📚 GCSE CIE Biology: Plant Hormones – Essential Revision | GCSE CIE 生物：植物激素 考点精讲
Plant hormones control growth and responses to stimuli in plants. Unlike animals, plants do not have a nervous system; they rely on chemical messengers called plant hormones or plant growth regulators. Understanding how these hormones work helps us explain tropisms and apply them in agriculture and horticulture. This revision guide covers the key concepts required for CIE GCSE Biology.
植物激素控制植物的生长和对刺激的反应。与动物不同，植物没有神经系统；它们依赖被称为植物激素或植物生长调节剂的化学信使。了解这些激素的作用有助于解释向性运动，并将其应用于农业和园艺。本备考指南涵盖 CIE GCSE 生物学所需的关键概念。
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 part, where they cause a specific response. They are active at very low concentrations and do not travel through nerves but via the phloem or by cell-to-cell diffusion.
植物激素是在植物某一部分产生并被运输到另一部分，在那里引起特定反应的化学物质。它们在极低的浓度下具有活性，并非通过神经传播，而是通过韧皮部或细胞间扩散运输。
Unlike animal hormones, plant hormones are not made in specialised glands; they are produced in actively growing regions such as shoot tips and root tips. They typically affect cell division, elongation, or differentiation rather than regulating homeostasis.
与动物激素不同，植物激素并非由专门的腺体产生；它们是在活跃生长的区域（如茎尖和根尖）中合成的。它们通常影响细胞分裂、伸长或分化，而不是调节内稳态。
The main plant hormones you need to know for CIE GCSE are auxins (e.g., IAA), gibberellins, and ethene (ethylene). Each plays a distinct role in coordinating growth and development.
在 CIE GCSE 中你需要了解的主要植物激素有：生长素（如 IAA）、赤霉素和乙烯。它们在协调生长发育中各扮演不同角色。
2. Auxins – The Key Growth Hormone | 生长素——关键的生长激素
Indole-3-acetic acid (IAA) is the most important natural auxin. It is synthesised in the apical meristems at the tips of shoots and roots, and is then transported downwards through the plant.
吲哚-3-乙酸 (IAA) 是最重要的天然生长素。它在茎尖和根尖的顶端分生组织中合成，然后向下运输到植物体各个部位。
In shoots: Auxin stimulates cell elongation by increasing the plasticity of the cell wall, allowing cells to expand as they take up water. This effect is concentration-dependent; moderate levels strongly promote growth.
在茎中： 生长素通过增加细胞壁的可塑性来促进细胞伸长，使细胞在吸水时能够扩张。这种效应与浓度相关；适度浓度强烈促进生长。
In roots: Auxin also promotes cell elongation at very low concentrations. However, the concentration that promotes shoot growth actually inhibits root cell elongation. This difference in sensitivity is crucial for tropisms.
在根中： 生长素在非常低的浓度下也能促进细胞伸长。但是，促进茎生长的浓度反而会抑制根细胞的伸长。这种敏感性的差异对向性运动至关重要。
Auxin also suppresses the growth of lateral buds when it moves down from the shoot apex – a phenomenon called apical dominance. We will explore this later.
生长素从顶端向下运输时，还会抑制侧芽的生长——这一现象称为顶端优势，我们稍后会探讨。
3. Phototropism: How Shoots Grow Towards Light | 向光性：茎如何向光生长
Phototropism is a directional growth response in which a plant shoot bends towards a light source. This is positive phototropism and maximises light capture for photosynthesis.
向光性是一种定向生长反应，即植物茎向光源弯曲。这是正向光性，有助于最大限度地捕获光能以进行光合作用。
When a shoot receives light from one side, auxin produced at the tip is redistributed laterally towards the shaded side. This creates a higher auxin concentration on the darker side of the shoot.
当茎受到单侧光照射时，顶端产生的生长素会横向重新分布至背光侧。这使得茎背光一侧的生长素浓度更高。
Because shoot cells are stimulated by high auxin, the shaded side elongates faster than the illuminated side. This unequal growth causes the shoot to bend towards the light.
由于茎细胞受高浓度生长素刺激，背光侧的细胞比向光侧伸长得更快。这种不均匀的生长导致茎向光弯曲。
Experimental evidence: If the tip of an oat coleoptile is removed, no phototropic bending occurs. If the tip is placed on an agar block and then replaced asymmetrically, bending occurs in the dark, proving the tip produces a diffusible substance (later identified as auxin).
实验证据：如果去除燕麦胚芽鞘的尖端，向光性弯曲不再发生。若将尖端放在琼脂块上，然后不对称地放回，在黑暗中也会发生弯曲，证明尖端产生了一种可扩散的物质（后被确认为生长素）。
4. Gravitropism: Roots Down, Shoots Up | 向地性：根向下，茎向上
Gravitropism (also called geotropism) is a growth response to gravity. Roots grow towards the pull of gravity (positive gravitropism), while shoots grow away from gravity (negative gravitropism).
向地性（又称 geotropism）是对重力的生长反应。根朝着重力方向生长（正向地性），而茎则背离重力生长（负向地性）。
When a root is placed horizontally, gravity causes auxin to accumulate on the lower side. In roots, high auxin concentration inhibits cell elongation, so cells on the upper side elongate more, pushing the root downward.
将根水平放置时，重力使生长素积累在下侧。在根中，高浓度生长素抑制细胞伸长，因此上侧的细胞伸长更多，推动根向下弯曲。
In a horizontally placed shoot, auxin also accumulates on the lower side. However, shoot cells are stimulated by high auxin, so the lower side grows faster and the shoot bends upward.
在水平放置的茎中，生长素同样在下侧积累。然而，茎细胞受高浓度生长素刺激，因此下侧生长更快，茎向上弯曲。
This differential sensitivity ensures that roots anchor the plant and access water/minerals, while shoots reach light for photosynthesis.
这种敏感性差异确保了根能够固定植物并获取水分和矿物质，而茎则能接触光照进行光合作用。
5. Apical Dominance | 顶端优势
Apical dominance is the phenomenon where the main terminal bud of a plant inhibits the growth of lateral (side) buds. This is caused by auxin produced in the apical bud and transported downwards.
顶端优势是指植物的主要顶芽抑制侧芽生长的现象。这由顶芽产生并向下运输的生长素所致。
High auxin concentration in the stem suppresses the development of lateral buds, keeping them dormant. If the apical bud is removed (e.g., by pruning or animal grazing), lateral buds are released and side branches grow.
茎中的高浓度生长素抑制侧芽的发育，使其保持休眠。如果顶芽被去除（例如通过修剪或动物啃食），侧芽便被释放出来，侧枝开始生长。
Gardeners cut off the tips of plants to encourage bushy growth. This removal of the auxin source lets multiple side shoots develop, useful in crops like beans or in hedges.
园丁剪去植物顶端以促使其生长成灌丛状。去除生长素来源后，多个侧枝得以发育，这在豆类作物或树篱中非常有用。
6. Synthetic Auxins: Rooting Powders | 合成生长素：生根粉
Synthetic auxins such as IBA (indolebutyric acid) and NAA (naphthaleneacetic acid) are used in rooting powders. These mimic the effect of natural auxins to stimulate root formation on cuttings.
合成生长素如 IBA（吲哚丁酸）和 NAA（萘乙酸）被用于生根粉。它们模拟天然生长素的作用，刺激插条形成根系。
A gardener dips the cut end of a stem cutting into rooting powder and plants it in compost. The auxin promotes the development of adventitious roots from the stem base, allowing a new plant to grow rapidly.
园丁将茎插条的切口端蘸上生根粉，然后种入堆肥中。生长素促进茎基部长出不定根，使新植物快速成长。
This method is a form of asexual reproduction, producing clones that are genetically identical to the parent plant. It is widely used in commercial horticulture for shrubs, roses, and fruit bushes.
这种方法是一种无性繁殖，产生的克隆与亲本植物遗传上完全相同。它被广泛用于灌木、玫瑰和果树的商业园艺中。
7. Synthetic Auxins: Selective Weedkillers | 合成生长素：选择性除草剂
Synthetic auxins like 2,4-D and MCPA act as selective weedkillers. They are absorbed by broad-leaved weeds (dicots) and cause uncontrolled, excessive growth.
合成生长素如 2,4-D 和 MCPA 可作为选择性除草剂。它们被阔叶杂草（双子叶植物）吸收后，会引起失控的过度生长。
This rapid growth depletes the plant’s energy reserves, distorts tissues, and eventually kills the weed. Narrow-leaved crop plants like wheat and maize (monocots) are much less sensitive to these auxins, so they survive.
这种快速生长耗尽了植物的能量储备，使组织畸形，最终杀死杂草。而小麦和玉米等窄叶作物（单子叶植物）对这些生长素不那么敏感，因此得以存活。
Selective weedkillers are invaluable in agriculture because they remove competitors for light, water, and nutrients without harming the crop. This improves yield and reduces manual weeding.
选择性除草剂在农业中非常宝贵，因为它们能清除争夺光、水和养分的竞争植物而不伤害作物。这提高了产量，并减少了人工除草。
8. Gibberellins: Stem Growth and Seed Germination | 赤霉素：茎的生长与种子萌发
Gibberellins are a group of plant hormones that mainly stimulate stem elongation by promoting cell division and elongation in internodes. Spraying gibberellins on dwarf plants can make them grow to normal height.
赤霉素是一类植物激素，主要通过促进节间的细胞分裂和伸长来刺激茎的伸长。在矮生植物上喷施赤霉素可以使它们长到正常高度。
During germination, gibberellins produced by the embryo diffuse to the aleurone layer in cereal grains. They trigger the production of enzymes such as α-amylase, which break down starch into sugars for the growing seedling.
在萌发过程中，胚产生的赤霉素扩散到谷物籽粒的糊粉层。它们触发α-淀粉酶等酶的产生，这些酶将淀粉分解为糖，供幼苗生长使用。
This action is exploited in the malting industry: barley grains are allowed to germinate until gibberellin activates enzymes, then dried to produce malt. Gibberellins are also used to increase the size of seedless grapes and to delay fruit senescence.
这一作用被用于制麦工业：让大麦粒发芽，直到赤霉素激活酶，然后烘干制成麦芽。赤霉素还用于增大无籽葡萄的果实，并延缓水果的衰老。
9. Ethene and Fruit Ripening | 乙烯与果实催熟
Ethene (ethylene) is a gaseous plant hormone that plays a central role in fruit ripening. It is produced naturally by many fruits as they mature and by wounded tissues.
乙烯是一种气体植物激素，在果实成熟中起着核心作用。它在许多果实成熟过程中自然产生，也由受伤组织释放。
Ethene speeds up the breakdown of chlorophyll, softening of cell walls, and conversion of starch to sugars – all hallmarks of ripening. Commercially, fruits like bananas and tomatoes are harvested when green and firm, then exposed to ethene gas at their destination to trigger uniform ripening.
乙烯加速叶绿素的分解、细胞壁的软化以及淀粉向糖的转化——这些都是成熟的标志
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