 扩散到包围胚乳的活细胞薄层——糊粉层。在这里，它触发编码 α-淀粉酶的基因转录，该酶将储存在胚乳中的淀粉水解为可溶性糖（麦芽糖和葡萄糖）。
These sugars are then transported to the growing embryo to provide energy and carbon skeletons for growth until the seedling becomes photosynthetic. The involvement of GA can be demonstrated by removing the embryo: a de-embryonated seed half does not produce α-amylase, but if GA is applied, α-amylase production is restored.
这些糖随后被运输到生长中的胚，为生长提供能量和碳骨架，直到幼苗能够进行光合作用。赤霉素的参与可以通过去除胚来证明：去胚的半粒种子不产生 α-淀粉酶，但若施用 GA，则 α-淀粉酶生成恢复。
8. Cytokinins: Cell Division and Delaying Senescence | 细胞分裂素：促进分裂与延缓衰老
Cytokinins are a group of hormones that promote cytokinesis (cell division). They are produced in actively dividing tissues, particularly in root apical meristems, and are transported upwards through the xylem. Together with auxin, cytokinins control the cell cycle and influence organ differentiation in tissue culture: a high cytokinin-to-auxin ratio promotes shoot formation, whereas a low ratio favours root formation.
细胞分裂素是一类促进胞质分裂（细胞分裂）的激素。它们在活跃分裂的组织中产生，特别是根尖分生组织，并通过木质部向上运输。与生长素一起，细胞分裂素控制细胞周期，影响组织培养中的器官分化：细胞分裂素与生长素比例高则促进芽形成，比例低则促进根形成。
A notable effect of cytokinins is the delay of leaf senescence (the natural aging process in leaves). A detached leaf placed in a cytokinin solution stays green and maintains protein synthesis longer than a control in water. This is because cytokinins promote nutrient mobilisation and prevent chlorophyll breakdown.
细胞分裂素的一个显著作用是延缓叶片衰老（叶片自然的衰老过程）。将离体叶片置于细胞分裂素溶液中，比在水中保持绿色的时间更长，并维持蛋白质合成更久。这是因为细胞分裂素促进养分动员并防止叶绿素降解。
9. Abscisic Acid (ABA): Stomatal Closure and Seed Dormancy | 脱落酸：气孔关闭与种子休眠
Abscisic acid (ABA) is sometimes referred to as the ‘stress hormone’. Under drought conditions, roots sense the water deficit and produce ABA, which is transported via the xylem to the leaves. In guard cells, ABA binds to receptors, leading to an increase in cytosolic Ca²⁺ concentration. This triggers the opening of anion channels, allowing Cl⁻ and malate²⁻ to leave. At the same time, K⁺ channels are inhibited, causing a massive loss of K⁺ from the guard cells. The water potential of the guard cells rises (becomes less negative), water leaves by osmosis, and the guard cells become flaccid — the stomata close.
脱落酸（ABA）有时被称为“胁迫激素”。在干旱条件下，根感知水分亏缺并产生 ABA，它通过木质部运输到叶片。在保卫细胞中，ABA 与受体结合，导致胞质 Ca²⁺ 浓度升高。这触发了阴离子通道的开放，使 Cl⁻ 和苹果酸根离子外流。同时，K⁺ 通道被抑制，导致保卫细胞大量失去 K⁺。保卫细胞的水势上升（变得不那么负），水分通过渗透作用流出，保卫细胞变得松弛——气孔关闭。
ABA also plays a central role in seed dormancy. It promotes the synthesis of storage proteins and inhibits germination by blocking the production of enzymes such as α-amylase. The balance between ABA and gibberellins regulates the transition from dormancy to germination: a decline in ABA and an increase in GA are key for germination to proceed.
ABA 还在种子休眠中起核心作用。它促进贮藏蛋白的合成，并通过阻断 α-淀粉酶等酶的产生来抑制萌发。ABA 与赤霉素之间的平衡调控着从休眠到萌发的过渡：ABA 减少和 GA 增加是萌发进行的关键。
10. Ethene: Fruit Ripening and the Triple Response | 乙烯：果实成熟与三重反应
Ethene (ethylene) is unique among plant hormones because it is a gas. It is produced by most plant tissues, especially during abscission, senescence, and fruit ripening. Ethene promotes the ripening of climacteric fruits such as bananas, tomatoes, and apples. A burst of ethene production triggers the conversion of starch to sugar, softening of the fruit by cell wall digestion, and colour changes. Because ethene is a gas, one ripening fruit can signal neighbouring fruits to ripen.
乙烯（ ethene/ethylene ）在植物激素中独一无二，因为它是气体。大多数植物组织都能产生乙烯，尤其在脱落、衰老和果实成熟期间。乙烯促进呼吸跃变型果实（如香蕉、番茄和苹果）的成熟。乙烯产量的一次爆发触发了淀粉向糖的转化、细胞壁消化导致的果实软化以及颜色变化。由于乙烯是气体，一个成熟中的果实可以发出信号，促使邻近果实成熟。
Ethene also induces the ‘triple response’ in etiolated seedlings grown in the dark: inhibition of hypocotyl elongation, swelling of the hypocotyl (radial expansion), and horizontal growth (diagravitropism). This response helps the seedling navigate around obstacles in the soil. Commercially, ethene gas or compounds that release ethene (such as ethephon) are used to ripen fruit uniformly for market.
乙烯还会在暗处生长的黄化幼苗中诱导“三重反应”：抑制下胚轴伸长、下胚轴加粗（径向膨大）以及水平生长（横向向地性）。这一反应有助于幼苗绕过土壤中的障碍物。在商业上，乙烯气体或释放乙烯的化合物（如乙烯利）用于使水果成熟均匀，以便上市。
11. Hormonal Interactions and Commercial Applications | 激素间的相互作用与商业应用
Plant hormones do not act in isolation; they interact synergistically or antagonistically. For instance, auxin and gibberellins often act synergistically to promote stem elongation, while auxin and cytokinins show antagonism in apical dominance: auxin maintains it, cytokinins released from roots promote lateral bud outgrowth. The auxin-to-cytokinin ratio in tissue culture determines whether roots or shoots develop. Similarly, ABA and gibberellins act antagonistically in seed germination.
植物激素并非单独作用；它们之间存在协同或拮抗的相互作用。例如，生长素和赤霉素常协同促进茎伸长，而生长素与细胞分裂素在顶端优势中表现拮抗：生长素维持顶端优势，来自根的细胞分裂素则促进侧芽生长。组织培养中生长素与细胞分裂素的比例决定是长根还是长芽。同样，ABA 与赤霉素在种子萌发中相互拮抗。
| Hormone | Key Effects | Key Commercial Uses |
| Auxin (IAA) | Cell elongation, tropisms, apical dominance, root initiation | Rooting powder, selective herbicides (2,4-D), parthenocarpic fruits |
| Gibberellins | Stem elongation, seed germination (α-amylase), flowering | Breaking seed dormancy, boosting grape size (seedless grapes), increasing sugarcane yield |
| Cytokinins | Cell division, delay of leaf senescence, shoot differentiation | Tissue culture, extending shelf-life of leafy vegetables |
| ABA | Stomatal closure under stress, seed dormancy maintenance | Not directly applied commercially; research in drought tolerance |
| Ethene | Fruit ripening, leaf abscission, triple response | Fruit ripening (bananas, tomatoes), ethephon as a spray to synchronise fruit maturity |
By combining knowledge of hormone synthesis, transport, perception, and interactions, you can explain how plants adapt to their environment and how humans manipulate these processes for agriculture. Remember to link experiments (Went, half-seed GA test) to the underlying concepts in your exam answers.
通过结合激素合成、运输、感知和相互作用的知识，你可以解释植物如何适应环境，以及人类如何利用这些过程为农业服务。在考试答案中，务必记得将实验（温特实验、GA 半粒种子实验）与基本概念联系起来。
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