📚 Plant Hormones: IB CIE Biology Revision | 植物激素：IB CIE 生物考点精讲
In both IB and CIE Biology syllabuses, plant hormones — also called plant growth regulators — are a key topic that bridges cell signalling, coordination and response, and plant physiology. Understanding how auxins, gibberellins, cytokinins, abscisic acid and ethylene control growth, development and responses to environmental stimuli is essential for exam success. This article breaks down every major concept, experimental evidence and application you need to master.
在IB和CIE生物课程大纲中，植物激素——也称植物生长调节剂——是连接细胞信号、协调与响应以及植物生理学的核心主题。理解生长素、赤霉素、细胞分裂素、脱落酸和乙烯如何控制生长、发育以及对环境刺激的响应，是考试成功的关键。本文分解了所有你需要掌握的主要概念、实验证据和应用。
1. Introduction to Plant Hormones | 植物激素概述
Plant hormones are organic compounds that act at very low concentrations to regulate plant growth and development. Unlike animal hormones, they are not produced in specialised glands; instead, they are synthesised in various tissues such as shoot tips, root tips, and developing seeds. They can act locally or be transported through the vascular system. The five classical plant hormone groups are auxins, gibberellins, cytokinins, abscisic acid (ABA) and ethylene. Their effects are often synergistic or antagonistic, meaning a particular response depends on the balance of several hormones rather than a single one.
植物激素是在极低浓度下起作用的有机化合物，调节植物的生长和发育。与动物激素不同，它们不在专门的腺体中产生，而是在茎尖、根尖和发育中的种子等多种组织中合成。它们可以局部作用，也可以通过维管系统运输。五类经典的植物激素分别是生长素、赤霉素、细胞分裂素、脱落酸和乙烯。它们的效果往往具有协同或拮抗作用，这意味着特定反应取决于多种激素的平衡，而不是单一激素。
Key characteristics of plant hormones include: they are effective in minute quantities, they are often transported from the site of synthesis to the site of action, and a single hormone can trigger multiple responses depending on the tissue and developmental stage. This flexibility is a hallmark of plant growth regulation and explains why plants can adapt their growth to changing conditions without a central nervous system.
植物激素的关键特征包括：它们在极微量时就能起作用，通常从合成部位运输到作用部位，并且同一种激素依赖组织和发育阶段的不同可引发多种反应。这种灵活性是植物生长调节的标志，也解释了为什么植物能够在不具备中枢神经系统的情况下使生长适应变化的环境。
2. Auxin Discovery: The Went Experiment | 生长素的发现：温特实验
Auxins were the first plant hormones to be discovered. In the 1920s, Frits Went built on earlier work by Charles Darwin and Boysen-Jensen. He cut off the tips of oat coleoptiles and placed them on agar blocks, allowing a water-soluble chemical to diffuse into the agar. When these agar blocks were placed asymmetrically on decapitated coleoptiles, they caused bending even in the dark, demonstrating that a chemical messenger — later named auxin — was responsible for the phototropic response. This is the classic Went experiment and forms the basis of the Cholodny–Went hypothesis.
生长素是最早发现的植物激素。20世纪20年代，弗里茨·温特在查尔斯·达尔文和博伊森-詹森早期工作的基础上进行了实验。他切下燕麦胚芽鞘的尖端，将其放在琼脂块上，让一种水溶性化学物质扩散到琼脂中。当这些琼脂块不对称地放在去顶的胚芽鞘上时，即使在黑暗中也引起了弯曲，这表明一种化学信使——后来被命名为生长素——负责向光性反应。这就是经典的温特实验，也是Cholodny–Went假说的基础。
The predominant natural auxin is indole-3-acetic acid (IAA). Synthesised mainly in the shoot apical meristem and young leaves, auxin is transported from cell to cell in a polar fashion. This polar transport requires energy (ATP) and is mediated by PIN efflux carrier proteins located on the basal side of cells. This unidirectional flow creates concentration gradients that direct growth patterns in roots and shoots.
主要的天然生长素是吲哚-3-乙酸（IAA）。主要在茎顶端分生组织和幼叶中合成，生长素以极性方式在细胞间运输。这种极性运输需要能量（ATP），并由位于细胞基底侧膜的PIN外排载体蛋白介导。这种单向流动形成浓度梯度，指导根和茎的生长模式。
3. Auxin and Phototropism | 生长素与向光性
Phototropism is the directional growth of a plant shoot towards light. According to the Cholodny–Went hypothesis, unilateral light causes a lateral redistribution of auxin from the illuminated side to the shaded side of the shoot tip. The higher auxin concentration on the shaded side stimulates more rapid cell elongation, causing the shoot to bend towards the light source. Auxin promotes cell wall loosening by activating H⁺-ATPases, acidifying the cell wall and activating expansins that break hydrogen bonds between cellulose microfibrils. This acid growth hypothesis explains how auxin-induced elongation occurs quickly.
向光性是植物茎朝光源方向生长的现象。根据Cholodny–Went假说，单侧光引起生长素从茎尖的向光侧横向重新分布到背光侧。背光侧较高的生长素浓度刺激细胞更快伸长，导致茎弯向光源。生长素通过激活H⁺-ATP酶、酸化细胞壁并激活扩张蛋白，这些蛋白可打断纤维素微纤维之间的氢键，从而促进细胞壁松弛。这个酸生长假说解释了生长素诱导的伸长是如何快速发生的。
Exam questions frequently ask students to explain the role of auxin in phototropism. Be specific: the receptor (phototropin) detects blue light, lateral transport is via PIN proteins, and the elongation response involves increased wall extensibility. Although the root is less studied in this context, roots exhibit negative phototropism or are generally insensitive, with auxin sometimes inhibiting root elongation.
考题经常要求学生解释生长素在向光性中的作用。要准确回答：光受体（向光蛋白）检测蓝光，横向运输通过PIN蛋白进行，伸长反应涉及细胞壁伸展性的增加。尽管根在此背景下研究较少，但根表现出负向光性或者通常不敏感，而生长素有时会抑制根伸长。
4. Auxin and Gravitropism | 生长素与向地性
Gravitropism (also called geotropism) is the growth response of a plant to gravity. When a root or shoot is placed horizontally, auxin redistributes to the lower side under the influence of gravity. The mechanism involves the sedimentation of starch-filled statoliths in root cap cells, which triggers a signal cascade leading to asymmetric auxin flow. In shoots, the higher auxin concentration on the lower side promotes elongation, so the stem bends upward. In roots, however, the lower side is inhibited because root cells are more sensitive to auxin; high auxin levels suppress elongation, causing the root to bend downward.
向地性（也称向重力性）是植物对重力的生长反应。当根或茎水平放置时，生长素在重力影响下重新分布到下方。机制涉及根冠细胞中含淀粉的平衡石的沉降，这触发了信号级联，导致不对称的生长素流动。在茎中，下方较高的生长素浓度促进伸长，因此茎向上弯曲。然而在根中，下侧的生长受到抑制，因为根细胞对生长素更敏感；高浓度生长素抑制伸长，导致根向下弯曲。
This different sensitivity is a crucial concept. In the root, the optimal auxin concentration for growth is much lower; a supra-optimal concentration on the lower side inhibits expansion, reinforcing the downward curvature. Many CIE A-level questions ask students to compare the gravitropic responses of roots and shoots, so ensure you can describe the Cholodny–Went hypothesis in relation to gravity, not just light.
这种不同的敏感性是关键概念。在根中，适宜生长的生长素浓度要低得多；下方超适宜的浓度抑制了细胞扩张，加强了向下弯曲。许多CIE A-level考题要求学生比较根与茎的向地性反应，因此要确保你能描述与重力（而不仅是光）相关的Cholodny–Went假说。
5. Apical Dominance: Auxin–Cytokinin Interaction | 顶端优势：生长素与细胞分裂素的相互作用
Apical dominance is the phenomenon where the shoot apex inhibits the growth of lateral (axillary) buds. Auxin produced by the apical bud travels downwards and suppresses bud outgrowth. If the apex is removed (decapitation), auxin levels drop and lateral buds start to grow. However, apical dominance is not a simple auxin story: cytokinins, produced in the root tips and transported upwards, promote bud growth. The balance between auxin and cytokinin determines whether a lateral bud remains dormant or grows out.
顶端优势是茎顶端抑制侧芽生长的现象。顶芽产生的生长素向下运输并抑制侧芽生长。如果切除顶端（去顶），生长素水平下降，侧芽便开始生长。然而，顶端优势并不是简单的生长素作用：细胞分裂素在根尖产生并向上运输，促进芽的生长。生长素与细胞分裂素之间的平衡决定了侧芽是保持休眠还是萌发生长。
Recent research also points to the role of strigolactones, but for IB and CIE Biology, the auxin–cytokinin interaction is the central model. Directly applied cytokinins can over-ride apical dominance and stimulate branching. This knowledge is used in horticulture to produce bushier plants through cytokinin sprays or “pinching out” the apical bud.
近期研究还指出了独脚金内酯的作用，但在IB和CIE生物中，生长素–细胞分裂素的相互作用是核心模型。直接施加细胞分裂素可以抵消顶端优势并促进分枝。园艺中利用这一知识，通过喷洒细胞分裂素或“摘心”的方法来培育更茂密的株形。
6. Gibberellins: Stem Elongation and Seed Germination | 赤霉素：茎伸长与种子萌发
Gibberellins (GAs) are a large family of diterpenoid compounds. They were first discovered in a fungus (Gibberella fujikuroi) that causes “foolish seedling” disease in rice, resulting in excessive stem elongation. The most common active form is gibberellic acid (GA₃). Gibberellins are synthesised in young leaves, roots and developing seeds. Their best-known function is to stimulate stem elongation by promoting both cell division and cell elongation. This is dramatically shown when gibberellin is applied to dwarf varieties of pea or maize: they grow to normal heights, indicating that the dwarf phenotype is due to a defect in gibberellin biosynthesis.
赤霉素是一大类二萜类化合物。它们最初是在导致水稻“恶苗病”的真菌（藤仓赤霉）中发现的，该病引起茎过度伸长。最常见的活性形式是赤霉酸（GA₃）。赤霉素在幼叶、根和发育中的种子中合成。它们最著名的功能是通过促进细胞分裂和细胞伸长来刺激茎的伸长。当赤霉素施用于豌豆或玉米的矮生品种时，它们能长到正常高度，这表明矮生表型是由赤霉素生物合成缺陷引起的。
In seed germination, gibberellins play an indispensable role. In cereal grains such as barley, the embryo releases GA that diffuses to the aleurone layer. GA triggers the synthesis of α-amylase and other hydrolytic enzymes. These enzymes break down starch and proteins stored in the endosperm into soluble sugars and amino acids, which are then transported to the growing embryo. This process is exploited in the malting industry, where gibberellin is added to accelerate malt production.
在种子萌发中，赤霉素起着不可或缺的作用。在大麦等谷物中，胚释放赤霉素并扩散至糊粉层。赤霉素触发α-淀粉酶和其他水解酶的合成。这些酶将储存在胚乳中的淀粉和蛋白质分解为可溶性糖和氨基酸，然后运输至生长的胚。这一过程在麦芽工业中被利用，通过添加赤霉素加速麦芽生产。
7. Cytokinins: Cell Division and Delaying Senescence | 细胞分裂素：细胞分裂与延缓衰老
Cytokinins are adenine derivatives that promote cell division (cytokinesis). They are primarily synthesised in the root tips and transported upward through the xylem. The most common natural cytokinin is zeatin. In tissue culture, the ratio of auxin to cytokinin determines organogenesis: a high auxin to cytokinin ratio favours root formation, whereas a high cytokinin to auxin ratio promotes shoot formation. An equal ratio leads to undifferentiated callus growth. This is a classic IB/CIE concept for micropropagation.
细胞分裂素是促进细胞分裂（胞质分裂）的腺嘌呤衍生物。它们主要在根尖合成，并通过木质部向上运输。最常见的天然细胞分裂素是玉米素。在组织培养中，生长素与细胞分裂素的比例决定器官发生：高的生长素/细胞分裂素比有利于形成根，而高的细胞分裂素/生长素比促进形成芽。等比例则导致未分化的愈伤组织生长。这是IB/CIE中关于微繁殖的经典概念。
Cytokinins also delay leaf senescence. A detached leaf treated with cytokinin retains its green colour and protein content longer. This Richmond–Lang effect is explained by cytokinin mobilising nutrients towards the treated area and inhibiting the breakdown of chlorophyll. Cut flower arrangements often benefit from cytokinin treatment to keep leaves fresh.
细胞分裂素还能延缓叶片衰老。一片离体叶片经细胞分裂素处理后，能更长时间保持绿色和蛋白质含量。这种Richmond–Lang效应可以解释为细胞分裂素将营养物质向处理区域调动，并抑制叶绿素的分解。切花保鲜常常受益于细胞分裂素处理。
8. Abscisic Acid (ABA): Stress and Dormancy | 脱落酸：胁迫与休眠
Despite its name, abscisic acid (ABA) is not the primary trigger of organ abscission; that role belongs mainly to ethylene. ABA is a key hormone in plant stress responses. During drought stress, ABA accumulates in leaves and triggers stomatal closure to reduce water loss by transpiration. ABA binds to receptors in guard cells, leading to a signalling
Published by TutorHao | IB Biology Revision Series | aleveler.com
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