📚 Plant Hormones in IGCSE WJEC Biology | IGCSE WJEC 生物：植物激素考点精讲
Plants may not have a nervous system, but they still respond to their environment in precise and coordinated ways. These responses are controlled by chemical messengers called plant hormones. For IGCSE WJEC Biology, you need to understand how auxins control directional growth (tropisms), how other hormones like gibberellins and ethene influence plant development, and how we use plant hormones commercially. This article breaks down every key concept, experiment, and application you need to master.
植物虽然没有神经系统，但依然能够对环境做出精确协调的反应。这些反应由化学信使——植物激素控制。在IGCSE WJEC生物学中，你需要理解生长素如何控制向性生长、赤霉素和乙烯等其他激素如何影响植物发育，以及植物激素的商业化应用。本文逐一拆解你需要掌握的每一个核心概念、实验和实际应用。
1. What Are Plant Hormones? | 什么是植物激素？
Plant hormones are chemical substances produced in one part of a plant and transported to another, where they trigger a response even at very low concentrations. Unlike animal hormones, they are not made in specialised glands but in various tissues, often in growing regions like shoot tips, root tips, and developing seeds. They travel through the plant via the phloem or by cell‑to‑cell diffusion.
植物激素是在植物某个部位产生并运输到另一个部位的化学物质，极低浓度就能引发反应。与动物激素不同，它们并不是由专门的腺体产生，而是在多种组织中形成，常见于生长区域如茎尖、根尖和发育中的种子。它们通过韧皮部或细胞间扩散在植物体内运输。
The key plant hormones you need to know for WJEC are auxins, gibberellins, and ethene. Auxins are the main players in tropisms and apical dominance, while gibberellins control seed germination and stem elongation. Ethene is a gaseous hormone responsible for fruit ripening and leaf fall.
你需要为WJEC考试掌握的主要植物激素有：生长素、赤霉素和乙烯。生长素是向性反应和顶端优势的主角，赤霉素控制种子萌发和茎的伸长，乙烯是气态激素，负责果实成熟和落叶。
2. Auxins and Phototropism | 生长素与向光性
Phototropism is the growth of a plant in response to unidirectional light. Shoots exhibit positive phototropism – they grow towards the light. This response is driven by auxin, mainly indole‑3‑acetic acid (IAA). When light shines on one side of a shoot tip, auxin is redistributed to the shaded side. The higher auxin concentration on the dark side stimulates faster cell elongation, causing the shoot to bend towards the light.
向光性是植物对单向光的生长反应。茎表现出正向光性——向着光生长。这一反应由生长素（主要是吲哚‑3‑乙酸，IAA）驱动。当光照射到茎尖一侧时，生长素会重新分布到背光侧。背光侧较高的生长素浓度刺激细胞更快伸长，导致茎向光弯曲。
An important detail: auxin is produced in the shoot tip. If the tip is removed or covered, phototropism does not occur. This is one of the classic experimental observations that established the role of auxin. In the exam, you may be asked to interpret results from experiments where shoot tips are removed, covered with opaque caps, or separated by agar blocks.
关键细节：生长素在茎尖产生。如果去除茎尖或将其遮住，向光性就不会发生。这是确立生长素作用的经典实验观察之一。在考试中，你可能需要解读去除茎尖、用不透明帽覆盖或用琼脂块隔开等实验的结果。
3. Auxins and Gravitropism (Geotropism) | 生长素与向地性
Gravitropism is directional growth in response to gravity. Roots show positive gravitropism – they grow downwards, towards the pull of gravity. Shoots exhibit negative gravitropism – they grow upwards, against gravity. Auxin mediates both, but the response differs because roots and shoots have different sensitivities to auxin concentration.
向地性是植物对重力的定向生长反应。根表现为正向地性——向下生长，顺着重力方向。茎表现为负向地性——向上生长，逆着重力方向。生长素参与了这两种反应，但由于根和茎对生长素浓度的敏感度不同，最终效果相反。
In a horizontal root, auxin accumulates on the lower side due to gravity (statoliths in root cap cells settle downwards). In roots, a high auxin concentration inhibits cell elongation, so the lower side grows more slowly than the upper side, causing the root to bend downwards. In a horizontal shoot, auxin also settles on the lower side, but here high auxin promotes cell elongation, so the lower side grows faster, bending the shoot upwards.
在水平放置的根中，生长素由于重力作用在下侧积累（根冠细胞内的平衡石沉降）。对根而言，高浓度生长素抑制细胞伸长，因此下侧比上侧生长慢，导致根向下弯曲。在水平放置的茎中，生长素同样在下侧积累，但此时高浓度生长素促进细胞伸长，于是下侧生长更快，使茎向上弯曲。
4. Mechanism of Auxin Action | 生长素的作用机制
How does auxin actually make cells elongate? Auxin stimulates the active transport of hydrogen ions (H⁺) into the cell wall space, lowering the pH. This acidity activates enzymes called expansins, which loosen the bonds between cellulose microfibrils. The cell wall becomes more flexible, allowing the cell to take up water by osmosis and expand under turgor pressure.
生长素究竟怎样促使细胞伸长？它刺激氢离子通过主动运输进入细胞壁空间，降低pH值。这种酸性环境激活名为扩张蛋白的酶，它们松动纤维素微纤丝之间的键。细胞壁变得更有弹性，细胞通过渗透吸水并在膨压作用下扩张。
This is known as the acid‑growth hypothesis. It explains why auxin‑induced bending occurs only in the growing region (zone of elongation) just behind the tip. In the WJEC exam, you are not required to recall the acid‑growth hypothesis in full molecular detail, but understanding how auxin promotes cell elongation strengthens your answers on tropisms.
这就是酸生长假说。它解释了为什么生长素引起的弯曲只发生在尖端后方的伸长区。在WJEC考试中，你无需完整复述酸生长假说的分子细节，但理解生长素如何促进细胞伸长能帮助你更有力地回答向性相关问题。
5. Apical Dominance | 顶端优势
Auxin is also responsible for apical dominance – the phenomenon where the main shoot tip suppresses the growth of side branches. If the terminal bud is intact, auxin moves downwards and inhibits the outgrowth of lateral buds. When the tip is removed (pruned), auxin levels drop, and side branches begin to grow. This is why gardeners regularly pinch out the tips of bushy plants to encourage a fuller shape.
生长素还负责顶端优势——主茎顶端抑制侧枝生长的现象。如果顶芽完好，生长素向下运输并抑制侧芽的萌发。当顶端被摘除（修剪）时，生长素水平下降，侧枝才开始生长。这就是为什么园丁经常掐掉丛生植物的尖端，以促使株形更饱满。
In an exam, you might need to explain why decapitation (removing the shoot tip) leads to bushy growth. Link it to the reduced flow of auxin and the release of lateral buds from inhibition. This is a clear example of negative feedback‑like regulation by plant hormones.
考试中可能要求解释为何去顶（切除茎尖）会导致植株丛生。回答时需联系到生长素流量减少和侧芽解除抑制。这是植物激素实现类似负反馈调控的典型案例。
6. Classic Experiments on Phototropism | 向光性经典实验
Boysen‑Jensen (1913) and Went (1928) carried out elegant experiments to prove that a mobile chemical signal controls phototropism. Boysen‑Jensen inserted thin mica sheets or gelatin between the tip and the growing region. When mica was placed on the shaded side, the signal was blocked and no bending occurred. Gelatin allowed the signal to pass.
博伊森‑詹森（1913年）和温特（1928年）进行了精巧的实验，证明光向性由可移动的化学信号控制。博伊森‑詹森在尖端和生长区域之间插入薄云母片或明胶。当云母插在背光侧时，信号被阻断，不发生弯曲；明胶则允许信号通过。
Went’s experiment is perhaps the most famous. He cut off shoot tips and placed them on agar blocks, allowing auxin to diffuse into the agar. He then placed the agar blocks asymmetrically on decapitated shoots. The shoots bent away from the side with the auxin‑loaded block, even in the dark. This directly demonstrated that auxin alone could cause bending.
温特的实验或许最为著名。他切下茎尖放在琼脂块上，让生长素扩散到琼脂中。随后将琼脂块不对称地放在去顶的茎上。即使处于黑暗中，茎也会背着含生长素的琼脂块方向弯曲。这直接证明了生长素单独就能引起弯曲。
These experiments are highly examinable. You should be able to describe the procedures and explain what each control proves, such as the need for an intact tip, the diffusion of auxin, and the directionality of the response.
这些实验是高频考点。你需要能够描述实验步骤，并解释每个对照组证明了什么，例如完整茎尖的必要性、生长素的扩散性、反应的方向性等。
7. Gibberellins and Their Roles | 赤霉素及其作用
Gibberellins are another class of plant hormone, first discovered in a fungus that caused excessive stem growth in rice. In plants, gibberellins promote stem elongation (internode growth), stimulate the production of digestive enzymes in germinating seeds, and can induce flowering in some long‑day plants.
赤霉素是另一类植物激素，最初是在一种导致水稻茎杆过度生长的真菌中发现的。在植物中，赤霉素促进茎的伸长（节间生长），刺激萌发种子产生消化酶，并能在某些长日照植物中诱导开花。
During seed germination, water uptake triggers the embryo to release gibberellins. These travel to the aleurone layer, where they switch on genes that code for amylase. The amylase breaks down starch in the endosperm into sugars, providing energy for the growing embryo. This is a classic example of hormonal regulation of gene expression.
种子萌发时，吸水促使胚释放赤霉素。赤霉素移动到糊粉层，在那里开启编码淀粉酶的基因。淀粉酶将胚乳中的淀粉分解为糖，为生长中的胚提供能量。这是激素调控基因表达的经典实例。
8. Ethene and Its Roles | 乙烯及其作用
Ethene (C₂H₄) is unique among plant hormones because it is a gas. It diffuses easily through tissues and even between neighbouring plants. Its main functions are to accelerate fruit ripening, promote leaf abscission (shedding), and stimulate the breakdown of cell walls in the abscission zone.
乙烯（C₂H₄）在植物激素中很独特，因为它是一种气体。它易于通过组织扩散，甚至能在相邻植株之间传递。其主要功能是加速果实成熟、促进叶片脱落，并刺激离层细胞壁的分解。
Ethene production is often triggered by stress, injury, or the onset of ripening in climacteric fruits such as bananas and tomatoes. Once one fruit starts releasing ethene, it can trigger ripening in surrounding fruits – which is why placing an unripe banana with an apple helps it ripen faster.
乙烯的产生常由胁迫、损伤或跃变型果实（如香蕉、番茄）开始成熟引发。一旦某个果实开始释放乙烯，就能触发周围果实成熟——因此将生香蕉与苹果放在一起能让它更快变熟。
9. Commercial Uses of Auxins | 生长素的商业用途
Synthetic auxins are widely used in agriculture and horticulture. Rooting powders contain artificial auxins like NAA (naphthaleneacetic acid) or IBA (indolebutyric acid). When a cutting is dipped into rooting powder, auxin stimulates the development of adventitious roots, making propagation much faster and more reliable.
合成生长素在农业和园艺中广泛应用。生根粉含有萘乙酸（NAA）或吲哚丁酸（IBA）等人工生长素。将插条蘸取生根粉后，生长素刺激不定根发育，使繁殖更快、更可靠。
Selective weedkillers are another major application. Many broad‑leaved weeds are dicots and are far more sensitive to synthetic auxins like 2,4‑D than narrow‑leaved monocot crops such as wheat. The auxin causes uncontrolled growth and eventually death in the weeds, while the cereal crop is relatively unharmed. This is an excellent example of selective toxicity based on hormonal sensitivity.
选择性除草剂是另一重大应用。许多阔叶杂草是双子叶植物，对2,4‑D等合成生长素远比小麦等窄叶单子叶作物敏感。生长素导致杂草异常生长并最终死亡，而谷类作物相对不受伤害。这是基于激素敏感性实现选择性毒性的绝佳案例。
Other uses include inducing fruit set without fertilisation (parthenocarpy), preventing premature fruit drop in orchards, and controlling the timing of flowering in some crops.
其他用途包括无需受精即可诱导坐果（单性结实）、防止果园过早落果，以及控制某些作物的开花时间。
10. Commercial Uses of Other Hormones | 其他激素的商业应用
Gibberellins are used to produce seedless grapes by spraying clusters to elongate the fruit stalks and increase berry size. They are also used in brewing to speed up malting: gibberellin promotes amylase production, so barley starch is converted to sugar more quickly. In cherry and apple production, gibberellins can delay fruit ripening and skin ageing, extending shelf life.
赤霉素用于生产无籽葡萄，喷洒果穗可以拉长果梗并增大果粒。在酿造中用于加速制麦：赤霉素促进淀粉酶生成，使大麦淀粉更快转化为糖。在樱桃和苹果生产中，赤霉素能延缓果实成熟和果皮老化，延长货架期。
Ethene is used commercially to ripen fruit in a controlled way. Bananas are transported green and then exposed to ethene gas in ripening rooms before sale. The same hormone can be blocked (using compounds like 1‑MCP) to prevent over‑ripening of fruits in storage. In cotton and cherry orchards, ethene‑releasing compounds are used to promote leaf drop for easier mechanical harvesting.
乙烯在商业上用于果实受控催熟。香蕉在青绿时运输，上市前在催熟室内用乙烯气体处理。同一种激素也能被阻断（使用1‑MCP等化合物），防止储藏中的果实过熟。在棉花和樱桃生产中，使用释放乙烯的化合物促进落叶，以便机械采收。
11. Comparing Plant and Animal Hormones | 植物激素与动物激素比较
The hormone concept is universal in multicellular organisms, but the machinery differs between plants and animals. The table below highlights key contrasts that are useful for WJEC extended‑answer questions.
激素概念在多细胞生物中普遍存在，但植物和动物中的运作机制不同。下表突出了一些关键对比，对WJEC扩展型题目很有帮助。
| Feature | 特征 | Plant Hormones | 植物激素 | Animal Hormones | 动物激素 |
|---|---|---|
| Production site | 产生部位 | No dedicated glands; produced in meristems, root tips, seeds, etc. | 无专门腺体；产生于分生组织、根尖、种子等 | Endocrine glands (e.g. pancreas, thyroid) | 内分泌腺（如胰腺、甲状腺） |
| Transport | 运输方式 | Phloem, xylem, or cell‑to‑cell diffusion | 韧皮部、木质部或细胞间扩散 | Bloodstream | 血液循环 |
| Chemical nature | 化学性质 | Small organic molecules (e.g. IAA, gibberellins) or gas (ethene) | 小分子有机物（如IAA、赤霉素）或气体（乙烯） | Proteins, steroids, amino acid derivatives | 蛋白质、类固醇、氨基酸衍生物 |
| Speed of response | 反应速度 | Generally slower, sustained | 通常较慢、持续长 | Can be rapid (seconds to hours) | 可能很快（数秒至数小时） |
| Examples | 实例 | Auxin in phototropism, ethene in ripening | 向光性中的生长素，催熟中的乙烯 | Insulin in glucose control, adrenaline in fight‑or‑flight | 控制血糖的胰岛素，逃跑反应的肾上腺素 |
A common exam question asks you to compare plant and animal hormonal responses. Refer to the table above, but always link your points to specific examples – the examiner expects named hormones and their effects, not just generic statements.
常见的考题要求比较植物和动物的激素反应。可参考上表，但一定要用具体实例支撑论点——考官期望看到具体激素名称及其效应，而非空泛陈述。
12. Summary and Exam Tips | 总结与考试技巧
For IGCSE WJEC Biology, plant hormones appear in both short‑answer and extended‑response questions. Focus on the three main hormones (auxin, gibberellin, ethene) and their specific roles. Always relate the effect of auxin concentration to the tissue – a high concentration promotes growth in shoots but inhibits it in roots.
在IGCSE WJEC生物学中，植物激素在简答和拓展题中都会出现。重点掌握三种主要激素（生长素、赤霉素、乙烯）及其具体功能。始终将生长素浓度的效应与组织类型相联系——高浓度在茎中促进生长，在根中则抑制生长。
Learn the Went experiment well: what it shows, the setup, the control group, and how it provides evidence for a chemical messenger. When tackling commercial‑use questions, name the hormone, the product, and the target organism or crop. Use the correct terminology: phototropism, gravitropism, apical dominance, selective weedkiller, rooting powder, fruit ripening.
认真学习温特实验：它说明了什么、装置如何、对照组怎样设置，以及它如何为化学信使提供证据。解答商业化应用题目时，要写出激素名称、产品名称以及针对的作物或生物。使用正确术语：向光性、向地性、顶端优势、选择性除草剂、生根粉、果实催熟。
Finally, remember that ethene is a gas – this often surprises students. It is easy to forget, but examiners like to test this unique feature. And do not confuse gibberellin with auxin: gibberellin promotes seed germination and stem elongation, while auxin is more about tropisms and apical dominance.
最后，记住乙烯是气体——这一点常让学生感到意外。很容易忘记，但考官乐于考这个独特特征。也不要混淆赤霉素和生长素：赤霉素促进种子萌发和茎的伸长，而生长素主要参与向性反应和顶端优势。
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