📚 IGCSE AQA Biology: Plant Hormones Revision Guide | IGCSE AQA 生物：植物激素 考点精讲
Plants may not have a nervous system, but they are far from passive. They detect and respond to stimuli such as light and gravity using chemical messengers called plant hormones. This revision guide covers every key point you need for the AQA IGCSE Biology specification, from auxin-controlled tropisms to the commercial use of plant hormones. Mastering these concepts will help you explain experimental results and answer application-based exam questions with confidence.
植物虽然没有神经系统，但它们绝非被动地生长。植物利用被称为植物激素的化学信使来探测和响应光照、重力等刺激。本考点精讲覆盖了 AQA IGCSE 生物大纲中每一个关键点，从生长素调控的向性运动到植物激素的商业应用。掌握这些概念将帮助你从容解释实验结果，并自信地应对应用类考题。
1. Introduction to Plant Hormones | 植物激素简介
Plant hormones are chemical substances produced in one part of a plant and transported to another, where they exert a specific effect at very low concentrations. Unlike animal hormones, they are not produced in specialised glands; instead, they are made in actively growing regions such as root and shoot tips. The most important plant hormones you need to know are auxins, gibberellins and ethene.
植物激素是在植物某个部位产生并被运输到另一部位的化学物质，在极低浓度下即可发挥特定作用。与动物激素不同，植物激素并不由专门的腺体制造，而是在根尖和茎尖等活跃生长的区域产生。你需要掌握的最重要的植物激素有生长素、赤霉素和乙烯。
2. Tropisms: Phototropism and Gravitropism | 向性：向光性与向地性
A tropism is a directional growth response in which a plant part grows towards or away from a stimulus. Phototropism is a response to light: shoots grow towards light (positive phototropism) while roots generally grow away from light (negative phototropism). Gravitropism (also called geotropism) is a response to gravity: roots grow downwards in the direction of gravity (positive gravitropism) and shoots grow upwards against gravity (negative gravitropism).
向性是一种方向性的生长反应，植物器官会朝向或背离刺激而生长。向光性是对光的响应：茎向光生长（正向光性），根通常背光生长（负向光性）。向地性（也称向重力性）是对重力的响应：根顺着重力方向向下生长（正向地性），茎则逆着重力向上生长（负向地性）。
3. Auxin: The Key Plant Hormone | 生长素：关键的植物激素
Auxin is the principal hormone controlling tropisms. The most common natural auxin is IAA (indole-3-acetic acid). Auxin is produced in the tips of shoots and roots and moves through the plant by active transport and diffusion. In shoots, auxin promotes cell elongation, making cells on one side grow longer than the other. In roots, high concentrations of auxin inhibit cell elongation, which is crucial for gravitropism.
生长素是控制向性的主要激素。最常见的天然生长素是吲哚-3-乙酸（IAA）。生长素在茎尖和根尖产生，并通过主动运输和扩散在植物体内移动。在茎中，生长素促进细胞伸长，使一侧细胞比另一侧长得更长；在根中，高浓度的生长素会抑制细胞伸长，这对向地性至关重要。
4. How Auxin Controls Phototropism in Shoots | 生长素如何控制茎的向光性
When light shines on a shoot from one side, auxin produced at the tip redistributes to the shaded side. This results in a higher concentration of auxin on the side away from light. In shoots, a higher auxin concentration stimulates cell elongation more strongly, so the cells on the shaded side elongate faster than those on the illuminated side. This differential growth causes the shoot to bend towards the light.
当单侧光照射茎时，尖端产生的生长素会重新分布到背光一侧。这导致背光面生长素浓度更高。在茎中，较高的生长素浓度会更强烈地刺激细胞伸长，因此背光侧的细胞比向光侧的细胞伸长得更快。这种差异性生长使得茎向光弯曲。
5. How Auxin Controls Gravitropism in Roots and Shoots | 生长素如何控制根和茎的向地性
When a root or shoot is placed horizontally, auxin accumulates on the lower side due to gravity. In shoots, this higher auxin concentration on the lower side promotes cell elongation, so the lower side grows faster and the shoot bends upwards (negative gravitropism). In roots, however, the same high auxin concentration inhibits cell elongation. The cells on the lower side of the root grow less than those on the upper side, causing the root to bend downwards (positive gravitropism).
当根或茎水平放置时，生长素会因重力而累积到下方一侧。在茎中，下方较高的生长素浓度促进细胞伸长，使得下方长得更快，茎向上弯曲（负向地性）。然而在根中，同样高的生长素浓度反而抑制细胞伸长。根下方一侧的细胞生长少于上方一侧，导致根向下弯曲（正向地性）。
6. The Went Experiment: Discovering Auxin | 温特实验：发现生长素
Frits Went’s famous experiment in the 1920s provided strong evidence for the existence of a chemical messenger. He removed the tips of oat coleoptiles and placed them on agar blocks, allowing the chemical to diffuse into the agar. When he then placed the agar blocks asymmetrically on decapitated coleoptiles, the coleoptiles bent away from the side with the block even in the absence of the original tip. This proved that a diffusible substance — later identified as auxin — controlled growth curvature.
弗里茨·温特在20世纪20年代进行的著名实验为化学信使的存在提供了有力证据。他将燕麦胚芽鞘的尖端切下，放在琼脂块上，让化学物质扩散到琼脂中。然后他将琼脂块不对称地放在去顶的胚芽鞘上，结果胚芽鞘向远离琼脂块的一侧弯曲，即使原来的尖端已被切除。这证明了有一种可扩散的物质——后来被鉴定为生长素——控制着生长弯曲。
7. Uses of Plant Hormones: Weedkillers and Rooting Powder | 植物激素的用途：除草剂与生根粉
Synthetic auxins are widely used in agriculture and horticulture. Selective weedkillers contain synthetic auxin analogues that cause broad-leaved weeds to grow abnormally fast, distorting their tissues and eventually killing them, while leaving narrow-leaved crops like wheat unharmed. Rooting powder contains auxins that stimulate the formation of adventitious roots on stem cuttings. This allows gardeners to clone desirable plants quickly and reliably.
合成生长素被广泛应用于农业和园艺。选择性除草剂含有合成生长素类似物，能导致阔叶杂草异常快速生长，组织扭曲并最终死亡，而不伤害小麦等窄叶作物。生根粉含有生长素，能刺激茎插条上不定根的形成，使园艺工作者能够快速、可靠地克隆优良植株。
8. Gibberellins: Stem Elongation and Seed Germination | 赤霉素：茎的伸长与种子萌发
Gibberellins are another group of plant hormones. They promote stem elongation, especially in dwarf varieties, and help end seed dormancy by triggering the production of enzymes that break down food reserves inside the seed. In the brewing industry, gibberellins are used to stimulate barley seeds to germinate, converting starch into sugars ready for fermentation.
赤霉素是另一类植物激素。它们能促进茎的伸长，尤其在矮化品种中效果明显，并能通过触发分解种子内储存营养的酶的产生来打破种子休眠。在酿造工业中，赤霉素被用于刺激大麦种子萌发，将淀粉转化为糖，以便后续发酵。
9. Ethene and Fruit Ripening | 乙烯与果实成熟
Ethene is a gaseous plant hormone that plays a major role in fruit ripening and leaf abscission (dropping of leaves). It stimulates the breakdown of cell walls, the conversion of starch into sugars, and colour changes associated with ripening. Commercially, unripe fruits are transported in cool conditions with low ethene levels to delay ripening; when they reach their destination, they are exposed to ethene to ripen them evenly for sale.
乙烯是一种气态植物激素，在果实成熟和落叶过程中起重要作用。它刺激细胞壁的分解、淀粉转化为糖以及伴随成熟而发生的颜色变化。商业上，未成熟水果在低乙烯浓度的冷藏条件下运输以延迟成熟；到达目的地后，再用乙烯处理使其均匀成熟后上市销售。
10. Summary and Exam Tips | 总结与考试技巧
Make sure you can compare the effects of auxin in shoots and roots for both phototropism and gravitropism. Always refer to redistribution of auxin, not production on one side only. When describing experiments like Went’s, emphasise the role of the agar block as a carrier of a chemical signal. Understand the commercial applications and be ready to suggest why a specific plant hormone is used in a given scenario. Use precise vocabulary such as ‘inhibit’, ‘stimulate’, ‘elongation’, and ‘differential growth’.
务必能够比较生长素在茎和根中分别对向光性和向地性的作用。始终使用“生长素重新分布”，而不是“一侧产生得更多”。在描述温特实验时，要强调琼脂块作为化学信号载体的作用。理解商业应用，并准备好解释特定场景下使用某种植物激素的原因。使用精确的词汇，如“抑制”、“刺激”、“伸长”和“差异性生长”。
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