📚 Plant Hormones for IGCSE CCEA Biology | IGCSE CCEA 生物：植物激素 考点精讲
Plants may seem passive, but they constantly respond to their environment using chemical messengers called plant hormones. Unlike animals, plants do not have a nervous system. Instead, they rely on hormones to coordinate growth, development, and responses to stimuli such as light and gravity. Understanding how these hormones work is essential for IGCSE CCEA Biology, where you will explore auxins, gibberellins, ethylene, and abscisic acid, along with their practical and commercial applications.
植物看似静态，但它们通过称为植物激素的化学信使不断对环境做出反应。与动物不同，植物没有神经系统，而是依赖激素来协调生长、发育以及对光、重力等刺激的应答。理解这些激素的工作原理对 IGCSE CCEA 生物考试至关重要，你需要掌握生长素、赤霉素、乙烯和脱落酸的作用，以及它们在实际和商业中的应用。
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 specific responses even at very low concentrations. They help control processes such as cell elongation, seed germination, flowering, fruit ripening, and leaf fall.
植物激素是植物某个部位产生的化学物质，被运输到其他部位后，即使在极低浓度下也能引发特定反应。它们有助于控制细胞伸长、种子萌发、开花、果实成熟和落叶等过程。
Unlike animal hormones, plant hormones are not produced in specialised glands. They act locally or at a distance, and their effects often depend on the concentration and the target tissue. The main groups you need to know for CCEA are auxins, gibberellins, ethylene (ethene), and abscisic acid (ABA).
与动物激素不同，植物激素不在专门的腺体中产生。它们可以局部作用或远距离作用，其效果往往取决于浓度和靶组织。CCEA 要求学生掌握的主要激素类别是生长素、赤霉素、乙烯和脱落酸 (ABA)。
2. Auxin and Phototropism | 生长素与向光性
Auxin is a plant hormone that plays a key role in phototropism – the growth of a plant shoot towards light. When light shines on one side of a shoot, auxin produced in the tip moves away from the light and accumulates on the shaded side.
生长素是一种在向光性中起关键作用的植物激素——即植物茎向光生长的现象。当光照射到茎的一侧时，顶端产生的生长素会从照光一侧移开，聚集在背光一侧。
The higher concentration of auxin on the shaded side causes those cells to elongate more rapidly than cells on the illuminated side. This uneven cell elongation makes the shoot bend towards the light source. This response helps maximise photosynthesis by orienting leaves to capture more light.
背光侧较高的生长素浓度导致这些细胞比向光侧的细胞伸长更快。这种不均匀的细胞伸长使茎向光源弯曲。该反应有助于最大化光合作用，使叶片朝向光源以捕获更多光线。
Experiments with coleoptiles (shoots of grass seedlings) have shown that removing the tip stops phototropism, while placing an agar block containing auxin on a decapitated shoot restores growth curvature. This confirms that auxin is the signal molecule responsible.
对胚芽鞘（草苗茎）的实验表明，切除顶端会停止向光性，而将含有生长素的琼脂块放在去顶的茎上可恢复弯曲生长。这证实生长素是负责该反应的信号分子。
3. Auxin and Geotropism (Gravitropism) | 生长素与向地性
Geotropism, also known as gravitropism, is a plant’s directional growth response to gravity. Roots grow downwards (positive geotropism), while shoots grow upwards (negative geotropism). Auxin again plays a central role.
向地性，也称向重力性，是植物对重力的定向生长反应。根向下生长（正向地性），而茎向上生长（负向地性）。生长素再次发挥核心作用。
In roots, a high concentration of auxin on the lower side inhibits cell elongation. Since the auxin accumulates on the lower side of a horizontal root, cells there elongate less than those on the upper side, causing the root to bend downwards.
在根中，下侧的高浓度生长素抑制细胞伸长。由于生长素在水平放置的根的下侧积累，那里的细胞伸长少于上侧，导致根向下弯曲。
In shoots, the same higher auxin concentration on the lower side promotes cell elongation, making the shoot grow upwards. This opposite response in roots and shoots is due to different sensitivity to auxin concentration. The root tip is especially important for detecting gravity, as statoliths (dense starch grains) in root cap cells settle downwards and influence auxin distribution.
在茎中，同样下侧较高的生长素浓度反而促进细胞伸长，使茎向上生长。根和茎的这种相反反应源于它们对生长素浓度的敏感度不同。根尖对于重力感知尤其重要，因为根冠细胞中的平衡石（致密淀粉粒）会下沉积聚，并影响生长素的分布。
4. The Role of Auxin in Apical Dominance | 生长素在顶端优势中的作用
Apical dominance is the phenomenon where the main central stem grows more strongly than side shoots. The growing tip (apical bud) produces auxin, which travels down the stem and inhibits the growth of lateral buds further down.
顶端优势是指主茎比侧枝生长更旺盛的现象。生长的顶端（顶芽）产生生长素，沿茎向下运输，抑制下部侧芽的生长。
If you remove the apical bud, the auxin supply to the lateral buds is cut off, and they begin to grow into side branches. This is widely used in horticulture to encourage bushier growth in plants. Once the side shoots start growing, they in turn produce auxin, which can re-establish some dominance patterns.
如果摘除顶芽，运往侧芽的生长素供应中断，侧芽便开始生长成侧枝。这在园艺中广泛用于促进植株长得更茂密。一旦侧枝开始生长，自身也会产生生长素，可能重新建立一定程度的顶端优势。
Auxin-based weedkillers exploit a similar mechanism: synthetic auxins applied at high doses cause uncontrollable growth in broad-leaved weeds, eventually killing them, while narrow-leaved crops like grasses are less affected because of differences in auxin metabolism.
以生长素为基础的除草剂利用了类似的机制：高剂量施用的人工合成生长素会引起阔叶杂草不受控制的生长，最终致其死亡，而像禾本科这样的窄叶作物则因生长素代谢不同而受影响较小。
5. Investigating Auxin – The Went Experiment | 研究生长素——温特实验
Frits Went’s experiment in the 1920s was a landmark in plant hormone research. He cut off the tips of oat coleoptiles and placed them on agar blocks. Auxin diffused from the tips into the agar.
弗里茨·温特在 20 世纪 20 年代的实验是植物激素研究的里程碑。他切下燕麦胚芽鞘尖端并放置在琼脂块上，生长素从尖端扩散到琼脂中。
When he placed an auxin-containing agar block asymmetrically on a decapitated coleoptile, the shoot bent away from the side with the block, just as though it were responding to light. A plain agar block caused no bending. This proved that a chemical diffusing from the tip could control growth.
当他把含有生长素的琼脂块不对称地放在去顶胚芽鞘上时，茎向没有琼脂块的一侧弯曲，仿佛在向光性反应中一样。纯琼脂块则不引起弯曲。这证明从尖端扩散出来的化学物质可控制生长。
Went’s work gave rise to the name ‘auxin’ (from Greek auxein, meaning ‘to grow’). The experiment itself is a classic example that IGCSE candidates should be able to describe and interpret, linking results to the role of auxin in phototropism.
温特的工作催生了“生长素”这一名称（源自希腊语 auxein，意为“生长”）。该实验本身是一个经典案例，IGCSE 考生需要能够描述并解释，将结果与生长素在向光性中的作用联系起来。
6. Gibberellins and Stem Elongation | 赤霉素与茎的伸长
Gibberellins are another important group of plant hormones. They stimulate stem elongation by promoting both cell division and cell elongation. In IGCSE, you need to know their role in plants and agriculture.
赤霉素是另一类重要的植物激素。它们通过促进细胞分裂和细胞伸长来刺激茎的伸长。IGCSE 要求了解它们在植物和农业中的作用。
Applying gibberellins to dwarf varieties of plants, such as dwarf peas, can make them grow to the height of normal tall plants. This is because dwarf varieties often have a genetic mutation that reduces natural gibberellin production, but their cells still respond if gibberellin is supplied externally.
将赤霉素施用在矮生品种植物（如矮生豌豆）上，可以使它们长到正常高株的高度。这是因为矮生品种往往具有一个基因突变，减少了天然赤霉素的产生，但如果从外部供给赤霉素，其细胞仍能作出反应。
Commercial growers use gibberellins to increase the length of flower stalks in ornamental plants, improve fruit size in seedless grapes, and promote uniform flowering in crops such as sugarcane. The effect on stem internode elongation is especially striking.
商业种植者使用赤霉素来增加观赏植物花柄的长度、提高无籽葡萄的果实大小，以及促使用甘蔗等作物均匀开花。其对茎节间伸长的效果尤其显著。
7. Gibberellins in Seed Germination | 赤霉素在种子萌发中的作用
In seeds, gibberellins play a critical role in switching on the process of germination. When a seed absorbs water, the embryo begins to produce gibberellins, which diffuse to the aleurone layer – a layer of cells in the seed endosperm.
在种子中，赤霉素在启动萌发过程中起着关键作用。当种子吸水后，胚开始产生赤霉素，扩散到糊粉层——种子胚乳中的一层细胞。
The gibberellins signal the aleurone layer to synthesise enzymes such as amylase. These enzymes break down stored starch in the endosperm into soluble sugars, which are then transported to the growing embryo. This provides the energy and building blocks needed for growth until the seedling can photosynthesise.
赤霉素向糊粉层发出信号，使其合成淀粉酶等酶类。这些酶将胚乳中储存的淀粉分解为可溶性糖，运输到生长的胚中，为幼苗提供能量和构建材料，直至它能进行光合作用。
This mechanism is used industrially in malting, where barley grains are allowed to germinate under controlled conditions to produce enzymes that convert starch to sugars for brewing. Understanding gibberellin action helps optimise malt quality in the food and drink industry.
这种机制在制麦芽工业中得到应用，即让大麦粒在受控条件下萌发，产生将淀粉转化为糖的酶用于酿造。了解赤霉素的作用有助于在食品和饮料行业中优化麦芽质量。
8. Ethylene (Ethene) and Fruit Ripening | 乙烯与果实成熟
Ethylene is a simple gaseous plant hormone that plays a major role in the ripening of climacteric fruits such as bananas, tomatoes, and apples. These fruits show a burst of ethylene production at the start of ripening, which then accelerates the process.
乙烯是一种简单的气体植物激素，在香蕉、番茄、苹果等跃变型果实的成熟过程中起主要作用。这些果实在成熟开始时会出现乙烯产量的爆发，进而加速成熟过程。
Ethylene triggers changes in colour (chlorophyll breakdown, carotenoid accumulation), softening of the fruit (breakdown of pectin in cell walls), and conversion of starch into sugars, making the fruit sweet and palatable. It also stimulates more ethylene production in a positive feedback loop, which is why one ripe fruit can ripen others stored nearby.
乙烯引发颜色变化（叶绿素分解，类胡萝卜素累积）、果实软化（细胞壁中果胶的分解）以及淀粉转化为糖，使果实又甜又好吃。它还通过正反馈回路刺激产生更多乙烯，因此一个成熟的水果可催熟附近储存的其他水果。
Commercially, fruits are often picked unripe and transported cold. Before sale, they are exposed to low concentrations of ethylene gas in ripening rooms to ensure they reach the right stage of ripeness uniformly. Conversely, potassium permanganate sachets or low temperatures can be used to absorb or reduce ethylene and extend storage life.
商业上，水果通常在未成熟时采摘并冷藏运输。销售前，将其暴露在低浓度乙烯气体的催熟室中，确保均匀达到合适的成熟度。相反，可使用高锰酸钾袋或低温来吸收或减少乙烯，延长储存寿命。
9. Abscisic Acid (ABA) and Seed Dormancy | 脱落酸与种子休眠
Abscisic acid (ABA) is a plant hormone that often acts as an inhibitor of growth. It is particularly important in maintaining seed dormancy in adverse conditions and promoting leaf fall (abscission) in some species.
脱落酸 (ABA) 是一种常为生长抑制剂的植物激素。它在不利条件下维持种子休眠，以及促进某些物种落叶（脱离）方面尤为重要。
In seeds, ABA levels are high during dormancy, preventing germination even if water and warmth are present. Germination begins only when ABA concentrations drop and gibberellin levels rise, shifting the hormonal balance towards growth. This ensures that seeds only germinate when environmental conditions are favourable for seedling survival.
在种子中，休眠期间 ABA 水平很高，即使有水和适宜温度也会阻止萌发。只有当 ABA 浓度下降而赤霉素水平上升时，激素平衡转向生长，萌发才会启动。这确保种子仅在环境条件利于幼苗存活时才萌发。
ABA also helps plants survive drought by causing stomata to close. When water is scarce, roots produce ABA, which travels to leaves and triggers the guard cells to lose turgor, closing the stomatal pore and reducing water loss via transpiration. This response can be rapid and is vital for drought tolerance.
ABA 还通过引起气孔关闭来帮助植物在干旱中存活。缺水时根系产生 ABA，运输到叶片促使保卫细胞失去膨压，关闭气孔，减少蒸腾作用失水。这种反应可迅速发生，对耐旱性至关重要。
10. Commercial Uses of Plant Hormones | 植物激素的商业用途
Knowledge of plant hormones has been applied widely in agriculture and horticulture. Auxin-based rooting powders are used to stimulate root formation on stem cuttings. Dipping the cut end into auxin powder before planting increases the success rate of vegetative propagation.
植物激素知识在农业和园艺中得到了广泛应用。基于生长素的生根粉用于刺激插条生根。种植前将切割端蘸取生长素粉末可提高无性繁殖的成功率。
Synthetic auxins as selective herbicides (weedkillers) differentially kill broad-leaved weeds in cereal crops and lawns. These chemicals cause rapid, distorted growth in the weeds, depleting their energy reserves and leading to death. The narrow-leaved crop plants tolerate them much better.
人工合成生长素作为选择性除草剂，可差异化杀死谷类作物和草坪中的阔叶杂草。这些化学物质在杂草中引起快速、畸形的生长，耗尽能量储备导致死亡。窄叶作物对其耐受性高得多。
Gibberellins are used commercially to delay ripening and yellowing in citrus fruits, to increase the size of grapes, and to improve the yield of sugar cane by elongating the stem internodes. They also replace the cold or light treatment normally required to induce flowering in some biennials (bolting).
赤霉素在商业上用于延缓柑橘类果实的成熟和变黄、增大葡萄果实、通过伸长蔗茎节间来提高甘蔗产量。它们还取代了某些二年生植物开花所需的光或冷处理（抽薹）。
Ethylene-releasing compounds, such as ethephon, are sprayed on crops to promote uniform ripening, loosen fruit for mechanical harvesting, and stimulate latex flow in rubber trees. Meanwhile, inhibitors of ethylene action, like 1-MCP, are used to extend the shelf life of fruits and cut flowers.
乙烯释放化合物（如乙烯利）被喷洒在作物上以促进均匀成熟、使果实松动便于机械采收，以及刺激橡胶树胶乳流动。同时，乙烯作用抑制剂（如 1-MCP）用于延长水果和切花的货架期。
11. Comparing Plant Hormones and Animal Hormones | 植物激素与动物激素比较
Although both plant and animal hormones act as chemical messengers, there are important differences that often appear in IGCSE CCEA questions. Animal hormones are typically produced in endocrine glands, transported in the blood, and act on specific target organs often far from the site of production.
虽然植物和动物激素都作为化学信使发挥作用，但存在重要差异，常出现在 IGCSE CCEA 考题中。动物激素通常在内分泌腺中产生，经血液运输，作用于常远离产生部位的特定靶器官。
Plant hormones are not produced in glands but in ordinary cells (often at shoot and root tips). They may act locally by diffusion or be transported in the xylem and phloem. Their effects are often broad and depend more on concentration gradients and tissue sensitivity than on a single target.
植物激素不在腺体中产生，而是在普通细胞中产生（通常在茎尖和根尖）。它们可通过扩散局部作用，也可通过木质部和韧皮部运输。其作用通常较广泛，更多取决于浓度梯度和组织敏感性，而非单一靶标。
Furthermore, each plant hormone can have multiple, sometimes opposite, effects in different tissues or at different concentrations, as seen with auxin promoting shoot growth while inhibiting root growth at high concentrations. In contrast, animal hormones usually have more dedicated and specific roles.
此外，每种植物激素在不同组织或不同浓度下可能具有多种甚至相反的效果，例如生长素在低浓度促进茎生长，而高浓度抑制根生长。相比之下，动物激素通常具有更专一和特定的作用。
12. Key Concepts and Exam Tips | 核心概念与应试技巧
For IGCSE CCEA Biology, be ready to explain the role of auxin in phototropism and geotropism using the idea of unequal distribution. Describe Went’s experiment and how it supports the chemical theory. Link gibberellins to seed germination by explaining the enzyme induction cascade. For ethylene, relate it to ripening and commercial practices.
针对 IGCSE CCEA 生物，准备好用不均匀分布的观点解释生长素在向光性和向地性中的作用。描述温特实验及其如何支持化学理论。将赤霉素与种子萌发联系起来，解释酶诱导级联反应。对于乙烯，要联系果实成熟和商业实践。
Make sure you can compare plant and animal hormones in a table format. Here is a useful summary:
确保你能用表格对比植物与动物激素。以下是一个有用的总结：
| Feature 特征 | Plant Hormones 植物激素 | Animal Hormones 动物激素 |
|---|---|---|
| Production site 产生部位 | Growing regions (tips, immature leaves) 生长区域（顶端、幼叶） | Endocrine glands 内分泌腺 |
| Transport 运输方式 | Diffusion, xylem, phloem 扩散、木质部、韧皮部 | Bloodstream 血液 |
| Specificity 特异性 | Broad effects, concentration-dependent 作用广泛，依赖浓度 | Often highly specific to target organ 通常对靶器官高度特异性 |
| Speed of response 反应速度 | Relatively slow, growth-based 相对较慢，基于生长 | Can be rapid or slow 可快可慢 |
Remember to use scientific vocabulary precisely: coleoptile, aleurone layer, amylase, climacteric, statolith, abscission. Practice interpreting graphs showing auxin concentration vs growth rate in shoots and roots. Good luck with your revision!
记住准确使用科学词汇：胚芽鞘、糊粉层、淀粉酶、跃变、平衡石、脱离。练习解读显示茎和根中生长素浓度与生长速率关系的图表。祝备考顺利！
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