Plant Hormones | IGCSE 生物：植物激素考点精讲

📚 Plant Hormones | IGCSE 生物：植物激素考点精讲

Plant hormones are chemical messengers that coordinate growth and responses to stimuli in plants. Unlike animals, plants do not have a nervous system; instead, they rely on hormones to react slowly to their environment. In IGCSE Biology, you need to understand how auxins control directional growth (tropisms) and how gibberellins trigger seed germination. You should also be able to explain commercial uses such as selective weedkillers and rooting powders.

植物激素是在植物体内传递信号、协调生长和应对外界刺激的化学信使。与动物不同，植物没有神经系统，它们依赖激素对环境做出较慢的反应。在 IGCSE 生物中，你需要理解生长素如何控制定向生长（向性运动），以及赤霉素如何启动种子萌发。你还需要能够解释选择性除草剂和生根粉等商业用途。

1. What Are Plant Hormones? | 什么是植物激素？

Plant hormones are small organic molecules produced in one part of the plant and transported to another, where they bring about a specific response. They act at very low concentrations and control processes such as cell elongation, cell division, flowering, fruit ripening and leaf fall.

植物激素是植物某一部分产生并被运输到另一部分发挥作用的小分子有机物。它们在极低浓度下就能起作用，控制着细胞伸长、细胞分裂、开花、果实成熟和落叶等过程。

Two key hormones in the IGCSE syllabus are auxin and gibberellin. Auxin is primarily made in the shoot and root tips, while gibberellin is produced in young leaves and seeds. Both travel through the plant’s transport systems to reach target tissues.

IGCSE 教学大纲中的两种关键激素是生长素和赤霉素。生长素主要在茎尖和根尖合成，而赤霉素在幼叶和种子中产生。两者都通过植物的运输系统到达靶组织。

2. Auxin and Phototropism | 生长素与向光性

Phototropism is the growth of a plant shoot towards light. This occurs because auxin accumulates on the shaded side of the shoot tip when it is exposed to one-sided light. The higher concentration of auxin causes the cells on the shaded side to elongate more than those on the illuminated side, bending the shoot towards the light.

向光性是植物茎向光生长的现象。当茎尖受到单侧光照时，生长素会积累在背光一侧。生长素浓度更高使得背光侧的细胞比向光侧伸长得更多，从而使茎向光弯曲。

A classic experiment uses coleoptiles (young shoot tips) with an agar block to show that auxin diffuses from the tip and causes bending. Removing the tip or covering it with a light-proof cap stops the response, proving that the tip detects light and produces the chemical messenger.

经典实验使用胚芽鞘（幼苗茎尖）和琼脂块来证明生长素从尖端扩散并引起弯曲。如果去掉尖端或用避光的帽子遮盖，向光弯曲就不会发生，证明尖端是感受光并产生化学信使的部位。

3. Auxin and Gravitropism (Geotropism) | 生长素与向地性

Gravitropism is the growth response to gravity. In a horizontally placed root or shoot, auxin moves downwards and accumulates on the lower side. However, root and shoot respond differently to the same auxin concentration.

向地性是对重力的生长反应。在水平放置的根或茎中，生长素向下移动并积累在下侧。然而，根和茎对相同浓度的生长素反应不同。

In shoots, the higher auxin concentration on the lower side stimulates cell elongation, so the shoot bends upwards — this is negative gravitropism. In roots, the same higher auxin concentration inhibits cell elongation on the lower side, causing the cells on the upper side to grow faster and the root to bend downwards — this is positive gravitropism.

在茎中，下侧较高的生长素浓度促进细胞伸长，因此茎向上弯曲——这是负向地性。在根中，同样较高的生长素浓度却抑制下侧细胞伸长，导致上侧的细胞生长更快，根向下弯曲——这是正向地性。

This difference is due to the varying sensitivity of root and shoot cells to auxin. Root cells are more sensitive and are inhibited by concentrations that stimulate shoot cells.

这种差异是由于根细胞和茎细胞对生长素的敏感性不同。根细胞更为敏感，能够被促进茎细胞生长的生长素浓度所抑制。

4. The Role of Gibberellins | 赤霉素的作用

Gibberellins are plant hormones that stimulate stem elongation, flowering and breaking of seed dormancy. In IGCSE, the focus is on their role in triggering germination.

赤霉素是一类植物激素，能刺激茎的伸长、开花和打破种子休眠。在 IGCSE 中，重点是它们在启动萌发中的作用。

When water is absorbed by a seed, the embryo produces gibberellins. These gibberellins diffuse to the aleurone layer (in cereal grains), where they trigger the synthesis of amylase. Amylase then breaks down starch stored in the endosperm into maltose and glucose, providing energy for the growing embryo.

当种子吸水后，胚会产生赤霉素。这些赤霉素扩散到糊粉层（在谷物种子中），在那里触发淀粉酶的合成。淀粉酶进而将储存在胚乳中的淀粉分解为麦芽糖和葡萄糖，为胚的生长提供能量。

Commercial gibberellins can be sprayed on barley grains to speed up malt production for the brewing industry. They are also used to increase fruit size in seedless grapes.

商业上可以将赤霉素喷洒在大麦粒上，以加快酿造工业中麦芽的生产。它们还被用来增大无籽葡萄的果实。

5. Selective Weedkillers | 选择性除草剂

Selective weedkillers are synthetic auxins that cause broad-leaved weeds to grow so fast that their stems become twisted, their leaves yellow and the plants die due to disrupted transport systems. Grass plants (narrow-leaved) are much less affected, so these chemicals can be used to kill weeds in lawns and cereal crops without harming the grass.

选择性除草剂是人工合成的生长素，它们使阔叶杂草生长过快，导致茎扭曲、叶片变黄，并且因运输系统被扰乱而死亡。禾本科植物（窄叶）受到的影响要小得多，因此这些化学药剂可以用于草坪和谷物田除草而不伤害禾草。

This selective action relies on the different sensitivity between monocots (grasses) and dicots (broad-leaved plants) to the synthetic auxin. The weedkiller is absorbed through the leaves and translocated to the growing points, where it unleashes uncontrolled growth.

这种选择性作用依赖于单子叶植物（禾草）和双子叶植物（阔叶植物）对人工合成生长素敏感性的不同。除草剂通过叶片吸收并被运输到生长点，在那里引发失控的生长。

A common example is 2,4-D (2,4-dichlorophenoxyacetic acid), which is widely used in agriculture. IGCSE candidates should be able to explain the mechanism and the reason for its selectivity.

一个常见的例子是 2,4-D（2,4-二氯苯氧乙酸），在农业上被广泛使用。IGCSE 考生应能解释其作用机制和选择性的原因。

6. Rooting Powders | 生根粉

Rooting powders contain synthetic auxins that stimulate root growth from stem cuttings. When a gardener dips the cut end of a stem cutting into rooting powder before planting, the auxin encourages undifferentiated cells to form root primordia, leading to faster and more reliable root development.

生根粉含有能刺激插条生根的人工合成生长素。园丁在扦插前将插条的下切口蘸上生根粉，生长素就能促进未分化的细胞形成根原基，从而更快、更可靠地长出根来。

This is a hormone-based commercial application that uses auxin’s natural role in cell elongation and differentiation. The concentrated auxin at the cut end mimics the action of the plant’s own hormone and overcomes the cutting’s low natural auxin level.

这是利用生长素在细胞伸长和分化中的天然作用的一项商业应用。切口处高浓度的生长素模拟了植物自身激素的作用，弥补了插条天然生长素水平低的不足。

7. Comparing Nervous and Hormonal Control | 神经控制与激素控制的比较

Plants lack a nervous system, but the concepts of chemical coordination are similar to animal hormones. A useful table can help consolidate differences between the two systems in flowering plants and mammals.

植物没有神经系统，但化学协调的概念与动物激素类似。一个有用的表格可以帮助巩固开花植物和哺乳动物中两种系统的区别。

Feature	Plant Hormones	Animal Nerves
Chemical used	Auxin, gibberellin, etc.	Neurotransmitters
Speed	Slow	Rapid
Transmission	Via phloem or cell-to-cell	Via nerve fibres (neurones)
Response	Growth or development change	Muscle contraction or gland secretion
Duration	Long-lasting	Short-lived

特征	植物激素	动物神经
使用的化学物质	生长素、赤霉素等	神经递质
速度	慢	快
传递方式	通过韧皮部或细胞间	通过神经纤维（神经元）
反应	生长或发育改变	肌肉收缩或腺体分泌
持续时间	持久	短暂

8. Designing Experiments on Tropisms | 设计向性实验

IGCSE exam questions often ask you to describe or evaluate experiments investigating phototropism or gravitropism. Variables must be carefully controlled: the direction of light, even distribution of moisture, use of a clinostat to cancel gravity, and covering tips with opaque caps or aluminium foil.

IGCSE 考试题目经常要求你描述或评估研究向光性或向地性的实验。实验中必须仔细控制变量：光照方向、水分均匀分布、使用回转器消除重力影响，以及用避光帽或铝箔遮盖尖端。

In a gravitropism experiment, seedlings are placed horizontally in a petri dish and kept in the dark. After a few days, roots curve downward and shoots curve upward. A clinostat slowly rotates the plant, making the direction of gravity constant relative to the plant; this prevents the uneven distribution of auxin and the plant grows straight.

在向地性实验中，幼苗被水平放置在培养皿中并置于暗处。几天后，根向下弯曲，茎向上弯曲。回转器缓慢旋转植物，使重力方向相对于植物保持不变；这阻止了生长素的不均匀分布，植物就会直立生长。

To test the role of the root tip in gravitropism, you can carefully remove the root cap and observe that the root no longer responds to gravity. This demonstrates that the root cap is essential for sensing gravity.

为了检验根冠在向地性中的作用，你可以小心地切除根冠，观察到根不再对重力做出反应。这表明根冠对于感受重力是必不可少的。

9. Key Definitions for the Exam | 考试中的关键定义

It is essential to memorise precise definitions. Tropism is a directional growth response in which the direction of growth is determined by the direction of the external stimulus. Phototropism is growth in response to light; gravitropism is growth in response to gravity. If growth is towards the stimulus, it is called positive tropism; if away, it is negative.

记住精准的定义至关重要。向性是一种定向生长反应，其生长方向由外界刺激的方向决定。向光性是对光的生长反应；向地性是对重力的生长反应。如果朝着刺激方向生长，称为正向性；如果背离，则称为负向性。

A plant hormone is a chemical substance produced in small amounts in one region of a plant and transported to another region, where it produces a specific effect on growth or development. Auxin is a plant hormone that promotes cell elongation; it is produced in the tips of shoots and roots.

植物激素是在植物某一区域少量产生并被运输到另一区域，对生长或发育产生特定效应的化学物质。生长素是一种促进细胞伸长的植物激素；它在茎尖和根尖产生。

10. Exam Tips and Common Pitfalls | 考试贴士与常见误区

Many students confuse the site of auxin production (shoot tip) with the region of elongation (just behind the tip). In phototropism, auxin moves away from light, but it is not destroyed by light. The shaded side receives more auxin because auxin migrates laterally.

许多学生混淆了生长素产生的部位（茎尖）和伸长的区域（尖端稍后部位）。在向光性中，生长素从光照处移开，但它并不被光破坏。背光侧获得更多的生长素是因为生长素发生了横向迁移。

When describing commercial weedkillers, be careful not to say they kill all plants. Emphasise the term “selective” and explain that the effect on grasses is minimal because grasses are less sensitive to the synthetic auxin.

在描述商业除草剂时，注意不要说它们杀死所有植物。要强调“选择性”这个词，并解释由于禾草对人工合成生长素不太敏感，因此受到的伤害很小。

For seed germination, do not forget that gibberellins stimulate amylase production. The starch digestion products glucose and maltose are then used for respiration and growth. Be able to link this to a practical demonstration using barley grains and a starch agar plate.

对于种子萌发，不要忘记赤霉素刺激淀粉酶产生。淀粉被分解产生的葡萄糖和麦芽糖随后用于呼吸作用和生长。要能够将这一过程与使用大麦粒和淀粉琼脂平板的演示实验联系起来。

11. Advanced Context: Synthetic Plant Hormones in Horticulture | 拓展情境：园艺中的人工合成植物激素

Beyond your core syllabus, you may be given unfamiliar contexts in exam questions. For instance, synthetic auxins are used to delay fruit drop in orchards or to induce parthenocarpic fruit (fruit without seeds). Ethene gas is another plant hormone used to speed up fruit ripening during transport.

在核心大纲之外，考试题目中可能会给出不熟悉的情境。例如，人工合成生长素被用于延迟果园落果，或诱导单性结实（无籽果实）。乙烯气体是另一种用于在运输过程中加速果实成熟的植物激素。

Understanding the general principle — that hormones have multiple effects depending on concentration, target tissue, and interaction with other hormones — will help you analyse new data. Always apply the concepts of transport, receptor binding, and differential sensitivity.

理解一般原理——即激素的作用因浓度、靶组织以及与其他激素的相互作用而不同——将有助于你分析新数据。始终运用运输、受体结合和敏感性差异等概念。

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Plant Hormones | IGCSE 生物：植物激素 考点精讲