📚 IGCSE Edexcel Biology: Plant Hormones – Key Exam Points | IGCSE Edexcel 生物：植物激素考点精讲
Plant hormones are chemical messengers that coordinate growth and responses to environmental stimuli in plants. For the IGCSE Edexcel Biology exam, you must understand the roles of auxins, gibberellins, and ethene, and how they are used commercially. This article distills the essential points to help you score high marks.
植物激素是协调植物生长和对环境刺激作出反应的化学信使。在IGCSE爱德思生物考试中，你必须理解生长素、赤霉素和乙烯的作用及其商业用途。本文将提炼关键考点，助你取得高分。
1. What Are Plant Hormones? | 什么是植物激素？
Plant hormones (often called plant growth regulators) are chemical substances produced in one region of the plant and transported to target tissues, where they cause physiological responses such as cell elongation, cell division, or fruit ripening. They are active in very low concentrations and often interact with one another.
植物激素（常称植物生长调节剂）是在植物某一部位产生并运输到靶组织的化学物质，它们在那里引发生理反应，如细胞伸长、细胞分裂或果实成熟。它们在极低浓度下即有活性，并且常相互协同或拮抗。
Unlike animal hormones, plant hormones are not produced in specialised glands. Many are made in actively growing regions, such as shoot and root tips, and then diffuse or are actively transported to other parts. Their effect depends on the concentration and the sensitivity of the target cells.
与动物激素不同，植物激素并非由专门腺体产生。许多是在活跃生长的区域如茎尖和根尖制造，然后通过扩散或主动运输至其他部位。其效应取决于浓度和靶细胞的敏感性。
The key hormones for Edexcel IGCSE are auxins, gibberellins, and ethene. You should be able to explain their roles in controlling plant growth and their commercial applications.
爱德思IGCSE需要掌握的关键激素有生长素、赤霉素和乙烯。你应能解释它们在调控植物生长中的作用及其商业应用。
2. Auxin and Phototropism: Bending Towards Light | 生长素与向光性：向光弯曲
Phototropism is a directional growth response to light. In shoots, auxin is produced in the growing tip and diffuses downwards. When light shines from one side, auxin is redistributed and accumulates more on the shaded side of the shoot.
向光性是一种对光的方向性生长反应。在茎中，生长素在生长尖端产生并向下扩散。当单侧光照射时，生长素重新分布，在茎的背光一侧积累更多。
The higher concentration of auxin on the shaded side stimulates faster cell elongation on that side. As a result, the shaded cells lengthen more than the cells on the illuminated side, causing the shoot tip to bend towards the light. This is positive phototropism in shoots.
背光侧较高的生长素浓度刺激该侧细胞更快伸长。因此，背光侧细胞比向光侧细胞伸长得更多，导致茎尖弯向光源。这是茎的正向光性。
In roots, a high concentration of auxin actually inhibits cell elongation, so roots do not display the same positive phototropic response. You only need to know that shoots are positively phototropic.
在根中，高浓度生长素实际上会抑制细胞伸长，因此根不会表现出同样的正向光性。你只需知道茎具有正向光性即可。
Classic experiments: if the shoot tip is removed or covered with an opaque cap, the shoot no longer bends towards light, proving that the tip detects the light and produces auxin.
经典实验：若去除茎尖或用不透光帽盖住，茎就不再向光弯曲，证明茎尖是感光部位并产生生长素。
3. Auxin and Geotropism (Gravitropism): Roots Down, Shoots Up | 生长素与向地性：根朝下，茎朝上
Geotropism (or gravitropism) is a growth response to gravity. When a root is placed horizontally, statoliths (dense starch grains) settle to the lower side of root cap cells, causing auxin to accumulate on the lower side of the root tip.
向地性是对重力的生长反应。当根水平放置时，平衡石（致密的淀粉粒）沉降至根冠细胞的下侧，导致生长素在根尖下侧积累。
In roots, cells are more sensitive to auxin. A high auxin concentration on the lower side inhibits cell elongation there, so the cells on the upper side elongate faster. This causes the root tip to bend downwards – positive gravitropism.
根细胞对生长素更敏感。下侧的高浓度生长素抑制该处细胞伸长，因此上侧细胞生长更快。这使根尖向下弯曲——正向地性。
In shoots, the same gravity-induced auxin accumulation on the lower side stimulates cell elongation on that side. The lower side grows faster, and the shoot bends upwards – negative gravitropism.
在茎中，同样的重力导致的生长素在下侧积累却刺激该侧细胞伸长。下侧生长更快，茎向上弯曲——负向地性。
Removing the root cap prevents the root from detecting gravity, demonstrating the role of the tip in sensing the stimulus.
切除根冠会阻止根感知重力，证明根尖在感受刺激中的作用。
4. Mechanism of Auxin Action: Unequal Distribution and Cell Elongation | 生长素作用机制：分布不均与细胞伸长
Auxin binds to receptor proteins on the cell membrane, activating proton pumps that move hydrogen ions (H⁺) into the cell wall. This acidifies the wall and lowers the pH.
生长素与细胞膜上的受体蛋白结合，激活质子泵，将氢离子（H⁺）泵入细胞壁，使壁酸化、pH降低。
In the acidic environment, enzymes called expansins are activated. They loosen the cross-links between cellulose microfibrils, making the cell wall more flexible. Water enters the cell by osmosis, increasing turgor pressure, and the cell wall stretches, causing the cell to elongate.
在酸性环境中，称为扩张蛋白的酶被激活。它们松开了纤维素微纤维之间的交联，使细胞壁更加柔软。水分通过渗透进入细胞，增加膨压，细胞壁伸展，导致细胞伸长。
This acid-growth mechanism explains how auxin promotes rapid elongation without needing new wall materials to be synthesised immediately. When auxin concentration is high, elongation is promoted; when it is excessively high in roots, it can trigger inhibitory pathways, halting growth.
这种酸生长机制解释了生长素如何在不立即合成新壁材的情况下促进快速伸长。当生长素浓度高时促进伸长；在根中浓度过高时会触发抑制途径，停止生长。
5. Commercial Use: Auxins as Selective Weedkillers | 商业应用：生长素作为选择性除草剂
Synthetic auxins, such as 2,4-D (2,4-dichlorophenoxyacetic acid), mimic natural auxin but are more stable. When applied to fields, they are absorbed more readily by broad-leaved weeds (dicots) than by narrow-leaved crops like wheat or maize (monocots).
合成的生长素如2,4-D（2,4-二氯苯氧乙酸）模拟天然生长素但更稳定。喷洒到田间时，它们更容易被阔叶杂草（双子叶植物）吸收，而不易被小麦或玉米等禾本科作物（单子叶植物）吸收。
Inside the dicot weeds, the synthetic auxin causes uncontrolled, abnormal growth – stems twist, leaves curl, and vascular tissues are disrupted. The weed cannot transport nutrients effectively and eventually dies, while the cereal crop remains largely unharmed.
在双子叶杂草内部，合成生长素引起失控的异常生长——茎扭曲、叶片卷缩、维管组织被破坏。杂草无法有效运输养分并最终死亡，而谷物作物基本不受影响。
This selectivity makes auxin-based herbicides extremely useful in agriculture, reducing competition for light, water, and minerals and increasing crop yields.
这种选择性使得基于生长素的除草剂在农业中极为有用，能减少对光、水和矿物质的竞争，提高作物产量。
6. Commercial Use: Auxins in Rooting Powders | 商业应用：生长素在生根粉中的应用
When taking cuttings from a parent plant, the cut end can be dipped into a rooting powder containing a low concentration of synthetic auxin (usually IBA or NAA). This stimulates undifferentiated cells to develop into root primordia, giving rise to adventitious roots.
从母株上取插条时，可将切端蘸入含有低浓度合成生长素（通常是IBA或NAA）的生根粉中。这会刺激未分化细胞发育为根原基，进而形成不定根。
The new roots enable the cutting to absorb water and minerals, allowing it to grow into an independent plant quickly. This technique is widely used in horticulture to propagate roses, geraniums, and many other ornamentals, producing genetically identical plants.
新根使插条能够吸收水分和矿物质，迅速长成独立植株。该技术在园艺中广泛用于繁殖月季、天竺葵及许多其他观赏植物，生产出基因相同的植株。
Without auxin treatment, many cuttings fail to root or take much longer, so rooting powders greatly increase the success rate of vegetative propagation.
若不用生长素处理，许多插条无法生根或耗时更长，因此生根粉极大提高了营养繁殖的成功率。
7. Gibberellins: Ending Seed Dormancy and Promoting Germination | 赤霉素：打破种子休眠与促进萌发
Gibberellins are a group of hormones that play a key role in seed germination. When a seed absorbs water, the embryo produces gibberellins that diffuse to the aleurone layer surrounding the endosperm.
赤霉素是一类在种子萌发中起关键作用的激素。当种子吸水后，胚产生赤霉素，扩散至包围胚乳的糊粉层。
The gibberellins trigger the aleurone cells to synthesise digestive enzymes, especially α-amylase. These enzymes are secreted into the endosperm and hydrolyse stored starch into sugars such as glucose and maltose.
赤霉素促使糊粉层细胞合成消化酶，尤其是α-淀粉酶。这些酶分泌到胚乳中，将储存的淀粉水解为葡萄糖和麦芽糖等糖类。
The sugars are then absorbed by the growing embryo to fuel respiration and growth, breaking seed dormancy and allowing germination to proceed. This natural mechanism is exploited in the malting industry, where gibberellins are added to barley grains to speed up starch breakdown for beer production.
生长的胚吸收这些糖类，为呼吸和生长提供能量，从而打破种子休眠、开始萌发。这一自然机理被用于制麦芽工业，将赤霉素加入大麦粒中以加速淀粉分解，用于啤酒生产。
8. Gibberellins: Stem Elongation and Fruit Development | 赤霉素：茎伸长与果实发育
Gibberellins promote internode elongation in stems. They stimulate both cell division in the intercalary meristems and cell elongation, resulting in taller plants. Dwarf varieties of plants often lack the ability to produce sufficient gibberellins; spraying them with gibberellin restores normal height.
赤霉素促进茎的节间伸长。它们刺激居间分生组织的细胞分裂和细胞伸长，使植株增高。矮化品种通常缺乏产生足够赤霉素的能力，喷洒赤霉素可使其恢复正常高度。
In fruit production, gibberellins can be applied to grape flowers to promote the development of seedless (parthenocarpic) fruits. The grapes grow larger and have looser clusters, which reduces fungal infections and improves market quality.
在果实生产中，可将赤霉素施用于葡萄花，促进无籽（单性结实）果实的发育。葡萄个头更大，果穗疏松，可减少真菌感染，提高商品品质。
Gibberellins also delay senescence in citrus fruits. Spraying on orange trees can keep the rind from ageing, allowing fruits to be left on the tree longer and extending the harvesting window.
赤霉素还能延缓柑橘果实的衰老。喷洒在橙树上可防止果皮老化，使果实更久地留在树上，延长采收窗口期。
9. Ethene: Controlling Fruit Ripening | 乙烯：控制果实成熟
Ethene (ethylene) is a simple hydrocarbon gas that acts as a plant hormone. It is naturally produced by ripening fruits and triggers a cascade of changes: starch is converted into sugars, cell walls soften due to pectin breakdown, and chlorophyll is lost while other pigments develop.
乙烯是一种简单的碳氢气体，发挥植物激素的作用。它由正在成熟的果实自然产生，并触发一连串变化：淀粉转化为糖，细胞壁因果胶分解而软化，叶绿素消失而其他色素显现。
Commercially, fruits such as bananas and tomatoes are harvested when they are still green and hard. They are transported in refrigerated containers to slow ripening, then exposed to a controlled concentration of ethene gas in ripening rooms. This ensures uniform colour, texture, and sweetness, ready for retail display.
商业上，香蕉和西红柿等果实在仍青绿、坚实时采收。它们在冷藏箱中运输以延缓成熟，然后在催熟室中暴露于受控浓度的乙烯气体下。这确保了颜色、质地和甜度均匀一致，达到零售标准。
Ethene can also be removed from storage environments using potassium permanganate scrubbers to delay ripening and extend shelf life. This counteracts the ethylene naturally given off by stored fruit.
乙烯也可以利用高锰酸钾洗涤器从储存环境中移除，以延迟成熟、延长货架期。这抵消了储存果实自然释放的乙烯作用。
10. Summary and Exam Tips | 总结与考试技巧
The table below condenses the essential information about the three plant hormones you must know for the exam. Use it for last-minute revision.
下表浓缩了考试必须掌握的三种植物激素的关键信息，可用于考前快速复习。
| Hormone | Key Natural Functions | Commercial Applications |
|---|---|---|
| Auxin | Causes positive phototropism in shoots; causes positive gravitropism in roots and negative gravitropism in shoots by unequal distribution | Selective weedkillers (e.g. 2,4-D); rooting powders for cuttings |
| Gibberellins | Break seed dormancy by stimulating amylase synthesis; promote stem internode elongation; involved in fruit development | Malting of barley; producing seedless grapes; increasing fruit size; delaying citrus fruit aging |
| Ethene | Triggers fruit ripening: colour change, softening, conversion of starch to sugar | Controlled ripening of bananas, tomatoes, and other fruits; removal to extend shelf life |
中文版总结表：
| 激素 | 主要天然功能 | 商业应用 |
|---|---|---|
| 生长素 | 通过分布不均导致茎的正向光性；根的正向地性和茎的负向地性 | 更多咨询请联系16621398022（同微信）
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