Transpiration is the process by which water flows from living plantssurface(mainly leaves)Water vaporThe process of state loss into the atmosphere is related to physicsEvaporation processDifferent, transpiration is not only affected by external environmental conditions, but also regulated and controlled by plants themselves, so it is a complex physiological process.The main process is as follows: water in soil → root hair → root canal → stem canal → leaf canal → stomata → atmosphere. When plants are young, all surfaces exposed to the air can transpiration.
Transpiration of woody plants through lenticels of branches and cracks of cork tissue is called lenticels transpiration.However, the transpiration of lenticels is very small, accounting for about 0.1% of the total canopy transpiration.
Cuticle transpiration
Transpiration through the cuticle of leaves and herbaceous stems is called cuticle transpiration, accounting for 5%~10% of transpiration.The cutin transpiration of young leaves can reach 1/3 to 1/2 of the total transpiration.The horny transpiration of mature leaves of general plants accounts for 5%~10% of the total transpiration.Plants that grow under drought conditions for a long time have lower transpiration in the cuticle, and their total transpiration is less than 5%.
Stomatal transpiration
Transpiration through pores is calledStomatal transpirationStomatal transpiration is the main way of plant transpiration.
Stomata is an important gateway for gas exchange in plants.Water vapor (HtwoO）、carbon dioxide（CO₂）、oxygen（Otwo）The opening and closing of stomata will affect plant transpiration, photosynthesis, respiration and other physiological processes.
Stomata are small pores in the epidermal tissue of plant leaves, usually formed in pairsGuard cell(guard cell).Guard cells are surrounded by thin epidermisCellIf the adjacent epidermal cells are morphologically the same as other epidermal cells, they are called neighboring cells. If there are obvious differences, they are calledSecondary guard cell(subsidiary cell)。Guard cells are composed of adjacent cells or side guard cellsStomatal complex 。Guard cells are morphologically and physiologically related toEpidermal cellThere are significant differences.
physiological index
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transpiration rate
transpiration rate(transmission)transpiration rate Or transpiration rate.It refers to the amount of water lost by plants through transpiration per unit time and per unit leaf area.It is commonly used in g/m/h and mg/dm/h. The transpiration rate of most plants is 15~250g/m/h in the daytime and 1~20g/m/h at night.
Transpiration efficiency
Transpiration efficiencyTransmission ratio refers to the grams of dry matter formed by plants when they transpiration 1kg of water.Common unit: g/kg.The transpiration efficiency of general plants is 1-8g/kg.
Transpiration coefficient
Transpiration coefficient(Transmission coefficient), also known as water demand, refers to the grams of water consumed by plants for producing 1g dry matter.It is the reciprocal of transpiration efficiency.The transpiration coefficient of most plants is between 125 and 1000.The transpiration coefficient of woody plants is relatively low, such as about 40 for pine trees;The transpiration coefficient of herbaceous plants is relatively high, 370 for corn and 540 for wheat.The lower the transpiration coefficient, the higher the water utilization efficiency of plants.
influence factor
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inside
stomaStomatal frequencyFrequencyIt is greatly conducive to transpiration.
The size and diameter of stoma are large, the internal resistance is small, and the transpiration is fast.
The volume of the lower stomatal cavity is large, the vapor pressure inside and outside the leaves is different, and the transpiration is fast.
Stomatal opening is large and transpiration is fast;Otherwise, it will be slow.
external
The transpiration rate of external factors affecting transpiration depends on the magnitude of vapor pressure difference inside and outside leaves and diffusion resistance.Therefore, all external factors that affect the vapor pressure difference and diffusion resistance inside and outside the leaves will affect the transpiration rate.
(1) The first effect of light on transpiration is to cause stomatal opening, reduce stomatal resistance, and thus enhance transpiration.Secondly, light can increase the temperature of atmosphere and leaves, increase the vapor pressure difference inside and outside leaves, and accelerate the transpiration rate.
（2）temperatureIt has great influence on transpiration rate.When the air temperature rises, the leaf temperature is 2~10 ℃ higher than the air temperature, so the increase of the vapor pressure in the lower cavity of the stomata is greater than the increase of the air vapor pressure, which increases the vapor pressure difference inside and outside the leaves and the transpiration rate;When the temperature is too high, the leaves are excessively dehydrated, the stomata are closed, and transpiration is weakened.[2]
(3) Humidity When the temperature is the same, the greater the relative humidity of the atmosphere, theVapor pressureThe greater the value, the smaller the difference between the internal and external vapor pressure of the leaves, and the water vapor in the cavity under the stoma is not easy to diffuse out, so the transpiration is weakened;On the contrary, the lower the relative humidity of the atmosphere, the faster the transpiration rate.
(4) When the wind speed is high, the water vapor diffusion layer outside the leaf surface pores can be blown away and replaced by relativehumidityLower air not only reduces the diffusion resistance, but also increases the vapor pressure difference inside and outside the leaves, which can accelerate transpiration.Strong wind may cause stomata to close, internal resistance to increase, and transpiration to weaken[1]。
Physiological significance
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Environmental Science
Transpiration provides a large amount of water vapor for the atmosphere, keeps the local air moist, lowers the temperature, and makes the local rainwater abundant, forming a virtuous circle.
transport
Transpiration is a main power for plants to absorb and transport water, especially for tall plants. If there is no transpirationTranspiration pullThe process of water absorption caused by this can not be produced, and the higher part of the plant can not obtain water.
Since mineral salts (inorganic salts) can only be absorbed by plants and run in the body if they are dissolved in water, since transpiration is the power for water absorption and flow, minerals are also absorbed and distributed in various parts of plants along with water absorption and flow.Therefore, transpiration affects thePlant internal transportThey are all helpful.
cooling
Transpiration can reduce leaf temperature.When the sun shines on the leaves, most of the energy is converted into heat energy. If the leaves do not have the ability to cool down,Leaf temperatureIf it is too high, the blade will be burned.In the process of transpiration, water needs to absorb heat energy when it turns into steam (1 g of water needs energy when it turns into steam, 2444.9J at 20 ℃, 2430.2J at 30 ℃). Therefore, transpiration can reduce the temperature of leaf surface, so that leaves can conduct photosynthesis under strong light without being injured.
