Plant Hormones: WJEC A-Level Biology Exam Focus | 植物激素：WJEC A-Level 生物考点精讲

📚 Plant Hormones: WJEC A-Level Biology Exam Focus | 植物激素：WJEC A-Level 生物考点精讲

Plant hormones are chemical messengers that coordinate growth, development, and responses to environmental stimuli. Unlike animal hormones, they are not produced in specialised glands and often act locally. For WJEC A-Level Biology, you need to understand the roles of auxins, gibberellins, ABA, and ethene, and be able to link their mechanisms to experimental evidence.

植物激素是协调生长、发育及对环境刺激作出反应的化学信使。与动物激素不同，它们并非由特化的腺体分泌，且常在局部发挥作用。WJEC A-Level 生物要求你理解生长素、赤霉素、脱落酸和乙烯的作用，并能将其机制与实验证据联系起来。

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

Plant hormones (or plant growth regulators) are organic molecules that, at very low concentrations, influence physiological processes. They move via diffusion, active transport, or in the phloem, and can act synergistically or antagonistically.

植物激素（或称植物生长调节剂）是在极低浓度下就能影响生理过程的有机分子。它们通过扩散、主动运输或韧皮部进行运输，并具有协同或拮抗效应。

Key hormones studied in the WJEC specification include auxins (IAA), gibberellins, abscisic acid (ABA), and ethene. Each one triggers specific signal transduction pathways, altering gene expression and enzyme activity.

WJEC 考纲中的关键激素包括生长素（IAA）、赤霉素、脱落酸（ABA）和乙烯。每种激素都会启动特定的信号转导途径，改变基因表达和酶活性。

2. Auxin (IAA) and Phototropism | 生长素（IAA）与向光性

Phototropism is the directional growth of a plant shoot towards unilateral light. In the plant shoot tip, light causes auxin (indole-3-acetic acid, IAA) to redistribute laterally towards the shaded side.

向光性是植物的茎朝向单侧光方向生长的现象。在茎尖，光照会导致生长素（吲哚-3-乙酸，IAA）横向重新分布，积累在背光一侧。

Higher IAA concentration on the shaded side stimulates cell elongation via activation of expansin proteins and proton pumps (acid growth hypothesis). This causes greater growth on the shaded side, bending the shoot towards light.

背光侧较高的 IAA 浓度通过激活扩张素蛋白和质子泵（酸生长假说）刺激细胞伸长，导致背光侧生长更快，使茎向光弯曲。

Experimental evidence: Decapitated coleoptiles do not bend; replacing the tip unilaterally restores bending. Agar blocks containing IAA placed on one side of a decapitated coleoptile cause curvature away from that side. These support the role of diffusible auxin from the tip.

实验证据：去除胚芽鞘尖端后不再向光弯曲；将尖端单侧放回则恢复弯曲。将含有 IAA 的琼脂块放在去尖胚芽鞘的一侧会导致背向该侧的弯曲。这些实验支持了可扩散的生长素来自尖端的理论。

3. Auxin and Gravitropism | 生长素与向地性

In roots, gravitropism ensures growth downwards into the soil. When a root is placed horizontally, gravity causes amyloplasts (statoliths) in root cap cells to settle on the lower side, triggering IAA redistribution to the lower side.

根中，向地性确保根向下生长进入土壤。当根水平放置时，重力使根冠细胞中的淀粉体（平衡石）沉降至下方，触发 IAA 向下侧重新分布。

In roots, high IAA concentration inhibits cell elongation, whereas in shoots it promotes elongation. Therefore, the upper side of the root elongates more, causing the root to bend downwards.

值得注意的是，根部高浓度 IAA 抑制细胞伸长，而茎中则促进伸长。因此，根的上侧伸长更多，导致根向下弯曲。

Removal of the root cap abolishes the gravitropic response, while replacing it restores the response. This demonstrates that the root cap is the site of gravity perception.

摘除根冠会消除向地性反应，重新放回根冠则恢复反应，证明根冠是重力感知的部位。

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

Apical dominance is the suppression of lateral bud growth by the actively growing apical shoot tip. The shoot tip produces auxin which is transported downwards, maintaining high auxin concentration in the stem.

顶端优势是指活跃生长的茎尖抑制侧芽生长的现象。茎尖产生生长素并向下运输，使茎中维持高浓度的生长素。

High auxin levels inhibit lateral bud outgrowth. Decapitation removes the auxin source, allowing lateral buds to grow. Application of IAA to the decapitated stump restores inhibition.

高浓度生长素抑制侧芽萌发。去除顶端则移除了生长素来源，侧芽得以生长；将 IAA 处理在去顶残端上则恢复抑制。

Cytokinins, produced in roots, promote bud outgrowth. The balance between auxin and cytokinin determines whether lateral buds remain dormant or develop. This concept is often tested in data analysis questions.

根部分泌的细胞分裂素促进侧芽萌发。生长素与细胞分裂素的平衡决定侧芽的休眠或发育，这一概念常在数据分析题中考查。

5. Gibberellins and Seed Germination | 赤霉素与种子萌发

Gibberellins are a group of hormones that break seed dormancy and promote germination. When a seed absorbs water, the embryo releases gibberellins that diffuse to the aleurone layer surrounding the endosperm.

赤霉素是一类打破种子休眠并促进萌发的激素。种子吸水后，胚释放出赤霉素，扩散至包围胚乳的糊粉层。

Gibberellins trigger the aleurone cells to synthesise and secrete α-amylase and other hydrolytic enzymes. These enzymes digest stored starch in the endosperm into sugars, providing energy for the growing embryo.

赤霉素促使糊粉层细胞合成并分泌 α-淀粉酶及其他水解酶。这些酶将胚乳中储存的淀粉分解为糖类，为生长中的胚提供能量。

Experimental evidence: Mutant barley seeds lacking gibberellins do not germinate unless gibberellins are supplied. Adding gibberellin biosynthesis inhibitors prevents germination, which is reversed by adding gibberellins.

实验证据：缺乏赤霉素的突变大麦种子不萌发，除非施加赤霉素。添加赤霉素合成抑制剂可阻止萌发，而加入赤霉素可解除抑制。

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

Gibberellins stimulate stem elongation by promoting both cell division and cell elongation. They act on the intercalary meristems and internode tissues, leading to taller plants with longer internodes.

赤霉素通过促进细胞分裂和细胞伸长来刺激茎的伸长。它们作用于居间分生组织和节间组织，使植株增高、节间伸长。

GA-activated signalling pathways degrade DELLA repressor proteins, which normally inhibit growth-promoting transcription factors. Removal of DELLA allows gene expression for expansion and division.

赤霉素激活的信号通路降解 DELLA 阻遏蛋白，这类蛋白通常会抑制促生长转录因子。DELLA 被降解后，扩张和分裂相关基因得以表达。

Dwarf varieties of rice and wheat often have mutations in the gibberellin synthesis pathway. Application of gibberellins restores normal height, a fact exploited during the Green Revolution to produce high-yield semi-dwarf crops.

水稻和小麦的矮化品种通常存在赤霉素合成通路的突变。施加赤霉素可恢复株高，这一原理在绿色革命中被用于培育高产半矮秆作物。

7. Abscisic Acid (ABA) and Stress Responses | 脱落酸（ABA）与胁迫反应

Abscisic acid is a hormone that generally acts as a growth inhibitor, helping plants tolerate abiotic stress. It induces stomatal closure during drought by triggering guard cells to lose K⁺ ions, losing turgor and closing the stomatal pore.

脱落酸是一种通常是生长抑制剂的激素，帮助植物耐受非生物胁迫。在干旱时，它通过促使保卫细胞排出 K⁺ 离子，使细胞失水、气孔关闭。

ABA also promotes seed dormancy and inhibits precocious germination. Its levels rise during seed maturation, and it represses α-amylase production, opposing gibberellin action.

ABA 还促进种子休眠，抑制过早萌发。在种子成熟过程中 ABA 水平上升，抑制 α-淀粉酶的产生，与赤霉素的作用相互拮抗。

Leaf abscission involves a drop in auxin and an increase in ABA and ethene, leading to the formation of an abscission zone. Cells in this zone produce hydrolytic enzymes to weaken cell walls, causing leaf drop.

叶片脱落涉及生长素浓度下降及 ABA 和乙烯浓度升高，导致离区的形成。离区细胞产生水解酶，弱化细胞壁，导致落叶。

8. Ethene and Fruit Ripening | 乙烯与果实成熟

Ethene (ethylene) is a gaseous hydrocarbon hormone that promotes fruit ripening and leaf abscission. Climacteric fruits like tomatoes, bananas, and apples show a sharp rise in respiration and ethene production at the onset of ripening.

乙烯是一种气态烃类激素，促进果实成熟和叶片脱落。诸如番茄、香蕉、苹果等跃变型果实在成熟启动时表现出呼吸速率和乙烯产量的急剧上升。

Ethene triggers cell wall breakdown by activating enzymes like pectinase and cellulase, converting starch to sugars, and changing pigment composition. It also stimulates the production of aroma volatiles.

乙烯通过激活果胶酶和纤维素酶等酶来促进细胞壁分解，将淀粉转化为糖，并改变色素组成。它还能刺激香气挥发物的产生。

Commercial application: Fruits are often picked unripe and exposed to ethene gas during transport to ensure uniform ripening. Conversely, ethene inhibitors (e.g., 1-MCP) are used to extend shelf life in storage.

商业应用：果实常在未成熟时采摘，运输中接触乙烯气体以确保均匀成熟。相反，在储存中使用乙烯抑制剂（如 1-MCP）可延长货架期。

9. Commercial Uses of Plant Hormones | 植物激素的商业应用

Auxins are widely used as rooting powders to stimulate adventitious root formation in cuttings. Synthetic auxins like 2,4-D act as selective herbicides, causing broad-leaved weeds to exhibit uncontrolled growth and death.

生长素广泛用于生根粉，促进插条不定根的形成。合成生长素如 2,4-D 作为选择性除草剂，使阔叶杂草生长失控而死亡。

Gibberellins are sprayed on seedless grapes to increase fruit size and delay ripening. They also enhance malting in brewing by promoting amylase production.

赤霉素被喷施在无籽葡萄上以增大果粒并延迟成熟。在酿酒中，它们通过促进淀粉酶产生来增进麦芽化过程。

Ethene-releasing compounds (Ethephon) are used to synchronise fruit ripening and to promote latex flow in rubber trees. Cytokinins are sometimes used in tissue culture to promote shoot proliferation.

乙烯释放剂（如乙烯利）用于同步果实成熟，以及促进橡胶树胶乳的流动。细胞分裂素有时用于组织培养中促进芽的增殖。

10. Interactions Between Hormones | 激素间的相互作用

Plant responses rarely rely on a single hormone; instead, the ratio of different hormones determines the outcome. Apical dominance depends on the auxin-to-cytokinin ratio, while seed dormancy is controlled by the ABA-to-gibberellin balance.

植物反应很少依赖单一激素，更常见的是不同激素的比例决定结果。顶端优势取决于生长素与细胞分裂素的比例，而种子休眠由 ABA 与赤霉素的平衡控制。

During leaf senescence, a decrease in cytokinin and an increase in ABA and ethene triggers the breakdown of chlorophyll and nutrient remobilisation before leaf fall.

在叶片衰老过程中，细胞分裂素减少而 ABA 和乙烯增加，引起叶绿素降解和养分转运，随后落叶。

Understanding these interactions helps explain experimental data where multiple factors change simultaneously. WJEC exam questions often ask you to interpret graphs showing hormone changes during a process.

理解这些相互作用有助于解释实验数据中多个因子同时变化的现象。WJEC 考试常会要求你解释在某一过程中激素变化的关系图。

11. Experimental Skills and Common Pitfalls | 实验技能与常见陷阱

You should be able to design experiments to test hormone effects, such as using coleoptile curvature tests, pea epicotyl elongation, or seed germination assays. Controls must exclude the hormone entirely, and variables like light and temperature must be standardised.

你应当能设计实验检验激素效应，例如使用胚芽鞘弯曲试验、豌豆上胚轴伸长或种子萌发试验。对照必须完全排除激素，且光照、温度等变量必须标准化。

Common mistakes include confusing the direction of auxin movement (lateral redistribution, not active pumping across the whole shoot) and misapplying root vs. shoot responses to auxin concentration.

常见错误包括混淆生长素移动方向（横向重新分布，而非整个地上部的主动泵送），以及错误套用根与茎对生长素浓度的不同反应。

When interpreting data, pay attention to whether the graph shows relative growth, inhibition percentage, or absolute concentration. Be prepared to link the effect of a hormone to the genetic and enzymatic pathways it targets.

解读数据时注意图形展示的是相对生长、抑制百分比还是绝对浓度。要准备好将激素的作用与其靶向的基因和酶途径联系起来。

12. Exam Tips and Summary | 考试技巧与总结

For WJEC exams, focus on precisely stating the mechanism: for phototropism, IAA moves laterally to the shaded side, promoting elongation; for gravitropism in roots, high IAA inhibits elongation. Use precise terminology such as ‘statoliths’, ‘aleurone layer’, and ‘DELLA proteins’ where relevant.

在 WJEC 考试中，重点关注机制的准确表述：向光性中 IAA 横向移动到背光侧促进伸长；根向地性中高 IAA 抑制伸长。尽可能使用诸如“平衡石”“糊粉层”“DELLA 蛋白”等精确术语。

Learn the classic experiments (Darwin, Went, Boysen-Jensen) and be able to interpret the equipment used. Explain commercial uses with a concrete example. Finally, always link hormone action to cell-level changes – enzyme activity, cell wall plasticity, and gene expression.

掌握经典实验（达尔文、温特、博伊森-詹森），并能够解释所用装置。用具体例子说明商业应用。最后，始终将激素作用与细胞层面的变化——酶活性、细胞壁可塑性和基因表达——联系起来。

Published by TutorHao | WJEC Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）