📚 Plant Hormones – IGCSE OCR Biology | 植物激素考点精讲
Unlike animals, plants do not have specialised nervous systems. Instead, they rely on chemical messengers called plant hormones to coordinate growth and development in response to environmental stimuli. These hormones are produced in specific regions, such as the shoot tips and root tips, and then transported to target cells where they trigger responses like elongation, germination or ripening. Understanding how plant hormones work is essential for IGCSE OCR Biology, not only for explaining tropisms but also for appreciating their widespread commercial applications in agriculture and horticulture.
与动物不同，植物没有专门的神经系统，它们依靠一类名为植物激素的化学信使，来协调因应环境刺激的生长与发育过程。这些激素在茎尖、根尖等特定区域产生，随后被运输到靶细胞，引发诸如伸长、萌发或成熟等反应。理解植物激素的工作原理是 IGCSE OCR 生物的重要内容，既要解释向性运动，也要认识它们在农业与园艺中的广泛商业应用。
1. Introduction to Plant Hormones | 植物激素简介
Plant hormones are organic substances that regulate plant growth and development. Unlike animal hormones, which are often produced in endocrine glands and transported by the bloodstream, plant hormones move by diffusion and active transport from cell to cell. They act at very low concentrations, and a single hormone can have different effects depending on its concentration, the type of target tissue, and the developmental stage of the plant.
植物激素是调控植物生长与发育的有机物质。与动物激素通常由内分泌腺产生并经血液运输不同，植物激素通过扩散和主动运输在细胞间移动。它们在极低的浓度下发挥作用，而且同一种激素会因浓度、靶组织类型和植株发育阶段的不同，而产生不同的效应。
In IGCSE OCR Biology, the main plant hormones you need to know are auxins, gibberellins and ethene. Auxins are primarily responsible for cell elongation and tropic responses; gibberellins promote seed germination and stem elongation; and ethene controls fruit ripening and leaf fall.
在 IGCSE OCR 生物中，需要掌握的主要植物激素有生长素、赤霉素和乙烯。生长素主要负责细胞伸长和向性反应；赤霉素促进种子萌发和茎的伸长；乙烯则调控果实成熟与叶片脱落。
2. Tropisms: Plant Growth Responses | 向性：植物生长反应
A tropism is a directional growth response in which a plant part grows towards or away from a stimulus. If the growth is towards the stimulus, it is called a positive tropism; if it is away from the stimulus, it is a negative tropism. In IGCSE Biology, the two most important tropisms are phototropism (response to light) and gravitropism, also known as geotropism (response to gravity).
向性是指植物器官朝向或背离刺激方向生长的反应。如果朝着刺激方向生长，称为正向性；如果背离刺激方向生长，则称为负向性。在 IGCSE 生物中，最重要的两种向性是向光性（对光的反应）和向地性（对重力的反应），后者也称向重力性。
Tropisms are brought about by the uneven distribution of auxin, which causes one side of a root or shoot to grow faster than the other. Shoots normally show positive phototropism and negative gravitropism, whilst roots show negative phototropism and positive gravitropism. This combination of responses ensures that the plant grows in a way that maximises photosynthesis and secures anchorage and water uptake.
向性由生长素的不均匀分布引起，这种不均匀分布使得根或茎的一侧生长速度快于另一侧。通常，茎表现出正向光性和负向地性，而根则表现出负向光性和正向地性。这些反应相互配合，确保植株以最利于光合作用、稳定固着和吸收水分的方式生长。
3. Role of Auxin | 生长素的作用
Auxin is synthesised in the tips of shoots and roots, known as apical meristems. It then diffuses downwards through the plant. The key effect of auxin is to stimulate cell elongation by loosening the cellulose microfibrils in the cell wall, allowing the cell to expand in response to turgor pressure. At moderate concentrations, auxin promotes growth in shoots, but at the same concentration it can inhibit growth in roots, because roots are more sensitive to auxin.
生长素在茎尖和根尖（即顶端分生组织）中合成，然后向植株下部扩散。生长素的主要作用是刺激细胞伸长，其机制是松解细胞壁中的纤维素微纤丝，使细胞在膨压的作用下得以扩展。中等浓度的生长素能促进茎的生长，但同样的浓度却会抑制根的生长，因为根对生长素更为敏感。
This dual effect is crucial for understanding gravitropism in roots, where high auxin concentration on the lower side of a root tip inhibits cell elongation, causing the root to curve downwards. In shoots, the same concentration promotes elongation, leading to upward curvature.
这种双重效应对于理解根部的向地性至关重要：在根尖的下侧，较高的生长素浓度抑制了细胞伸长，使得根向下弯曲。而在茎中，同样较高的浓度反而促进伸长，导致向上弯曲。
4. Phototropism Explained | 向光性解析
When light shines on a shoot from one side, auxin produced at the tip moves away from the light and accumulates on the shaded side. As a result, cells on the shaded side elongate more rapidly than cells on the illuminated side. The differential growth causes the shoot to bend towards the light – a positive phototropic response that helps the plant capture more light for photosynthesis.
当光从一侧照射茎时，在茎尖产生的生长素会向背光一侧移动，并在阴暗侧积累。于是，背光侧的细胞比向光侧的细胞伸长得更快。这种不均匀的生长导致茎朝着光源弯曲，即表现出正向光性反应，这有助于植物截获更多光照用于光合作用。
If the tip of the shoot is removed or covered by a light‑proof cap, auxin cannot be redistributed, and the shoot will not bend towards the light. This classic experiment demonstrates that the tip is both the site of auxin production and the region where the light stimulus is perceived. If the tip is placed back onto a decapitated shoot via a permeable agar block, the bending response is restored, showing that a water‑soluble chemical (auxin) is responsible.
若切除茎尖或用不透光的小帽罩住茎尖，生长素便无法重新分布，茎也不会向光弯曲。这个经典实验表明，茎尖不仅是生长素的产生部位，也是感知光刺激的部位。如果用可渗透的琼脂块将切下的茎尖重新放置在去尖的茎上，弯曲反应又能恢复，这证明是一种水溶性化学物质（即生长素）在发挥作用。
5. Gravitropism (Geotropism) Explained | 向地性解析
Gravitropism is a growth response to gravity. In a horizontally placed seedling, auxin accumulates on the lower side of both the shoot and the root, because of the pull of gravity. In the shoot, the higher auxin concentration on the lower side stimulates cell elongation, making the shoot bend upwards – a negative gravitropic response. In the root, however, this higher concentration inhibits cell elongation, so cells on the upper side elongate more, and the root bends downwards – a positive gravitropic response.
向地性是对重力的生长反应。在一株水平放置的幼苗中，由于重力的作用，生长素在茎和根的下侧积累。在茎中，下侧较高的生长素浓度刺激细胞伸长，使茎向上弯曲，表现出负向地性；而在根中，较高的生长素浓度却抑制了细胞伸长，因此上侧的细胞伸长更多，导致根向下弯曲，表现为正向地性。
This difference in sensitivity to auxin between shoots and roots explains why roots grow down into the soil, anchoring the plant and absorbing water and minerals, while shoots grow upwards, reaching for light. Experiments using a clinostat – a rotating wheel that cancels the directional effect of gravity – can be used to demonstrate that gravitropism is indeed a response to the direction of gravity, not just to contact with a surface.
茎与根对生长素敏感度的差异，解释了为何根向下深入土壤，起到固定植株并吸收水分和矿物质的作用，而茎则向上生长以获取光照。使用旋转器（一种能消除重力方向性影响的旋转装置）进行的实验可以证明，向地性确实是对重力方向的反应，而不仅仅是与物体表面接触所致。
6. Investigating Tropisms: Key Experiments | 向性实验探究
The study of tropisms has a long history of elegant experiments. For the IGCSE OCR course, you should be able to describe simple investigations that show the role of auxin. One typical setup uses oat coleoptiles or mustard seedlings. By removing the tip, covering it with foil, or replacing it with an agar block containing auxin, you can show that a chemical messenger from the tip controls phototropism.
对向性的研究有着悠久的精妙实验史。在 IGCSE OCR 课程中，你需要能够描述显示生长素作用的简单探究实验。典型的实验材料包括燕麦胚芽鞘或芥菜幼苗。通过切除尖端、用锡箔包裹尖端，或用含有生长素的琼脂块替换尖端，可以证明来自茎尖的化学信使控制着向光性反应。
Similarly, gravitropism experiments often involve placing seedlings horizontally in a dark, moist chamber and observing the direction of growth over time. Using a clinostat allows you to rotate the plant slowly, ensuring that gravity acts equally on all sides, thereby preventing the tropic response. A control plant kept stationary will show clear bending, confirming that gravity is the stimulus.
类似地，向地性实验通常是将幼苗水平放置在黑暗潮湿的培养箱中，定时观察生长方向。借助旋转器缓慢转动植株，可以使重力均匀作用于各个侧面，从而抑制向性反应。保持静止的对照组则会表现出明显的弯曲，从而确认重力正是刺激来源。
When describing results, always link the observed growth pattern to the distribution of auxin. Use terms like ‘higher concentration on the shaded side’ or ‘accumulation on the lower side’ to provide a full biological explanation.
在描述实验结果时，请务必将观察到的生长模式与生长素的分布联系起来。使用“阴暗侧浓度较高”或“下侧积累”等表述，提供完整的生物学解释。
7. Commercial Uses of Auxins | 生长素的商业用途
Auxins, particularly synthetic analogues such as IAA (indole‑3‑acetic acid) and NAA (naphthalene acetic acid), have several valuable commercial applications. One of the most important is their use as selective weedkillers. Synthetic auxins like 2,4‑D are applied to lawns or cereal crops; they promote uncontrolled, abnormally rapid growth in broad‑leaved weeds, which exhausts their energy reserves and kills them, while the narrow‑leaved grasses and cereals are much less affected.
生长素，尤其是吲哚乙酸 (IAA) 和萘乙酸 (NAA) 等人工合成类似物，有着若干重要的商业用途。其中最重要的用途之一是作为选择性除草剂。将 2,4‑D 等合成生长素施用在草坪或谷类作物上，会促使阔叶杂草发生不受控制的异常快速生长，耗尽能量储备而死亡，而窄叶的禾本科杂草和谷类作物则基本不受影响。
Auxins are also widely used in rooting powders. When a stem cutting is dipped into a powder containing a low concentration of auxin, it stimulates the formation of adventitious roots at the cut end. This greatly increases the success rate of plant propagation, enabling gardeners and foresters to produce large numbers of identical plants quickly and economically.
生长素还广泛用于生根粉。将茎的插穗切口蘸取含有低浓度生长素的粉末，可刺激切端形成不定根。这大大提高了植物无性繁殖的成功率，使园艺师和林业工作者能够快速、经济地培育出大量遗传性状一致的植株。
Furthermore, spraying auxin onto unpollinated flowers can promote parthenocarpy – the development of fruit without fertilisation. This allows the production of seedless fruits, such as certain tomatoes, cucumbers and watermelons, which are highly valued in the fresh‑fruit market.
此外，将生长素喷洒在未授粉的花上，可以促进单性结实——即无需受精便能发育成果实。这样就可以生产出无籽水果，如某些品种的番茄、黄瓜和西瓜，在鲜果市场上备受欢迎。
8. Gibberellins and Seed Germination | 赤霉素与种子萌发
Gibberellins are another important group of plant hormones. They were first discovered in the fungus Gibberella fujikuroi, which causes ‘bakanae’ disease in rice, making the plants grow too tall and spindly. Today, commercial gibberellins are used to stimulate seed germination. Many seeds require a period of cold or light to break dormancy; application of gibberellins can bypass this requirement, allowing uniform and rapid germination.
赤霉素是另一类重要的植物激素，最初发现于引发水稻“恶苗病”的真菌 Gibberella fujikuroi 中，该病致使植株长得过高且纤细。如今，商业赤霉素被用于促进种子萌发。许多种子需要经历一段低温或光照才能打破休眠；施加赤霉素则能绕过这一需求，实现整齐快速的萌发。
A classic use of gibberellins is in the production of malt for brewing. Barley seeds are sprayed with gibberellins while they germinate, which activates genes coding for amylase enzymes. The amylase breaks down starch in the endosperm into maltose, providing sugars for yeast fermentation. This increases the yield of malt extract and shortens the malting process.
赤霉素的一个经典应用是在酿造麦芽的生产中。在大麦种子萌发时喷洒赤霉素，可以激活编码淀粉酶的基因。淀粉酶将胚乳中的淀粉分解为麦芽糖，为酵母发酵提供糖分。这样不仅提高了麦芽浸出物的产量，还缩短了制麦流程。
Gibberellins are also applied to table grapes to elongate the stem internodes, producing larger, looser clusters of fruit. This reduces the incidence of fungal diseases, as the improved air circulation dries the fruit surface quickly after rain.
食用葡萄上也施用赤霉素，以延长茎的节间距离，从而使果穗更大且果粒松散。这样可以改善通风，雨后果实表面干得更快，从而降低了真菌病害的发生率。
9. Ethene and Fruit Ripening | 乙烯与果实成熟
Ethene (C₂H₄) is a gaseous plant hormone that plays a central role in the ripening of fleshy fruits. It is produced by fruits themselves, particularly as they begin to ripen, and acts as a trigger for a cascade of changes: breakdown of chlorophyll, synthesis of pigments such as carotenoids, conversion of starch to sugars, and softening of cell walls. Once one fruit starts producing ethene, it stimulates neighbouring fruits to ripen, which is why ‘one bad apple spoils the barrel’.
乙烯 (C₂H₄) 是一种气体植物激素，在肉质果实的成熟过程中起着核心作用。它由果实自身产生，特别是在开始成熟时，并触发一系列变化：叶绿素分解、类胡萝卜素等色素的合成、淀粉转变成糖、细胞壁软化。一旦某一果实开始产生乙烯，就会刺激邻近的果实成熟，这就是“一个烂苹果坏了一整桶”的道理。
Commercially, ethene is used to control the ripening of fruit after transport. Bananas, for example, are shipped whilst still green and unripe. On arrival at their destination, they are exposed to a carefully controlled concentration of ethene gas in ripening rooms, which ensures that they develop an even yellow colour and the desired sweetness before being sent to retailers. In the same way, ripening can be delayed by removing ethene from the storage atmosphere, using chemicals that absorb the gas.
在商业上，乙烯被用来控制运输后果实的成熟。例如，香蕉在青涩尚未成熟时装运。抵达目的地后，在催熟室中施用精确浓度的乙烯气体，确保果实均匀转黄、达到理想甜度，再送往零售商。同理，通过使用吸收乙烯的化学物质将乙烯从贮藏环境中移除，就能延缓成熟。
Ethene also promotes leaf abscission – the natural shedding of leaves in autumn – by stimulating the breakdown of cell walls in the abscission layer. This is a valuable response that helps plants conserve water and survive winter.
乙烯还能促进叶片脱落，即秋天落叶的自然过程，它通过刺激离层细胞壁的分解来实现。这一反应有助于植物节约水分，度过冬季。
10. Comparing Plant Hormones: A Summary | 植物激素对比总结
The table below summarises the functions and commercial uses of the three principal plant hormones you need to know for IGCSE OCR Biology. These applications directly link the theoretical understanding of hormone action to real‑world practice in farming and horticulture.
下表总结了你需要为 IGCSE OCR 生物掌握的三类主要植物激素的功能和商业用途。这些应用将激素作用的理论理解与农业园艺实践紧密联系起来。
| Hormone 激素 | Main Functions in Plants 植物中的主要功能 | Commercial Uses 商业用途 |
|---|---|---|
| Auxin (e.g. IAA) 生长素 | Cell elongation; apical dominance; phototropism and gravitropism 细胞伸长、顶端优势、向光性和向地性 | Selective weedkillers; rooting powders; seedless fruit production 选择性除草剂、生根粉、无籽果实生产 |
| Gibberellin 赤霉素 | Promote seed germination; stem elongation; fruit development 促进种子萌发、茎伸长、果实发育 | Malt production in brewing; increasing grape size; breaking seed dormancy 啤酒麦芽生产、增大葡萄果粒、打破种子休眠 |
| Ethene (C₂H₄) 乙烯 | Fruit ripening; leaf abscission; flower wilting 果实成熟、叶片脱落、花朵凋谢 | Controlled ripening of bananas and other fruits; delayed ripening in storage 香蕉等果实的人工催熟、贮藏中延缓成熟 |
When writing about plant hormones in the exam, always be precise in your terminology, refer to the direction of auxin movement, and explain the different sensitivities of shoot and root cells. Remember that tropic responses are growth responses – not instantaneous movements – and that the differential growth is caused by unequal auxin distribution, not by one side simply being ‘pulled’ or ‘bent’.
在考试中回答植物激素相关问题时，请始终使用精确的术语，说明生长素的移动方向，并解释茎和根细胞对生长素的不同敏感性。记住，向性反应是生长反应而非瞬时运动，其不均匀生长是由生长素的不均匀分布引起的，而并非某一侧被简单地“拉拽”或“弯折”。
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