📚 IB WJEC Biology: Plant Hormones – Exam Clinic | IB WJEC 生物：植物激素考点精讲
Plant hormones are chemical signals that orchestrate growth, development, and responses to the environment in plants. Mastering their roles, transport mechanisms, and experimental evidence is essential for top marks in both IB and WJEC biology exams. This article breaks down every key hormone, links theory to classic experiments, and highlights the exact detail examiners expect.
植物激素是协调植物生长、发育和对环境响应的化学信号。掌握它们的作用、运输机制以及实验证据是在 IB 和 WJEC 生物考试中取得高分的关键。本文将拆解每一种重要激素，将理论与经典实验联系起来，并强调阅卷人所期望的准确细节。
1. Introduction to Plant Hormones | 植物激素简介
Plant hormones, or phytohormones, are organic compounds produced in small quantities that regulate physiological processes at low concentrations. Unlike animal hormones, they are not secreted by dedicated endocrine glands but are synthesised in meristems, leaves, fruits, and roots, often acting locally or being transported via vascular tissues.
植物激素又称植物生长物质，是微量的有机化合物，能在低浓度下调控生理过程。与动物激素不同，它们不由专门的内分泌腺分泌，而是在分生组织、叶片、果实和根中合成，常在局部起作用或通过维管组织运输。
The five classical groups are auxins, gibberellins, cytokinins, abscisic acid (ABA), and ethylene. Each hormone can exhibit multiple effects depending on concentration, tissue sensitivity, and interactions with other hormones.
五大经典类群为生长素、赤霉素、细胞分裂素、脱落酸（ABA）和乙烯。每种激素可表现出多种效应，取决于浓度、组织敏感性以及与其他激素的相互作用。
2. Auxin (IAA): The Master Growth Regulator | 生长素（IAA）：主要的生长调节剂
The most studied auxin is indole-3-acetic acid (IAA). It is synthesised primarily in shoot apical meristems and young leaves from the amino acid tryptophan. Polar transport from the shoot tip downwards in the phloem ensures directional growth responses.
研究最多的生长素是吲哚-3-乙酸（IAA）。它主要由色氨酸在茎端分生组织和幼叶中合成。从茎尖向下通过韧皮部的极性运输确保了定向生长反应。
Auxin promotes cell elongation in shoots and inhibits growth in roots at equivalent concentrations. It also stimulates lateral root formation, maintains apical dominance, and promotes fruit development.
生长素在等浓度下促进茎细胞伸长却抑制根的生长。它还能刺激侧根形成、维持顶端优势并促进果实发育。
3. Acid Growth Hypothesis and Cell Elongation | 酸生长假说与细胞伸长
The acid growth hypothesis explains auxin-induced cell elongation. IAA binds to a receptor and stimulates the plasma membrane H⁺-ATPase to pump protons (H⁺) into the cell wall space. This lowers the apoplastic pH, activating expansins that break hydrogen bonds between cellulose microfibrils.
酸生长假说解释生长素诱导的细胞伸长。IAA 与受体结合，刺激质膜 H⁺-ATP酶将质子（H⁺）泵出到细胞壁空间。这降低了质外体 pH，激活扩张蛋白，破坏纤维素微纤维之间的氢键。
The loosened cell wall allows turgor-driven water uptake and irreversible extension. This mechanism is important in tropisms and explains why auxin stimulates stem growth but often inhibits root elongation, where the cell wall response may differ.
松散的细胞壁允许由膨压驱动的水分吸收和不可逆伸展。该机制在向性中很重要，也解释了为什么生长素刺激茎生长但常抑制根伸长，因为根的细胞壁响应可能不同。
Auxin → H⁺ efflux → pH drop → expansin activation → cell wall loosening → elongation
生长素 → H⁺ 外排 → pH下降 → 扩张蛋白激活 → 细胞壁松弛 → 伸长
4. Phototropism: Role of Auxin | 向光性：生长素的作用
Phototropism is directional growth towards or away from light. In coleoptile and shoot tips, unidirectional blue light causes lateral redistribution of auxin. PIN proteins relocate to the shaded side, transporting IAA away from light, creating a higher auxin concentration on the shaded side.
向光性是指植物朝着或背离光源的方向性生长。在胚芽鞘和茎尖中，单向蓝光导致生长素侧向重新分布。PIN 蛋白重新定位到背光侧，将 IAA 运离光照，使背光侧生长素浓度更高。
Higher auxin on the shaded side promotes greater cell elongation, bending the shoot towards the light. In roots, phototropism is typically negative, and the growth inhibition at higher auxin concentration causes bending away from light.
背光侧较高的生长素浓度促进更强的细胞伸长，使茎向光弯曲。在根中，向光性通常为负，较高生长素浓度下的生长抑制导致根背离光源弯曲。
5. Gravitropism: Auxin Redistribution | 向地性：生长素的重新分布
Gravitropism (geotropism) orients plant organs relative to gravity. Statoliths (amyloplasts containing starch) in root cap columella cells settle under gravity, triggering auxin redistribution. In a horizontally placed root, auxin accumulates on the lower side.
向地性使植物器官相对于重力定向。根冠柱状细胞中的平衡石（含淀粉的造粉体）在重力下沉降，触发生长素重新分布。在水平放置的根中，生长素在下侧积累。
In roots, higher auxin inhibits elongation, so cells on the upper side grow faster, bending the root downwards. In shoots, the same auxin accumulation promotes elongation on the lower side, causing upward bending. This opposite response is known as differential sensitivity.
在根中，较高浓度的生长素抑制伸长，因此上侧细胞生长较快，使根向下弯曲。在茎中，同样的生长素积累促进下侧伸长，导致向上弯曲。这种相反的反应称为差异敏感性。
6. Gibberellins: Stem Elongation and Seed Germination | 赤霉素：茎伸长与种子萌发
Gibberellins are a large family of tetracyclic diterpenoids; the most active form is gibberellic acid (GA₃). They promote internode elongation by stimulating cell division and cell elongation. GA application can restore normal height in dwarf (mutant) plants that lack functional GA biosynthetic enzymes.
赤霉素是一个庞大的四环二萜家族，活性最强的形式是赤霉酸（GA₃）。它们通过刺激细胞分裂和伸长促进节间伸长。施用 GA 可以使缺乏功能性 GA 生物合成酶的矮化（突变体）植株恢复正常高度。
In cereal grain germination, GA released from the embryo diffuses to the aleurone layer, where it induces transcription of α-amylase mRNA. α-amylase hydrolyses starch into maltose, providing energy for the growing embryo. This is a classic gene expression control pathway examiners love.
在谷类种子萌发中，由胚释放的 GA 扩散至糊粉层，诱导 α-淀粉酶 mRNA 的转录。α-淀粉酶将淀粉水解为麦芽糖，为生长的胚提供能量。这是一条阅卷人青睐的经典基因表达调控途径。
GA → aleurone layer → transcription factor → α-amylase gene → starch → maltose
GA → 糊粉层 → 转录因子 → α-淀粉酶基因 → 淀粉 → 麦芽糖
7. Abscisic Acid (ABA): Stress Hormone | 脱落酸（ABA）：胁迫激素
ABA is synthesised from carotenoids, particularly in plastids. Despite its name, ABA has little direct role in abscission; its primary function is as a stress signal. During drought, ABA levels rise in leaves, triggering stomatal closure to reduce water loss.
ABA 由类胡萝卜素合成，特别是在质体中。尽管名为脱落酸，它在脱落中的作用很小；其主要功能是作为胁迫信号。干旱期间，叶片中 ABA 水平上升，触发气孔关闭以减少水分流失。
ABA binds to guard cell receptors, causing a cascade that opens Ca²⁺ channels and promotes efflux of K⁺ and Cl⁻. The resulting osmotic water loss reduces turgor, closing the stomatal pore. ABA also maintains seed dormancy in adverse conditions, inhibiting gibberellin-induced germination.
ABA 与保卫细胞受体结合，引发级联反应，打开 Ca²⁺ 通道并促进 K⁺ 和 Cl⁻ 的外流。由此产生的渗透失水降低膨压，关闭气孔。ABA 还在不利条件下维持种子休眠，抑制赤霉素诱导的萌发。
8. Ethylene: The Gaseous Hormone | 乙烯：气体激素
Ethylene (C₂H₄) is a simple gaseous hydrocarbon that acts as a hormone. It is produced from methionine via the Yang cycle and diffuses readily through tissues. Ethylene promotes fruit ripening, flower senescence, leaf abscission, and the triple response in etiolated seedlings.
乙烯（C₂H₄）是一种简单的气态碳氢化合物，起激素作用。它由甲硫氨酸通过杨氏循环产生，容易在组织中扩散。乙烯促进果实成熟、花朵衰老、叶片脱落和黄花苗的三重反应。
The triple response—reduced stem elongation, radial swelling, and horizontal growth—helps seedlings navigate around physical obstacles in the soil. Ethylene application is commercially used to synchronise ripening, while inhibitors such as silver thiosulfate delay senescence in cut flowers.
三重反应——茎伸长减少、径向膨大和水平生长——帮助幼苗绕开土壤中的物理障碍。乙烯在商业上用于同步催熟，而硫代硫酸银等抑制剂则用于延缓鲜切花衰老。
9. Cytokinins: Cell Division and Delaying Senescence | 细胞分裂素：细胞分裂与延缓衰老
Cytokinins are adenine derivatives synthesised in root tips and transported upwards via the xylem. They stimulate cytokinesis, promote chloroplast development, and delay leaf senescence by maintaining protein and chlorophyll synthesis.
细胞分裂素是腺嘌呤衍生物，在根尖合成，通过木质部向上运输。它们刺激胞质分裂，促进叶绿体发育，并通过维持蛋白质和叶绿素合成来延缓叶片衰老。
The ratio of auxin to cytokinin is critical in tissue culture: a high auxin:cytokinin ratio induces root formation, while a low ratio promotes shoot formation. This interaction underpins micropropagation techniques used in horticulture.
生长素与细胞分裂素的比例在组织培养中至关重要：高生长素/细胞分裂素比例诱导生根，而低比例促进生芽。这种相互作用支撑着园艺中使用的微繁技术。
10. Apical Dominance and Hormonal Interactions | 顶端优势与激素相互作用
Apical dominance is the suppression of lateral bud growth by the actively growing shoot apex. The classical model holds that auxin produced at the apex moves downwards and indirectly inhibits bud outgrowth. Cytokinins, synthesised in roots, promote bud release, counteracting auxin’s effect.
顶端优势是指活跃生长的茎尖对侧芽生长的抑制。经典模型认为，顶端产生的生长素向下移动，间接抑制侧芽萌发。根中合成的细胞分裂素促进芽的释放，对抗生长素的作用。
Strigolactones, a newer class of hormones, are also implicated in apical dominance. They act as a signal that reduces axillary bud growth. Hormonal control thus exemplifies both synergism (e.g., auxin and gibberellin in stem growth) and antagonism (e.g., ABA vs. GA in germination).
独脚金内酯是一类新发现的激素，也与顶端优势有关。它们作为减少腋芽生长的信号。由此可见，激素调控既有协同作用（如生长素和赤霉素在茎生长中的协同），也有拮抗作用（如 ABA 与 GA 在萌发中的拮抗）。
11. Classic Experiments: Went and Boysen-Jensen | 经典实验：Went 和 Boysen-Jensen
Peter Boysen-Jensen (1913) demonstrated that a chemical signal, not light directly, caused phototropism. He inserted a mica plate or gelatin block between the tip and the stump of oat coleoptiles. Mica on the shaded side blocked the signal and bending; gelatin allowed the signal to pass, confirming water-soluble chemical transmission.
Peter Boysen-Jensen（1913年）证明是化学信号而非光线本身导致向光性。他在燕麦胚芽鞘的尖端和残桩之间插入云母片或明胶块。背光侧的云母阻断了信号和弯曲；明胶则允许信号通过，证实了水溶性化学传递。
Frits Went (1926) developed the agar block technique. He collected hormone diffusing from coleoptile tips into agar and placed blocks asymmetrically on decapitated coleoptiles. This induced curvature even in darkness, proving the chemical (later named auxin) could replace the tip’s function and caused growth proportional to its concentration.
Frits Went（1926年）开发了琼脂块技术。他将胚芽鞘尖端扩散出的激素收集到琼脂中，然后将琼脂块不对称地放在去顶的胚芽鞘上。即使在黑暗中也诱发弯曲，证明了该化学物质（后命名为生长素）可以替代尖端的功能，并且引起的生长与其浓度成正比。
12. Exam Tips and Common Misconceptions | 考试要点与常见误区
IB and WJEC examiners expect precise terminology. State ‘IAA’ rather than simply ‘auxin’ when referring to the natural auxin. Use ‘polar transport’ for basipetal movement in shoots; explain PIN proteins and chemiosmotic gradients for top band marks. Never say auxin ‘directly inhibits’ lateral buds—it acts via second messengers or other hormones.
IB 和 WJEC 阅卷人期望看到精确的术语。当提及天然生长素时，使用“IAA”而非仅仅是“生长素”。在茎中向基部极性运输时用“极性运输”；解释 PIN 蛋白和化学渗透梯度有助于拿到高分。绝不要说生长素“直接抑制”侧芽——它通过第二信使或其他激素起作用。
Avoid confusing tropism mechanisms in roots and shoots. In roots, higher IAA inhibits elongation, while in shoots it promotes elongation—this is differential sensitivity, not a concentration inversion. Also, ethylene is a gas, not a vapour, and ABA does not accelerate leaf abscission in most cases; that role belongs to ethylene.
避免混淆根和芽中的向性机制。在根中，较高 IAA 抑制伸长，而在芽中则促进伸长——这是差异敏感性，而非浓度颠倒。另外，乙烯是气体，而非蒸气；在大多数情况下 ABA 并不加速叶片脱落，该作用属于乙烯。
When describing experiments, link them to hypotheses and control groups. Boysen-Jensen controlled for physical blocks; Went’s curvature test quantified auxin activity. Practise explaining the α-amylase induction as a signal transduction pathway: GA receptor → derepression of repressors → gene expression. Good luck!
描述实验时，要将其与假设和对照组联系起来。Boysen-Jensen 设置了物理阻隔对照组；Went 的弯曲实验量化了生长素活性。练习将 α-淀粉酶诱导解释为一条信号转导通路：GA 受体 → 去抑制阻遏蛋白 → 基因表达。祝好运！
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