Plant Hormones | 植物激素考点精讲

📚 Plant Hormones | 植物激素考点精讲

Plant hormones, also known as phytohormones, are chemical messengers that coordinate growth, development, and responses to environmental stimuli in plants. Unlike animal hormones, they are often produced in small amounts, can act locally or at a distance, and their effects frequently depend on interactions between multiple signalling molecules. For IB and CCEA Biology, understanding the key plant hormones—auxin, gibberellin, cytokinin, abscisic acid, and ethylene—is essential, especially their roles in tropisms, seed germination, apical dominance, and fruit ripening.

植物激素，也称植物荷尔蒙，是协调植物生长、发育和响应环境刺激的化学信使。与动物激素不同，它们通常只需微量即可发挥作用，可以在产生部位附近或远处起作用，并且其效应常常取决于多种信号分子的相互作用。在 IB 和 CCEA 生物课程中，理解关键植物激素——生长素、赤霉素、细胞分裂素、脱落酸和乙烯——的作用至关重要，尤其要掌握它们在向性运动、种子萌发、顶端优势和果实成熟中的功能。

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

Plant hormones are organic compounds synthesised in one part of the plant and transported to target cells, where they trigger specific physiological responses at very low concentrations. They do not act in isolation; rather, the final effect on a plant tissue is determined by the balance between several hormones. Five major classes dominate the examination specifications: auxins, gibberellins, cytokinins, abscisic acid (ABA), and ethylene.

植物激素是在植物某个部位合成并被运输到靶细胞的有机化合物，在极低浓度下即可引发特定的生理反应。它们并非单独起作用；相反，对植物组织的最终效果取决于多种激素之间的平衡。考试大纲主要涵盖五大类：生长素、赤霉素、细胞分裂素、脱落酸和乙烯。

Hormones can stimulate or inhibit processes. For example, auxin promotes cell elongation in shoots but inhibits it in roots at high concentrations. Understanding these dual roles is a common exam theme. Unlike animal systems, plants lack specialist endocrine glands; each cell can potentially produce hormones.

激素可以促进或抑制生理过程。例如，生长素在茎中促进细胞伸长，但在根中高浓度时却抑制伸长。理解这种双重作用是常见的考试主题。与动物系统不同，植物缺乏专门的内分泌腺；每个细胞都可能具有产生激素的能力。

2. Auxins and Phototropism | 生长素与向光性

Auxins (principally indole-3-acetic acid, IAA) are synthesised in shoot and root apical meristems and young leaves. In phototropism, unilateral light causes lateral redistribution of auxin towards the shaded side of a coleoptile or stem. The higher auxin concentration on the shaded side stimulates cell elongation more than on the illuminated side, causing the shoot to bend towards the light.

生长素（主要是吲哚-3-乙酸，IAA）在茎尖和根尖分生组织及幼叶中合成。在向光性中，单向光照导致生长素向胚芽鞘或茎的背光侧横向重新分布。背光侧较高的生长素浓度比向光侧更强烈地刺激细胞伸长，使茎朝光源弯曲。

The classic Went experiment demonstrated that the tip of a coleoptile produces a diffusible chemical (auxin) that promotes growth. If the tip is removed, growth ceases; if the tip is placed on an agar block and then the block is applied asymmetrically to a decapitated coleoptile, bending occurs in darkness, proving that a chemical messenger induces curvature independently of light.

经典的温特实验证明，胚芽鞘尖端产生一种可扩散的化学物质（生长素），能促进生长。如果切除尖端，生长停止；如果把尖端放在琼脂块上，再将琼脂块不对称地置于去顶的胚芽鞘上，在黑暗中也会发生弯曲，证明化学信使能在无光条件下诱导弯曲。

3. Auxins and Gravitropism | 生长素与向地性

In roots, gravity sensing occurs in root cap cells containing statoliths (dense starch grains). When a root is placed horizontally, statoliths sediment to the lower side, triggering a redistribution of auxin. In roots, high auxin concentration inhibits cell elongation, so the lower side grows more slowly than the upper side, causing the root to bend downwards.

在根中，对重力的感知发生在含有平衡石（致密淀粉粒）的根冠细胞内。当根水平放置时，平衡石沉降到下方，引发生长素的重新分布。在根中，高浓度生长素抑制细胞伸长，因此下侧生长慢于上侧，导致根向下弯曲。

This differential sensitivity is a key exam point: the same hormone, auxin, has opposite effects in shoots and roots at a given concentration. In shoots, the higher concentration on the lower side of a horizontal stem stimulates growth, making the stem bend upwards, a negative gravitropic response.

这种差异敏感性是重要考点：同一种激素——生长素，在某一浓度下对茎和根具有相反的效应。在茎中，水平放置的茎下侧较高的生长素浓度刺激生长，使茎向上弯曲，表现为负向地性反应。

4. Auxin in Apical Dominance | 生长素与顶端优势

Apical dominance is the phenomenon where the main shoot apex inhibits the growth of lateral buds. The terminal bud produces auxin, which moves downwards and suppresses the outgrowth of axillary buds further down the stem. When the apex is removed (decapitation), lateral buds are released from inhibition and start to grow.

顶端优势是指主茎顶芽抑制侧芽生长的现象。顶芽产生生长素，并向下运输，抑制下部腋芽的生长。当顶芽被切除（去顶）时，侧芽摆脱抑制并开始生长。

Cytokinins produced in roots antagonise this effect by promoting bud outgrowth. The balance between auxin from the shoot and cytokinin from the roots determines branching architecture. In agriculture, pruning or applying cytokinin sprays can be used to manipulate plant shape and improve crop yield.

根部产生的细胞分裂素通过促进侧芽生长来拮抗这一效应。来自茎的生长素和来自根的细胞分裂素之间的平衡决定了分枝结构。在农业上，修剪或喷施细胞分裂素可用于调控植株形态并提高作物产量。

5. Gibberellins and Stem Elongation | 赤霉素与茎伸长

Gibberellins are a large group of diterpenoid compounds. The most studied form, gibberellic acid (GA₃), promotes internode elongation by stimulating cell division and cell expansion. Dwarf varieties of many plants lack the ability to produce active gibberellins; when supplied with exogenous GA₃, they grow to normal heights, demonstrating the hormone’s role directly.

赤霉素是一大类二萜化合物。研究最多的形式——赤霉酸（GA₃）通过刺激细胞分裂和细胞扩张来促进节间伸长。许多植物的矮化品种缺乏产生活性赤霉素的能力；当外源施加 GA₃ 时，它们能长到正常高度，直接证明了该激素的作用。

Gibberellins also activate the breakdown of DELLA repressor proteins, which normally restrain growth. This molecular mechanism allows rapid responses to environmental signals such as light and temperature, and is frequently questioned in exams linking gene expression to plant development.

赤霉素还激活 DELLA 抑制蛋白的降解，这些蛋白质通常抑制生长。这种分子机制使得植物能快速响应光和温度等环境信号，考试中常将基因表达与植物发育联系起来考查。

6. Gibberellins in Seed Germination | 赤霉素与种子萌发

In cereal grains, germination begins with water uptake, which triggers the embryo to release gibberellins. Gibberellins diffuse to the aleurone layer, where they induce the synthesis of α-amylase. This enzyme hydrolyses starch in the endosperm into sugars, providing energy for the growing embryo.

在谷物种子中，萌发始于吸水，吸水促使胚释放赤霉素。赤霉素扩散到糊粉层，诱导 α-淀粉酶的合成。该酶将胚乳中的淀粉水解为糖，为胚的生长提供能量。

This is a classic example of a hormone-controlled enzyme induction. Exam questions often present experimental data: removing the embryo prevents α-amylase production, but adding gibberellin to embryoless seeds restores enzyme activity, proving the role of the embryo and gibberellin in germination.

这是激素控制酶诱导的经典例子。考试题目常常给出实验数据：去除胚会阻止 α-淀粉酶的产生，但向无胚种子添加赤霉素可恢复酶活性，从而证明胚和赤霉素在萌发中的作用。

7. Cytokinins and Cell Division | 细胞分裂素与细胞分裂

Cytokinins are adenine derivatives produced primarily in root tips and transported via xylem. They promote cytokinesis (cell division) in shoot apical meristems and interact with auxin in tissue culture: a high auxin-to-cytokinin ratio stimulates root formation, whereas a low ratio favours shoot development. An intermediate ratio leads to undifferentiated callus growth.

细胞分裂素是主要合成于根尖并通过木质部分运输的腺嘌呤衍生物。它们促进茎尖分生组织的胞质分裂（细胞分裂），并在组织培养中与生长素相互作用：高生长素/细胞分裂素比例刺激根形成，而低比例则利于芽发育。中间比例导致未分化愈伤组织生长。

Cytokinins also delay leaf senescence (ageing) by maintaining protein synthesis and chlorophyll content. Sprays containing cytokinins are sometimes used on cut flowers and leafy vegetables to prolong shelf life, a direct application examined in biology papers.

细胞分裂素还通过维持蛋白质合成和叶绿素含量来延缓叶片衰老。含有细胞分裂素的喷雾有时用于切花和叶菜以延长货架期，这是生物试题中常见的直接应用。

8. Abscisic Acid and Stress Responses | 脱落酸与逆境响应

Abscisic acid (ABA) is misnamed; it does not primarily induce abscission but acts as a stress hormone. During water deficiency, ABA accumulates in leaves and rapidly triggers stomatal closure by causing guard cells to lose turgor. This reduces transpiration and helps the plant survive drought conditions.

脱落酸（ABA）被误名；它并不主要引起脱落，而是作为一种胁迫激素起作用。在缺水时，ABA 在叶片中积累，通过使保卫细胞失水膨压下降迅速引发气孔关闭。这减少了蒸腾作用，帮助植物在干旱条件下存活。

ABA also promotes seed dormancy by inhibiting gibberellin-induced enzyme production. The balance of ABA to gibberellin determines whether a seed remains dormant or germinates. Cold stratification or light can lower ABA levels and break dormancy, a topic regularly featured in exam problem-solving contexts.

ABA 还通过抑制赤霉素诱导的酶产生来促进种子休眠。ABA 与赤霉素的平衡决定种子是保持休眠还是萌发。冷层积或光照可降低 ABA 水平并打破休眠，这是考试中经常在解题情境中出现的主题。

9. Ethylene and Fruit Ripening | 乙烯与果实成熟

Ethylene is a simple gaseous hydrocarbon (C₂H₄) that acts as a ripening hormone in climacteric fruits such as bananas, tomatoes, and apples. It triggers a cascade of reactions including starch hydrolysis, cell wall softening, colour changes, and aroma production. Ethylene is produced in autocatalytic bursts: a small initial amount stimulates further ethylene synthesis.

乙烯是一种简单的气态碳氢化合物（C₂H₄），在香蕉、番茄和苹果等跃变型果实中作为催熟激素。它引发一系列反应，包括淀粉水解、细胞壁软化、颜色变化和香气产生。乙烯以自催化爆发的方式产生：少量初始乙烯会刺激更多乙烯的合成。

The classic triple response of etiolated pea seedlings to ethylene—shortening and thickening of the hypocotyl, reduced root elongation, and exaggerated apical hook—allows plants to push through obstacles and is a standard experimental observation. Commercial use of ethylene or its inhibitors (e.g., silver thiosulfate) controls ripening and flower senescence in the horticultural industry.

黄化豌豆幼苗对乙烯的典型三重反应——下胚轴缩短增粗、根伸长减弱、顶端钩加剧——使植物能推开障碍物，是标准的实验观察。在园艺产业中，商业上利用乙烯或其抑制剂（如硫代硫酸银）来控制成熟和切花衰老。

10. Synergistic and Antagonistic Interactions | 激素间的协同与拮抗

Plant responses rarely depend on a single hormone. Instead, the ratio and interplay between hormones dictate the outcome. For instance, auxin and cytokinin antagonistically regulate axillary bud outgrowth. Auxin and gibberellin can act synergistically to promote stem elongation, while ABA and gibberellin are antagonistic in seed dormancy.

植物的反应很少依赖单一的激素。相反，激素之间的比例和相互作用决定了最终的结果。例如，生长素和细胞分裂素拮抗性地调控腋芽生长。生长素和赤霉素可以协同作用促进茎伸长，而ABA 和赤霉素在种子休眠中相互拮抗。

Exam questions often require students to predict outcomes when a hormone is added or its synthesis is blocked genetically. Constructing flow diagrams showing the balance of promoters and inhibitors helps clarify these networks. The concept of ‘hormonal crosstalk’ is increasingly important in current specifications.

考试题常要求学生预测当添加某种激素或从遗传上阻断其合成时会出现什么结果。构建显示促进因子与抑制因子平衡的流程图有助于阐明这些网络。“激素互作”的概念在当前大纲中日益重要。

11. Commercial Applications and Ethical Considerations | 商业应用与伦理考量

Understanding plant hormones allows manipulation of crop growth, yield, and post-harvest quality. Auxin-based rooting powders stimulate adventitious root formation in cuttings. Gibberellins are sprayed on grapes to increase berry size and on sugarcane to boost stem length and sugar yield. Ethylene is used in ripening chambers, while inhibitors of ethylene (like 1-MCP) extend the shelf life of fruits and flowers.

了解植物激素使得我们能够调控作物生长、产量和采后品质。基于生长素的生根粉可刺激插条的不定根形成。赤霉素被喷洒在葡萄上以增大浆果，喷在甘蔗上以增加茎长和糖产量。乙烯用于催熟室，而乙烯抑制剂（如 1-MCP）则延长水果和花卉的货架期。

These applications raise questions about food safety and environmental impact. Regulators assess maximum residue limits to ensure consumer safety. In the IB and CCEA curriculum, students should be able to evaluate the benefits and risks of using plant growth regulators in agriculture.

这些应用带来了食品安全和环境影响方面的问题。监管机构评估最大残留限量以确保消费者安全。在 IB 和 CCEA 课程中，学生应能评价在农业中使用植物生长调节剂的益处和风险。

12. Exam Tips and Common Mistakes | 考试贴士与常见错误

Many marks are lost by confusing the sites of synthesis or the direction of auxin movement. Remember: auxin is synthesised in apical meristems and moves polarly towards the base in shoots; in tropisms, lateral redistribution is driven by environmental cues, not by destruction of auxin on the illuminated side. Avoid saying auxin is ‘destroyed by light’—the modern consensus is that light triggers redistribution via PIN proteins.

很多失分是因为混淆了合成部位或生长素的移动方向。记住：生长素在顶端分生组织合成，在茎中极性向下运输；在向性运动中，侧向重新分布是由环境信号驱动的，而不是光照一侧的生长素被破坏。避免说生长素“被光破坏”——现代共识是光通过 PIN 蛋白触发重新分布。

Another common error is assuming that all hormones are exclusively promoters. ABA inhibits growth, and high auxin concentrations inhibit root elongation. Also, when describing the effect of gibberellin on dwarf plants, clarify that it restores wild-type height rather than making plants ‘taller than normal’. Always think in terms of dose, tissue sensitivity, and hormone interactions.

另一个常见错误是以为所有激素都只起促进作用。ABA 抑制生长，高浓度生长素抑制根伸长。此外，在描述赤霉素对矮化植物的影响时，要说明它恢复野生型株高，而不是让植株“高于正常”。始终要从剂量、组织敏感性和激素相互作用的角度思考。

Draw labelled diagrams of the Went experiment and the starch statolith hypothesis wherever possible. Use precise terminology: ‘phototropism’ not ‘growing towards light’, ‘apical dominance’ not ‘top of plant inhibits bottom’. Linking the molecular action of DELLA proteins to gibberellin responses can impress examiners and demonstrate a deeper understanding.

尽可能绘制带标注的温特实验图和淀粉平衡石假说图。使用精确的术语：“向光性”而非“朝着光生长”，“顶端优势”而非“植物顶部抑制底部”。将 DELLA 蛋白的分子作用与赤霉素反应联系起来会让考官印象深刻，并展示出更深层的理解。

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Plant Hormones | 植物激素 考点精讲