📚 2.81 Factors Affecting the Rate of Transpiration | 影响蒸腾速率的因素
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata in the leaves. In Edexcel IGCSE Biology, you need to know how four environmental factors change the rate at which this water loss happens. This article gives you a clear, exam-ready explanation of each factor, with English and Chinese paired together so you can revise effectively.
蒸腾作用是指植物地上部分（主要是叶片上的气孔）以水蒸气形式散失水分的过程。在 Edexcel IGCSE 生物考试中，你需要掌握四个环境因素如何改变植物失水的速率。本文将每个因素都作了清晰且适合考试的解释，并用中英双语对照的方式帮助你高效复习。
1. What Is Transpiration? | 什么是蒸腾作用？
Water is absorbed by root hair cells and moves up the plant through xylem vessels. It eventually reaches the leaves, where it evaporates from the surfaces of mesophyll cells into the air spaces inside the leaf.
水分通过根毛细胞被吸收，并沿木质部导管向上运输。最终到达叶片，在叶肉细胞表面蒸发进入叶片内部的空气间隙。
The water vapour then diffuses out of the leaf through tiny pores called stomata. This diffusion from the moist internal leaf surface to the drier outside air is the process of transpiration.
水蒸气随后通过称为气孔的微小孔隙扩散出叶片。水分从湿润的叶片内部表面扩散到较干燥的外部空气，这就是蒸腾作用。
Stomata must be open to allow carbon dioxide to enter for photosynthesis. However, when the stomata are open, water vapour is inevitably lost. Transpiration is therefore an unavoidable consequence of gas exchange in leaves.
气孔必须张开才能让二氧化碳进入以进行光合作用。然而，当气孔张开时，水蒸气也不可避免地会散失。因此，蒸腾作用是叶片进行气体交换时不可避免的后果。
2. The Transpiration Stream | 蒸腾流
→ diffusion through stomata. This whole movement is often called the transpiration stream.
水分在植物体内沿一条连续路径移动：根毛细胞 → 根皮层 → 根和茎中的木质部 → 叶片中的木质部 → 叶肉细胞中蒸发 → 通过气孔扩散。这整个流动过程通常称为蒸腾流。
Xylem vessels are long, hollow tubes made of dead cells. Their walls are thickened with lignin, and their open ends allow water to pass through easily.
木质部导管是由死细胞组成的长而中空的管道。它们的细胞壁因木质素而加厚，并且细胞首尾相通，便于水分顺利通过。
Water molecules are cohesive, meaning they stick together. When water evaporates from the leaves, it pulls the water column up from the roots. This is called the cohesion-tension theory.
水分子具有内聚力，即它们会相互吸附。当叶片中的水分蒸发时，会向上拉动根部的水柱。这被称为“内聚力-张力学说”。
Transpiration does not require energy from the plant. It is driven by the evaporation of water and the diffusion of water vapour out of the leaf.
蒸腾作用不需要植物消耗能量。它由水分蒸发和水蒸气扩散出叶片所驱动。
3. How Is Transpiration Rate Measured? | 如何测定蒸腾速率？
In the laboratory, you can measure transpiration rate using a potometer. A potometer measures the uptake of water by a cut shoot, which is used as an approximation of the water lost by transpiration.
在实验室中，可以使用蒸腾计来测定蒸腾速率。蒸腾计测量切下的枝条对水分的吸收量，并以这个吸收量近似代表因蒸腾而散失的水量。
In a simple potometer, a cut shoot is sealed into a tube filled with water. An air bubble is introduced into a narrow capillary tube. As the plant takes up water, the bubble moves along the capillary.
在简易蒸腾计中，将切下的枝条密封在充满水的管中，并在细毛细管中引入一个气泡。当植物吸水时，气泡会沿毛细管移动。
To calculate the rate, measure the distance the bubble travels in a set time, such as 10 minutes. The result is usually recorded as distance per unit time, for example cm/minute.
要计算速率，需要测量气泡在固定时间内（例如10分钟）移动的距离。结果通常以单位时间的距离表示，例如厘米/分钟。
Another method is to weigh a leafy plant or a cut leaf on a sensitive balance at intervals. The loss of mass is due to water loss. The faster the mass decreases, the higher the transpiration rate.
另一种方法是每隔一段时间用精密天平称量带叶植物或离体叶片的重量。质量的减少来自水分流失。质量下降越快，蒸腾速率越高。
4. Stomata and Guard Cells: The Gatekeepers | 气孔与保卫细胞：门卫
Stomata are pores, usually found mainly on the lower surface of dicotyledonous leaves. Each stoma is surrounded by two guard cells. These cells control whether the pore is open or closed.
气孔是叶片上的小孔，双子叶植物的气孔通常主要分布在叶片下表面。每个气孔由两个保卫细胞包围，这些细胞控制气孔的张开或关闭。
When guard cells are turgid, they bend apart and the stoma opens. When they lose water and become flaccid, they straighten and the stoma closes.
当保卫细胞膨胀时，它们弯曲分开，气孔张开。当保卫细胞失水变软时，它们恢复伸直，气孔关闭。
In many plants, potassium ions (K⁺) move into guard cells, causing water to enter by osmosis. This makes the guard cells turgid and opens the stoma.
在许多植物中，钾离子（K⁺）进入保卫细胞，使水分通过渗透作用进入细胞。这使保卫细胞膨胀，气孔张开。
Because stomata open to allow CO₂ entry, they also allow water vapour to escape. Therefore, any factor that changes stomatal opening will affect transpiration rate.
由于气孔张开是为了让二氧化碳进入，同时也会让水蒸气散失。因此，任何改变气孔开度的因素都会影响蒸腾速率。
5. Factor 1: Light Intensity | 因素一：光照强度
In general, as light intensity increases, the rate of transpiration increases. This is because light stimulates the guard cells to open the stomata wider.
一般来说，光照强度增大，蒸腾速率也会增大。这是因为光照会刺激保卫细胞，使气孔张得更大。
In bright light, the plant photosynthesises rapidly and needs more carbon dioxide. The stomata open wider to allow CO₂ in, but this also provides a larger exit for water vapour.
在强光下，植物光合作用旺盛，需要更多二氧化碳。气孔张得更大以让二氧化碳进入，但这也为水蒸气提供了更大的出口。
In darkness, most stomata close because photosynthesis is not occurring. With stomata closed, very little water vapour can escape, so the transpiration rate is very low.
在黑暗中，大多数气孔会关闭，因为光合作用无法进行。气孔关闭后，水蒸气几乎无法逸出，因此蒸腾速率很低。
However, at very high light intensity, all stomata may already be fully open. Increasing light further may then cause no additional rise in transpiration rate.
然而，当光照强度非常高时，所有气孔可能已经全部张开。此时继续增大光照可能不会再使蒸腾速率进一步上升。
6. Factor 2: Temperature | 因素二：温度
Temperature has a major effect on transpiration because it changes the rate of evaporation and diffusion. Higher temperature generally increases transpiration rate.
温度对蒸腾作用有重大影响，因为它会改变蒸发速率和扩散速率。温度升高通常会使蒸腾速率增大。
At higher temperatures, water molecules have more kinetic energy. They evaporate more quickly from the leaf cell walls and diffuse faster out of the stomata.
在较高温度下，水分子具有更大的动能。它们更快地从叶肉细胞壁蒸发，并更快地通过气孔扩散出去。
Warm air can hold more water vapour than cool air. Therefore, on a warm day, the air outside the leaf is drier relative to its capacity, so the concentration gradient between the leaf and the air is steeper.
暖空气比冷空气能容纳更多水蒸气。因此，在温暖的天气里，叶外空气相对其容纳能力来说更干燥，叶片与空气之间的浓度梯度更大。
However, if the temperature becomes extremely high, the plant may close its stomata to conserve water. This can cause the transpiration rate to fall. Guard cells can also be damaged by very high temperatures.
然而，如果温度变得过高，植物可能关闭气孔以节约水分，这会导致蒸腾速率下降。极高温度还可能损伤保卫细胞。
7. Factor 3: Humidity | 因素三：湿度
Humidity is a measure of how much water vapour is present in the air. High humidity means the air is already moist, while low humidity means the air is dry.
湿度是衡量空气中含有多少水蒸气的指标。高湿度意味着空气已经较为湿润，低湿度则意味着空气干燥。
Transpiration occurs because there is a higher concentration of water vapour inside the leaf than in the surrounding air. Water vapour diffuses down this concentration gradient.
蒸腾作用发生的原因是叶片内部水蒸气浓度高于周围空气。水蒸气会沿浓度梯度向外扩散。
When humidity is low, the concentration gradient between the leaf and the air is large, so water vapour leaves the leaf quickly. Transpiration rate is high.
当湿度较低时，叶片与空气之间的浓度梯度大，水蒸气迅速离开叶片，蒸腾速率高。
When humidity is high, the outside air contains a lot of water vapour, so the concentration gradient is small. Water vapour diffuses out more slowly, and transpiration rate is low.
当湿度较高时，外部空气中含有大量水蒸气，浓度梯度小。水蒸气扩散出去的速度较慢，蒸腾速率低。
8. Factor 4: Air Movement | 因素四：空气流动（风）
Air movement, or wind, affects transpiration by removing the water vapour that accumulates just outside the stomata.
空气流动（风）通过移走气孔外侧积聚的水蒸气来影响蒸腾作用。
Without wind, water vapour builds up in the still air around the leaf, reducing the concentration gradient. Transpiration slows down.
在没有风的情况下，水蒸气会在叶片周围的静止空气中积聚，使浓度梯度减小，蒸腾作用变慢。
Wind blows this moist layer away, replacing it with drier air. The concentration gradient is restored and transpiration rate increases.
风会吹走这层湿润空气，用较干燥的空气取而代之。浓度梯度得以恢复，蒸腾速率增大。
The stronger the wind, the faster the moist air is removed, so the higher the transpiration rate, up to a limit.
风越强，湿润空气被移走的速度越快，因此蒸腾速率越高，但这种效果也存在上限。
9. Summary Table: Environmental Factors and Transpiration | 总表：环境因素与蒸腾作用
| Factor | Change | Effect on transpiration rate | Key reason |
|---|---|---|---|
| Light intensity | Increases | Increases | Stomata open wider; more water vapour escapes |
| Temperature | Increases | Increases, but decreases if very hot | Faster evaporation and diffusion; stomata may close at extremes |
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