📚 Plant Hormones: A-Level OCR Biology Key Points | A-Level OCR 生物：植物激素 考点精讲
Plant hormones, also known as phytohormones, are chemical messengers that coordinate plant growth, development and responses to environmental stimuli. Unlike animal hormones, they are often produced in one region and act locally or in another part of the plant via diffusion or transport systems, controlling processes such as cell elongation, division and differentiation. In OCR A-Level Biology, you are expected to understand the roles of key plant hormones including auxins, gibberellins, cytokinins, abscisic acid and ethene, and to explain experimental evidence for their actions.
植物激素，也称为植物激素，是协调植物生长、发育及对环境刺激响应的化学信使。与动物激素不同，植物激素通常在一个区域产生，并通过扩散或转运系统在局部或植物另一部分起作用，控制细胞伸长、分裂和分化等过程。在OCR A-Level生物中，要求你理解关键植物激素的作用，包括生长素、赤霉素、细胞分裂素、脱落酸和乙烯，并解释其作用的实验证据。
1. Introduction to Plant Hormones | 植物激素简介
Plant hormones are organic molecules produced in small quantities that act as signalling molecules to trigger specific physiological responses. They are synthesised in meristems, young tissues and developing fruits, and can exert their effects at the site of synthesis or be transported via the phloem, xylem or parenchyma cells. The five core plant hormone groups covered in the OCR specification are auxins (primarily IAA), gibberellins, cytokinins, abscisic acid (ABA) and ethene.
植物激素是微量生产的有机分子，作为信号分子触发生理反应。它们在分生组织、幼嫩组织和发育中的果实中合成，可以在合成部位起作用，或通过韧皮部、木质部或薄壁细胞运输。OCR大纲涵盖的五类核心植物激素是生长素（主要是IAA）、赤霉素、细胞分裂素、脱落酸（ABA）和乙烯。
| Hormone | 激素 | Key Role | 关键作用 |
|---|---|---|---|
| Auxin (e.g. IAA) | 生长素（如吲哚乙酸） | Cell elongation, tropisms, apical dominance | 细胞伸长、向性、顶端优势 |
| Gibberellins | 赤霉素 | Stem elongation, seed germination | 茎伸长、种子萌发 |
| Cytokinins | 细胞分裂素 | Cell division, delay senescence | 细胞分裂、延缓衰老 |
| Abscisic acid (ABA) | 脱落酸 | Stomatal closure, seed dormancy | 气孔关闭、种子休眠 |
| Ethene | 乙烯 | Fruit ripening, leaf abscission | 果实成熟、叶片脱落 |
2. Auxins: IAA and Its Production | 生长素：吲哚乙酸及其生成
Indole-3-acetic acid (IAA) is the most abundant natural auxin. It is produced mainly in the shoot apical meristem, young leaves and developing seeds. IAA moves through the plant by a unique polar transport system: specific PIN efflux carrier proteins localise at the basal end of cells, directing auxin flow from shoot apex to root tip. This unidirectional movement establishes auxin concentration gradients that regulate growth patterns.
吲哚-3-乙酸（IAA）是最丰富的天然生长素。它主要在茎尖分生组织、幼叶和发育中的种子中产生。IAA通过独特的极性运输系统在植物体内移动：特定的PIN外排载体蛋白定位在细胞的基部，引导生长素从茎尖流向根尖。这种单向移动建立起调节生长模式的生长素浓度梯度。
A classic experiment by Frits Went (1926) provided strong evidence for a diffusible growth-promoting chemical. He cut coleoptile tips of oat seedlings and placed them on agar blocks for a period, allowing the substance to diffuse into the agar. When an agar block was placed asymmetrically on a decapitated coleoptile, the shoot curved away from the side with the block, demonstrating that the chemical could stimulate cell elongation on that side. This bioassay underpinned the discovery of auxin.
弗里茨·温特（1926）的经典实验为可扩散的生长促进化学物质提供了有力证据。他将燕麦胚芽鞘尖端切下，放在琼脂块上一段时间，使物质扩散到琼脂中。当琼脂块不对称地放置在去尖的胚芽鞘上时，茎便会向远离琼脂块的一侧弯曲，证明该化学物质能刺激该侧细胞伸长。这项生物测定为生长素的发现奠定了基础。
3. Phototropism: How Plants Grow Towards Light | 向光性：植物如何向光生长
Phototropism is the directional growth of a plant shoot towards unilateral light. Darwin’s experiments using grass coleoptiles (1880) showed that the tip of the coleoptile is essential for sensing light: removing the tip or covering it with an opaque cap prevented bending, while a transparent cap allowed bending. This indicated that the signal is perceived at the tip but the growth response occurs in the elongation zone below.
向光性是植物地上部分朝向单侧光的定向生长。达尔文使用禾本科胚芽鞘进行的实验（1880）表明，胚芽鞘尖端是感光的关键：去除尖端或用不透明帽盖住尖端阻止了弯曲，而透明帽则允许弯曲。这表明信号在尖端被感知，但生长反应发生在下方的伸长区。
Boysen-Jensen (1913) inserted a thin sheet of mica into the shaded side of coleoptile tips, blocking bending, but mica on the illuminated side did not. A gelatin strip, which allows diffusion, permitted bending. This suggested a water-soluble chemical moves from the tip down the shaded side. The current model explains that light triggers redistribution of auxin from the illuminated side to the shaded side, causing cells on the shaded side to elongate faster and the shoot to bend towards the light.
博伊森-延森（1913）将薄薄的云母片插入胚芽鞘尖端的背光侧，弯曲被阻断，而云母片插在向光侧则没有影响。允许扩散的明胶条则没有阻碍弯曲。这表明一种水溶性化学物质从尖端沿着背光侧向下移动。现有模型解释称，光触发生长素从向光侧重新分布到背光侧，使得背光侧细胞伸长更快，茎便向光弯曲。
4. Geotropism: Root and Shoot Responses | 向地性：根和茎的反应
Geotropism (graviptropism) is growth in response to gravity. Shoots exhibit negative geotropism (grow upwards), while roots display positive geotropism (grow downwards). The root cap contains statocytes with starch-filled amyloplasts that sediment to the lower side, causing a redistribution of auxin. In roots, higher auxin concentration on the lower side inhibits elongation, so the upper side grows faster, bending the root downwards. In shoots, the lower side’s higher auxin promotes elongation, bending the shoot upwards.
向地性是对重力的生长反应。茎表现出负向地性（向上生长），而根表现出正向地性（向下生长）。根冠含有含淀粉体的平衡细胞，淀粉体沉降到下方，引起生长素重新分布。在根中，下方较高的生长素浓度抑制伸长，因此上方生长更快，使根向下弯曲。在茎中，下方较高的生长素促进伸长，使茎向上弯曲。
Experimental evidence: if the root cap is removed, the root loses its ability to respond to gravity, although the elongation zone remains intact. When seedlings are placed horizontally, auxin accumulates on the lower side due to gravity-directed transport; application of auxin transport inhibitors abolishes the bending response, confirming the role of polar auxin movement in geotropism.
实验证据：如果去除根冠，根便失去对重力的反应能力，尽管伸长区完好。当幼苗水平放置时，生长素因重力定向运输而积聚在下侧；施加生长素运输抑制剂可消除弯曲反应，证实了生长素极性移动在向地性中的作用。
5. Apical Dominance | 顶端优势
Apical dominance is the phenomenon where the main shoot tip suppresses the growth of lateral buds. The growing apex produces auxin that is transported downwards, inhibiting axillary bud outgrowth. If the shoot tip is removed (decapitation), the lateral buds are released and begin to grow. Application of IAA to the cut stump restores inhibition, demonstrating that auxin from the apex maintains dominance.
顶端优势是指主茎顶端抑制侧芽生长的现象。生长中的顶端产生生长素并向下运输，抑制腋芽萌发。如果去除茎尖（打顶），侧芽便被释放并开始生长。在切面上施加IAA可恢复抑制，证明来自顶端的生长素维持着优势。
Cytokinins are known to antagonise this effect: they promote lateral bud growth. If cytokinins are applied directly to a bud, it can overcome auxin-induced inhibition. Therefore, apical dominance is thought to be controlled by the balance between auxin from the shoot tip and cytokinins produced in roots, an interplay that is often tested in synoptic data analysis questions.
已知细胞分裂素可拮抗这种效应：它们促进侧芽生长。若将细胞分裂素直接施加到芽上，便能克服生长素引起的抑制。因此，顶端优势被认为是由茎尖产生的生长素与根中产生的细胞分裂素之间的平衡所控制，这种相互作用常在综合性数据分析题中出现。
6. Gibberellins: Stem Elongation and Seed Germination | 赤霉素：茎的伸长和种子萌发
Gibberellins are a group of plant hormones that promote stem elongation through cell division and cell elongation. They are particularly effective in dwarf plant varieties, which often lack functional gibberellin biosynthesis. Application of gibberellic acid (GA) to dwarf plants restores normal height. Gibberellins also stimulate bolting (rapid stem elongation) in long-day plants and can increase fruit size in grapes, producing seedless varieties.
赤霉素是一类通过促进细胞分裂和细胞伸长来促进茎伸长的植物激素。它们对矮化植物品种特别有效，这些品种通常缺乏功能性赤霉素合成。施用赤霉酸（GA）可恢复矮化植物的正常高度。赤霉素还刺激长日植物的抽薹，并可增大葡萄果实，生产无籽品种。
In seed germination, gibberellins play a central role in mobilising food reserves. After water uptake, the embryo releases gibberellin, which diffuses to the aleurone layer. There it triggers the synthesis of α-amylase and other hydrolytic enzymes. These enzymes break down starch in the endosperm into soluble sugars, providing energy for the growing embryo. Experimental evidence: if embryos are removed from barley grains, no α-amylase is produced; adding exogenous gibberellin restores enzyme production, proving the hormone triggers the response.
在种子萌发中，赤霉素在调动食物储备方面发挥核心作用。吸水后，胚释放赤霉素，扩散到糊粉层。在那里，赤霉素触发α-淀粉酶和其他水解酶的合成。这些酶将胚乳中的淀粉分解为可溶性糖，为生长中的胚提供能量。实验证据：如果从大麦粒中去除胚，则不产生α-淀粉酶；添加外源赤霉素可恢复酶的产生，证明该激素触发了反应。
7. Cytokinins and Cell Division | 细胞分裂素与细胞分裂
Cytokinins are hormones that stimulate cytokinesis and cell division, particularly in roots and shoots. They are synthesised in root tips and transported via the xylem to shoot tissues. Cytokinins also delay leaf senescence by maintaining protein synthesis and preventing chlorophyll breakdown. In tissue culture, a high cytokinin-to-auxin ratio promotes shoot formation, while a high auxin-to-cytokinin ratio favours root development.
细胞分裂素是刺激胞质分裂和细胞分裂的激素，特别是在根和茎中。它们在根尖合成，通过木质部运输到地上组织。细胞分裂素还通过维持蛋白质合成和防止叶绿素分解来延缓叶片衰老。在组织培养中，高细胞分裂素/生长素比例促进芽的形成，而高生长素/细胞分裂素比例则有利于根的分化。
Commercial applications exploit these properties: cytokinin sprays are used to keep cut flowers and leafy vegetables fresh, and in micropropagation to rapidly multiply elite plants. Understanding the antagonistic interaction between cytokinins and auxin is essential for questions on apical dominance and plant development.
商业应用利用了这些特性：细胞分裂素喷洒剂用于保持切花和叶菜的鲜度，并在微繁殖中快速增殖优良植株。理解细胞分裂素与生长素之间的拮抗作用，对于解决有关顶端优势和植物发育的问题至关重要。
8. Abscisic Acid (ABA) and Stress Responses | 脱落酸与应激反应
Abscisic acid (ABA) is often called the stress hormone. It is produced in roots and mature leaves, and its levels rise dramatically under drought, waterlogging and cold. ABA induces stomatal closure by causing guard cells to lose potassium ions and water, thereby reducing transpirational water loss. This is one of the most direct hormone-mediated stress responses tested in OCR exams.
脱落酸（ABA）常被称为胁迫激素。它在根和成熟叶片中产生，并在干旱、水涝和寒冷条件下水平急剧升高。ABA通过使保卫细胞流失钾离子和水分，诱导气孔关闭，从而减少蒸腾失水。这是OCR考试中最常见的激素介导的胁迫反应之一。
ABA is also a key regulator of seed dormancy. It inhibits germination by blocking gibberellin-induced enzyme production. The balance between ABA and gibberellin determines whether a seed remains dormant or germinates. Desert plant seeds often maintain high ABA levels until heavy rains leach the inhibitor, an ecological adaptation frequently cited in applied questions.
ABA还是种子休眠的关键调节因子。它通过阻断赤霉素诱导的酶产生来抑制萌发。ABA与赤霉素的平衡决定了种子是保持休眠还是萌发。沙漠植物种子常保持高ABA水平，直到大雨淋洗掉这种抑制物，这是一种常在应用题中提及的生态适应。
9. Ethene: Fruit Ripening and Leaf Abscission | 乙烯：果实成熟与叶片脱落
Ethene (ethylene) is a simple gaseous hydrocarbon that acts as a plant hormone. It is produced in large amounts by ripening fruits, aging leaves and flowers. Ethene promotes fruit ripening by stimulating the conversion of starch to sugars, softening cell walls and producing characteristic colour and aroma changes. The ripening process is autocatalytic: a small amount of ethene triggers its own further production, leading to a rapid, coordinated ripening in climacteric fruits such as bananas and tomatoes.
乙烯是一种简单的气态烃，作为植物激素起作用。它在成熟果实、衰老叶片和花朵中大量产生。乙烯通过刺激淀粉转化为糖、软化细胞壁并产生特有的颜色和香气变化来促进果实成熟。成熟过程是自催化的：少量乙烯会触发自身进一步产生，导致跃变型果实（如香蕉和番茄）的快速、协调成熟。
Ethene also accelerates leaf and fruit abscission by promoting the formation of an abscission layer at the base of the petiole. This leads to programmed shedding of organs, a critical process in seasonal plants. In horticulture, ethene is used to ripen fruits uniformly before marketing. Inhibitors of ethene action, like silver ions, can prolong shelf life.
乙烯还通过促进叶柄基部离层的形成来加速叶片和果实的脱落，导致器官的程序性脱落，这是季节性植物的关键过程。在园艺中，乙烯被用来在销售前均匀地催熟果实。乙烯作用抑制剂，如银离子，可以延长货架期。
10. Commercial Applications of Plant Hormones | 植物激素的商业应用
Synthetic auxins such as NAA (naphthaleneacetic acid) and IBA (indolebutyric acid) are widely used as rooting powders to encourage adventitious root formation in cuttings. High concentrations of synthetic auxins like 2,4-D selectively kill broad-leaved weeds by causing uncontrolled growth, making them effective herbicides in cereal crops.
合成生长素如NAA（萘乙酸）和IBA（吲哚丁酸）被广泛用作生根粉，以促进插条的不定根形成。高浓度的合成生长素如2,4-D通过引起失控生长选择性地杀死阔叶杂草，使其成为谷类作物中的有效除草剂。
Gibberellins are applied commercially to increase grape size for seedless varieties, improve malting in barley by promoting uniform germination, and delay ripening in citrus to extend the harvest window. Cytokinins are used to maintain post-harvest freshness in broccoli and asparagus. Ethene gas is used in ripening rooms to bring climacteric fruit to market-ready condition. ABA analogues are explored for inducing drought tolerance and extending seed dormancy in storage.
赤
Published by TutorHao | A-Level Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导