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MindMap Gallery Plant Physiology Chapter 2 Water Metabolism
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Plant Physiology Chapter 2 Water Metabolism
Plant Physiology, Plants absorb water from the surrounding environment on the one hand, and continuously lose water to the environment on the other hand. The process of plant absorption, transportation, utilization and loss of water is called plant water metabolism.
Edited at 2024-03-19 10:48:00
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Plant Physiology Chapter 2 Water Metabolism
- Valentine's Day marketing campaign
This Valentine's Day brand marketing handbook provides businesses with five practical models, covering everything from creating offline experiences to driving online engagement. Whether you're a shopping mall, restaurant, or online brand, you'll find a suitable strategy: each model includes clear objectives and industry-specific guidelines, helping brands transform traffic into real sales and lasting emotional connections during this romantic season.
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- Outline
Chapter two water metabolism
concept On the one hand, plants absorb water from the surrounding environment, and on the other hand, they continuously lose water to the environment. The process of plant absorption, transportation, utilization and loss of water is called plant water metabolism.
Section 1 The role of water in plant life activities
Physical and chemical properties of water related to plant life activities
Water has a higher specific heat capacity
Water has a high heat of vaporization
The cohesion, adhesion and surface tension of water
Cohesion: the intermolecular attraction between similar molecules
Adhesion: the mutual attraction between the liquid phase and the solid phase
Water is a good solvent
Water is a transparent liquid
plant water content
Moisture content = [(fresh material quality - dry material quality) / fresh material quality] x 100%
Different plants have different water contents
Generally, the water content of plant tissue is 75%-90%
The water content of aquatic plants can reach 95%
The water content of lower plants such as lichens and mosses that grow in arid environments is only 5% to 7%.
Different organs and tissues of the same plant have different water contents
Generally, vigorously growing organs and tissues (such as tender stems, young roots, new leaves, and developing fruits) have a higher water content of 80%-90%
Organs and tissues that tend to age and become dormant have lower water content, generally below 60%.
⛛The moisture content of dry seeds is 10%~14%
The same organs and tissues have different water contents at different stages of growth
The water content of leaves is relatively high during the growth period, but the water content decreases after growth and finalization.
The moisture content of cereal seeds can reach 90% in the early stages of development, and drops to less than 25% when mature.
The same plant grows in different environments with different water contents.
Plants growing in shaded, humid environments have higher water content than plants growing in sunny, dry environments.
Throughout the day, plants generally have higher water content in the morning than at noon and afternoon.
The more vigorous the life activities, the higher the water content.
The state of water present in plants
Bound water refers to water that is tightly bound to cellular components and cannot move freely. The bound water content is relatively stable, does not evaporate easily, and does not act as a solvent or participate in chemical reactions.
Close to hydrophilic colloidal particles
Adsorbed by protoplasmic components
Free water refers to water that has weak adsorption to cell components and can move freely. Free water content changes and can participate in various metabolic activities.
Stay away from hydrophilic particles
Not adsorbed by protoplasmic components
When the free water/bound water ratio is high, the cell protoplasm is in a sol-like state, the plant has strong metabolism, grows quickly, and has weak stress resistance. On the contrary, the cell protoplasm is in a gel state, has low metabolic activity, slow growth, and is weak in stress resistance. Strong sex.
The role of water in plant life activities
Water is the main component of cell protoplasm
Water plays an important role in the physiological activities of plants
Water is a metabolic reactant
Water is the medium for material transport, absorption and biochemical reactions
Water maintains cell turgor and promotes growth
Water keeps plants in their proper position
Water has important ecological significance for plant survival
Regulation of plant body temperature by water
Regulation of water on plant living environment
The light transmittance of water allows the light-demanding reactions of aquatic plants to proceed normally
Physiological water requirement refers to the water required for plant life activities and maintaining water balance in plants.
Ecological water requirement refers to the water needed to regulate the ecological environment of plants by utilizing the physical and chemical properties of water.
*Section 3 Water absorption by plant roots
The part of the root system that absorbs water
The parts of the root system that absorb water are mainly at the tip of the root, including the root cap, meristematic zone, elongation zone and root hair zone. Among them, the root hair zone has the strongest water absorption capacity.
The reason why root hairs have strong ability to absorb water
There are many root hairs in the root hair area, which increases the absorption area.
The outer layer of the root hair cell wall is covered with pectin, which is highly sticky and hydrophilic, which is conducive to adhesion to soil colloidal particles and water absorption.
The conductive tissue in the root hair area is well developed and has little resistance to water movement, so water transfer is fast.
How roots absorb water
The water absorbed by the roots enters the vessels through root hairs, cortex, endodermis, and pericycle parenchyma cells.
apoplastic pathway
Refers to the movement of water through the apoplast composed of cell walls, intercellular spaces, and xylem vessels.
symplast pathway
Refers to the movement of water from the cytoplasm of one cell into the cytoplasm of another cell through plasmodesmata.
transmembrane pathway
Refers to the transport pathway of water through cell membranes
The mechanism of water absorption by roots
Actively absorb water
The phenomenon of active water absorption
root pressure
Root pressure is the positive pressure in the xylem
Plant roots continuously absorb ions from the soil and transport them to the endodermis of the roots, causing the solutes in the stele cells and vessels to increase and the solute potential to decrease. When the water potential in the endodermis is lower than the soil water potential, the water in the soil penetrates from the outer apoplast through the endodermis into the stele and conduits along the water potential gradient, where the endodermis acts as a selectively permeable membrane. Furthermore, the upper part of the conduit is in an open state and does not generate pressure, so the water column moves toward the aboveground part under the upward pressure, thus forming root pressure.
Injury
Bleeding is the phenomenon of fluid escaping from plant wounds.
The quantity and composition of wound fluid can be used as root physiological indicators of activity
Injury and vomiting can also prove the existence of root pressure.
Spit water
Spitting is the phenomenon of droplets of water overflowing from the water holes at the tips or edges of the leaves of the plant.
In an environment with sufficient soil moisture, humid warmth, and weak transpiration, it is easy to observe the phenomenon of plants spitting water.
The water spitting phenomenon can be used as an indicator of root physiological activity and crop transplant survival.
Active water absorption mechanism
Plants use metabolic energy to actively absorb external solutes, causing the water potential of the solution in the endothelial layer to be lower than the water potential of the external solution. Water passively enters the catheter from the outside along the water potential gradient.
Passive water absorption
Root water absorption caused by transpiration pull is called passive water absorption
Transpiration pull refers to the force of the water potential gradient generated by leaf transpiration to cause the water in the plant to rise.
Primary mode of plant, independent of metabolic activity
The proportion of active water absorption and passive water absorption
Plants undergoing transpiration, especially tall trees, mainly absorb water passively. Active water absorption becomes the main method of water absorption only during the seedling stage, after the tree leaves have not expanded or fallen, and at night when the transpiration rate is very low. In addition, if the soil water potential is between 0.2 and -0.1 MPa or below, active water absorption will be ineffective, but passive water absorption can still proceed until the soil moisture approaches the permanent wilting point (about 1.5 MPa). Plants cannot use the water in the soil.
Soil conditions that affect root water uptake
Soil moisture status
Physical state of soil moisture and its availability
Water in soil can be divided into bound water, capillary water and gravity water according to its physical state.
Bound water refers to water adsorbed by soil colloidal particles. The water potential is lower than -3.1MPa and cannot be used by plants.
Gravity water refers to water that is not adsorbed by soil particles and capillaries after precipitation or irrigation, and can leak out from top to bottom under the action of gravity. The water potential is higher than -0.01MPa.
Capillary water refers to the water held in the capillaries between soil particles. The water potential is -3.10~-0.01MPa. It is the main source of water absorbed by plants.
soil water holding capacity
The indicator of soil water holding capacity is field water capacity
It refers to the soil moisture content when all capillary water and bound water are retained after all gravity water in the soil is eliminated, expressed as the proportion of water to soil dry matter mass (%).
The soil water potential is 0. 03 ~ 0.01MPa. Usually, when the soil moisture content is about 70% of the field water capacity, it is most suitable for farming and root water absorption.
Plant performance of soil water shortage
When less water is absorbed than lost, the cells will lose turgor pressure and the plant will wilt. If the transpiration rate decreases, the wilted plants can return to normal, and this wilting is called temporary wilting.
If the wilted plant still cannot return to normal after the transpiration rate is reduced, such wilting is called permanent wilting.
Permanent wilting can only be eliminated by increasing the water available to the soil. Permanent wilting can cause plant death if it continues.
The permanent wilting coefficient refers to the proportion (%) of the water remaining in the soil to the dry matter mass of the soil when the plant wilters permanently.
soil temperature
Effect of low temperature
Low temperatures reduce water absorption by roots
reason ① The viscosity of water increases at low temperature and the diffusion speed decreases. At the same time, due to the increase in the viscosity of the cell protoplasm, the water diffusion resistance increases. ② The metabolic activity of the root system is weakened and the active water absorption is reduced. ③ The root system grows slowly, which affects the increase in water absorption area.
Effect of high temperature
Excessively high soil temperature is also detrimental to water absorption by the root system. It will increase the degree of lignification of the roots, accelerate root aging, and cause casein denaturation and malaise.
Root hairs fall off and water absorption area decreases
Generally, thermophilic plants and vigorously growing plants absorb water and are affected by low temperatures.
Soil aeration status
When the moisture in the soil is appropriate, gas exchange is smooth, and oxygen is sufficient, it can promote aerobic respiration of the roots and increase active water absorption. It is also conducive to the growth of the roots and the expansion of the water absorption area.
If the soil is hardened or has too much water, insufficient oxygen, and carbon dioxide accumulation, it will affect the normal respiration of the root system. Not only will the absorption of water and fertilizer be reduced, but more harmful substances will accumulate in the anaerobic respiration system, which will poison the root system.
Measures such as cultivating fields and draining and drying fields can improve soil aeration and enhance the absorption capacity of roots.
When the oxygen content in the soil is less than 5%, the water absorption capacity of the roots is significantly reduced.
soil solution concentration
The concentration of soil solution determines the water potential of the soil. Generally, the concentration of soil solution is low and will not affect the normal water absorption of the roots.
There are two situations that can cause the soil solution concentration to be too high: ① Too much or too concentrated chemical fertilizers are used, causing the local soil water potential to decrease; ② In saline-alkali land, because there are more salt ions in the soil solution, the soil solution concentration increases and the water potential decreases. . Under these circumstances, the roots cannot absorb water and even reverse osmosis of water occurs, causing "seedling burning". Measures such as irrigation and salt washing can be used to reduce the concentration of soil solution
To absorb water in the soil, the water potential of its cells must be lower than the water potential of the soil solution.
Section 5 water transport in plants
Pathways and speeds of water transport in plants
In the soil-plant-atmosphere continuum, the water transport pathway is: soil hair, cortex, endodermis, pericycle, root vessel, stem vessel, petiole vessel, vein vessel, mesophyll cells, leaf intercellular space, stomata Lower chamber - stomata and atmosphere
The mechanism by which water rises in plants
The force of moisture rising along the conduit
The power of water to rise along the conduit (or tracheid) is determined by the pressure potential difference between the two ends of the conduit.
The establishment of pressure potential difference mainly comes from two aspects, one is root pressure (positive pressure potential), and the other is transpiration pull force (negative pressure potential).
water column continuity
Transpiration causes the leaves to lose water, the water potential decreases, and water is absorbed into the conduit. The upper end of the conduit is subject to upward transpiration pull, and the gravity of the water column itself and the water flow resistance pull the water column downward, so that the water column of the conduit is subject to a kind of tension. .
The higher the part of the plant is from the ground, the greater the tension on the water column of the conduit.
Since the cohesive force of water is much greater than the tension of the water column, and at the same time, there is adhesion between water molecules and cellulose molecules on the catheter wall (mutual attraction between the liquid phase and the solid phase), the continuity of the water column in the catheter is maintained, causing the water to continue to rise. . Therefore, this theory should be called the transpiration flow-cohesion-tension theory.
Gas will be dissolved in the liquid flow of the catheter. When the tension of the water column increases, the dissolved gas will escape from the water and form bubbles. This is called cavitation, and the bubbles will continue to expand under the action of tension. Large bubbles can block pipes, called embolisms
The cavitation phenomenon will reduce the water conduction ability of the xylem to a certain extent, but it is not entirely a bad thing for the water balance of the plant. Because cavitation mainly occurs at the ends of material conduction tissues with high tension and low water potential (leaves and small branches), it can effectively reduce water transpiration and help prevent dehydration of plant trunks and roots.
Section 6 The physiological basis of rational irrigation
Reasons for rational irrigation to increase production
Reasonable irrigation is the most effective way to maintain water balance in plants. Reasonable irrigation can meet the physiological water needs of plants, especially ensuring the water supply during the critical period of crop water, making plants grow faster, expanding leaf area, and increasing photosynthetic area; it enhances root activity, is conducive to the absorption of water and fertilizer, and ensures transpiration. Proceed normally, increase the transport rate of water and nutrients, thereby speeding up the photosynthetic rate, reducing the phenomenon of photosynthetic nap, improving the distribution and utilization of photosynthetic products, and increasing yield. Reasonable irrigation can also meet the ecological water needs of plants and improve the soil and climate conditions of the cultivation environment. For example, it can promote fertilizer dissolution, wash salt and alkali, keep warm and prevent cold, cool down and moisturize, supply oxygen and detoxify, and relieve atmospheric and soil drought.
Crop water requirements
Different crops have different water requirements
The transpiration coefficient can be used to estimate the water requirement of crops, that is, the product of the crop's biological yield and the transpiration coefficient is used as the theoretical minimum water requirement.
The same crop has different water requirements at different growth stages.
In the life cycle of plants, the most sensitive and vulnerable period to water deficiency is called the water critical period.
Crops with limited water stress effects
Water stress is not entirely a negative effect. At specific development stages of crops, limited water stress is beneficial to improving yield and quality. The impact of water shortage on crops involves a process from adaptation to damage. As long as the water shortage does not exceed the adaptive range, compensation effects on water utilization and growth can often occur after rewatering, which is beneficial or harmful to the final yield. This is the limited water shortage effect of crops.
Irrigation index
soil index
Generally speaking, the soil moisture content in the active layer of crop roots (0~90cm) should account for 60%~80% of the field water capacity.
morphological indicators
Symptoms of crop water shortage include: wilting of young stems and leaves; reduced growth rate; relative increase in chlorophyll content, darker stems and leaves; and some stems and leaves turning red.
Physiological indicators
Leaves are the most sensitive parts that reflect physiological changes in plants. The relative water content, osmotic potential, water potential, cell sap concentration and stomata opening of leaves can all be used as physiological indicators for irrigation.
The relative water content of leaves refers to the ratio (%) of actual water content to water content when water is saturated. Usually the relative moisture content of leaves is 85% to 95%. If the relative moisture content is lower than the critical value (about 50%), the leaves will generally wither.
When the water balance in the plant body is out of balance, the solute potential of the leaves decreases, the water potential decreases, the cell sap concentration increases, and the stomata opening decreases or even closes.
Irrigation method
Common irrigation methods
Flood irrigation
Also known as surface irrigation, water forms a water layer or flow on the farmland surface through ditches and seeps into the soil.
It has the advantages of simple and convenient operation and low operating cost.
Its disadvantage is that it wastes a lot of water resources, and causes soil erosion, fertility loss and other disadvantages. The key to improving surface irrigation is to improve land flatness and improve field water distribution.
sprinkler irrigation
It refers to spraying water into the air with the help of power equipment to form water droplets and land on plants and soil.
This method can effectively relieve atmospheric drought and premature soil drying, save manpower, occupy less cultivated land, and has strong adaptability to terrain.
Its disadvantages are that it is greatly affected by wind and requires high investment in equipment.
drip irrigation
A method of localized irrigation that delivers irrigation water and nutrients directly to around plant roots through a network of pipes buried or placed in the ground.
Drip irrigation has almost no evaporation losses and deep seepage, allowing crop roots to always be in a good state of moisture, air and nutrients. Drip irrigation can be used in a variety of terrain and soil conditions, and is especially suitable for areas with water shortages or high salt content.
The disadvantage is that the dripper orifice is easily clogged, so the irrigation water must be filtered.
Several new water-saving irrigation methods
precise irrigation
It is an intelligent water-saving irrigation method based on the actual water needs of crops and using information technology such as computer automatic control and remote sensing.
The precise irrigation system can greatly improve water utilization and save fresh water resources; it can automatically supply water according to the needs of plants in a timely manner, greatly improving the quality of plant growth. Precision irrigation is a trend in green irrigation in the future
regulated deficit irrigation
It is a water-saving irrigation method based on the physiological characteristics of crops, with moderate water loss during the peak vegetative growth period, sufficient water supply during the critical period of water demand, and a combination of promotion and control.
Artificially exerting a certain degree of water stress at certain growth stages can regulate the distribution ratio of photosynthetic products among different tissues and organs, regulate the growth dynamics of aboveground and underground parts, control vegetative growth, and promote reproductive growth, thereby increasing economic output. Achieve the goals of water conservation, high efficiency, high yield and high quality, and increase the irrigation area.
Controlled root zone alternating irrigation
Alternate irrigation, for short, is a water-saving irrigation method that only irrigates some areas during the irrigation process, while keeping other areas dry, so that the root systems in different areas are alternately subjected to water stress.
① The roots in the local dry area transmit abscisic acid to the above ground to adjust the stomata opening of the plant, thereby reducing the extravagant transpiration water loss of the entire plant. ② Subjecting some roots to a certain degree of water stress can stimulate the compensatory function of the roots and improve root conductivity. ③Photosynthesis and transpiration respond differently to stomatal opening. The photosynthetic rate increases with the stomatal opening. When the stomatal opening reaches a certain value, the increase in photosynthetic rate is no longer obvious; while the transpiration rate increases with the stomatal opening. And increases linearly. Similarly, the stomatal opening becomes smaller, the photosynthetic rate decreases less, and the transpiration water loss will be greatly reduced. Therefore, it is theoretically feasible to achieve the maximum without sacrificing crop photosynthetic product accumulation. ④ Reduce soil evaporation between plants and deep penetration in the root zone, improve the effectiveness of water stored in the root zone, and improve water utilization efficiency.
*Section 4 Transpiration
concept The process by which water in plants loses its gaseous state to the atmosphere
The physiological significance, methods and indicators of transpiration
Physiological significance of transpiration
The transpiration pull generated by transpiration is the main driving force for plants to absorb and transport water.
Transpiration promotes transport of substances in xylem sap
Transpiration can lower the temperature of plants
Normal transpiration is beneficial to the assimilation of carbon dioxide
mode of transpiration
As the plant grows, corks form on the surface of the stems and branches, and there are lenticels in the uncorked parts, allowing for lenticel transpiration, but the amount of lenticel transpiration is very small, accounting for only about 0.1% of the total transpiration. Plant transpiration is mainly carried out by leaves.
There are two ways of transpiration in leaves. One is transpiration through the corn layer, which is called corn layer transpiration. The other is transpiration through the stomata, which is called stomatal transpiration.
Stomatal transpiration is the main mode of transpiration in mesophytic and early-growing plants.
Indicators of transpiration
Transpiration rate
Also known as transpiration intensity, it refers to the amount of water lost by plants through transpiration per unit time and per unit leaf area. Commonly used units are g/(m^2·h), mg/(dm^2·h)
Transpiration efficiency
Also known as transpiration productivity, it refers to the amount of dry matter formed by plants transpiration per kilogram of water. The common unit is g/kg.
Transpiration coefficient
Also known as water demand, it refers to the amount of water (g) transpired by plants per 1g of dry matter produced. The unit is g/g.
The smaller the transpiration coefficient, the higher the efficiency of utilizing water vapor.
transpiration ratio
Refers to the amount of water (mol) required for transpiration and dissipation per 1 mol of carbon dioxide fixed by plant photosynthesis.
Generally, the transpiration ratio of woody plants is smaller than that of herbaceous plants.
stomatal transpiration
Morphological structure and physiological characteristics of stomata
Stomata is the general name for the two guard cells on the epidermal tissue and the pores formed by them.
Guard cells are surrounded by epidermal cells. If the adjacent epidermal cells are morphologically the same as other epidermal cells, they are called adjacent cells. If they are obviously different, they are called accessory cells.
The guard cells, accessory cells and the pores in the middle are together called the stomatal organ, also known as the stomatal complex.
Usually, monocot leaves have stomata on the upper and lower epidermis, dicot leaves have stomata mainly on the lower epidermis, and floating plants have stomata on the upper epidermis.
The smaller the hole, the more obvious the edge effect and the faster the diffusion rate.
The guard cells of dicots and most monocots are kidney-shaped, and the guard cells of grasses are dumbbell-shaped.
The mechanism of stomatal movement
Stomatal movement is caused by changes in turgor pressure of guard cells
Inorganic ion pump theory
Driven by the H electrochemical potential gradient, potassium ions enter the guard cells from surrounding cells through the inward K* channel on the guard cell plasma membrane, and then further enter the vacuole. The K* concentration increases and the water potential decreases, causing the guard cells to absorb water and open the stomata. open. Chloride ions may enter guard cells through the CI-H* co-transporter. In the dark, photosynthesis stops, H* ATPase activity decreases, and the plasma membrane of the guard cells depolarizes, driving K* to transfer to surrounding cells through the outward K* channel, accompanied by the release of anions, causing the water potential of the guard cells to increase, and the water Move outward and close the stomata
Malic acid metabolism theory
Under light, the carbon dioxide in the guard cells is utilized, the pH rises to 8.0~8.5, and the phosphoenolpyruvate carboxylase in the cytoplasm is activated and catalyzes the phosphoenolpyruvate produced by the degradation of starch. It combines with bicarbonate to form oxaloacetate, which is reduced to malic acid by NADPH (or NADH). Malic acid dissociates into 2H and malate radicals. Driven by the H*/K* pump, H* exchanges with K*. Malate radicals enter the vacuole and Cl* maintains electrical balance with K* together. At the same time, the presence of malic acid can also reduce the water potential, prompting guard cells to absorb water and open stomata. When the leaves move from light to darkness, the malic acid content decreases, causing the guard cells to lose water and close the stomata.
Starch and sugar transformation theory
When starch is converted into soluble sugar, the osmotic potential decreases and the stomata open; when soluble sugar is converted into starch, the osmotic potential increases and the stomata close.
External factors influencing stomatal movement
Light
Light is the main environmental factor affecting stomatal movement. The stomata of most plants open in the light and close in the dark.
There are two effects of light promoting stomatal opening: ① The indirect effect that occurs through photosynthesis, which can be inhibited by the photosynthetic electron transfer inhibitor dichlorophenyldimethylurea; ② The direct effect that occurs through photoreceptors that sense light signals. effect, which is not inhibited by dichlorophenyldimethylurea. Both red light and blue light can cause stomata to open, with red light acting through an indirect effect; blue light is the most effective light quality to regulate stomata opening. It can activate the plasma membrane H*-ATPase, pump protons out of the cell, and directly open the stomata. kick in. In addition, light can increase the temperature of the atmosphere and leaves, increase the vapor pressure difference between the inside and outside of the leaves, and accelerate the transpiration rate.
carbon dioxide
Low concentrations of carbon dioxide promote opening of stomata, while high concentrations of carbon dioxide promote closing of stomata.
temperature
The stomata opening generally increases as the temperature rises
Moisture
When the water content of the plant decreases, the opening of the stomata decreases. When the plant loses water severely, the stomata will close even in the light.
wind
Breeze facilitates stomata opening and transpiration
Plant hormone
Cytokinin (CTK) promotes stomatal opening, while abscisic acid (ABA) promotes stomatal closing.
Internal and external factors affecting transpiration
Internal influencing factors of transpiration
The internal influencing factors of stomatal transpiration are mainly leaf structure, especially the morphological structure of the stomatal apparatus and its surrounding cells.
External factors affecting transpiration
illumination
Light is the most important external condition that affects transpiration. Light causes the opening of stomata and reduces stomatal resistance; light increases the temperature of the atmosphere and leaves, increases the vapor pressure difference between the inside and outside of the leaves, and accelerates the transpiration rate.
temperature
When the temperature rises, the leaf temperature can be 2 to 10°C higher than the air temperature. The increase in vapor pressure in the mesophyll cells is greater than the increase in air vapor pressure. In this way, the vapor pressure difference between the inside and outside of the leaf increases and transpiration is enhanced. When the temperature is too high, the leaves lose excessive water and the stomata close, weakening transpiration.
humidity
When the temperature is the same, the greater the relative humidity of the atmosphere, the greater the vapor pressure. The vapor pressure inside and outside the leaves will become smaller, and the water vapor in the lower cavity of the stomata will not easily diffuse out, and the transpiration will be weakened. On the contrary, when the relative humidity of the atmosphere is low, the transpiration will be reduced. The speed increases.
wind speed
When the wind speed is high, the water vapor diffusion layer outside the stomata on the leaf surface can be blown away and replaced by air with lower relative humidity, which not only reduces the diffusion resistance, but also increases the vapor pressure difference between the inside and outside of the leaf, which can accelerate transpiration. Strong ventilation may cause the stomata to close or reduce their opening, increase internal resistance, and weaken transpiration.
soil condition
The continuous transpiration of the above-ground parts of plants depends on the continuous absorption of water by the roots from the soil, and the amount of water lost through transpiration and the amount of water absorbed by the roots are equal under normal circumstances. Therefore, any soil conditions that affect root water absorption (such as soil moisture content, temperature, gas, solution concentration, etc.) can indirectly affect transpiration.
Ways and measures to reduce transpiration
Reduce transpiration area
When transplanting plants, some branches and leaves can be removed to reduce the transpiration area and water loss through transpiration to improve the survival rate.
Reduce transpiration rate
Avoid external conditions that promote transpiration, transplant plants in the evening or on cloudy days, and spray water for shading after planting; cultivating plants in greenhouses or greenhouses or using mulch films, sunshade nets, straw coverings and other measures can increase environmental humidity and diffusion resistance. , reduce the transpiration rate
Use anti-transpiration agents
Substances that can reduce the transpiration rate of plants but have little impact on their photosynthesis and growth are called anti-transpiration agents
Some anti-transpiration agents can affect the expansion of guard cells and reduce stomatal opening, such as abscisic acid, atrazine, etc.; some can form a protective film after being applied to the leaf surface to prevent water loss, such as silicone, latex, polyethylene Wax, etc.; some can increase the reflection of light on the leaf surface, lower leaf temperature, and reduce transpiration, such as kaolin clay. Some multi-functional anti-transpiration agents (such as polymer film-forming agents, transpiration inhibitors, plant growth regulators and trace elements, etc.) have begun to be promoted in production and can be used in tree transplanting, lawn flowers, and fruit tree medicinal materials. , field crops, highway slopes, roof gardens, etc., it has the effects of reducing transpiration, preventing wind and cold, enhancing plant stress resistance, high survival rate and survival quality.
*Section 2 absorption of water by plant cells
The concept of water potential
chemical potential
concept Chemical potential: It is the free energy per partial mole of substance, represented by the Greek letter μ.
Substances always move spontaneously from places with high chemical potential to places with low chemical potential. When the chemical potentials are equal, there is a dynamic equilibrium.
chemical potential of water
concept When the temperature, pressure and the amount of other substances (except water) remain unchanged, the change in the free energy of the system caused by the amount of water (molar increment) is expressed in μw.
Water always moves from areas of high chemical potential to areas of low chemical potential
water potential
concept Chemical potential difference per partial molar volume of water
The free energy of water in a solution is lower than that of pure water, and the water potential of the solution is negative. The more concentrated the solution, the lower the water potential.
Bound energy: energy that cannot be converted into work
Free energy: energy used to do work at a constant temperature
Water potential of pure water = 0
Water potential components of aqueous systems
The water potential of pure water ψw
Pure water has the highest water potential
Solute potential ψs
concept Refers to the numerical value that reduces the water potential of the system due to the presence of solute particles.
The water potential of a solution is equal to its solute potential, which is always negative. The more solute a solution has, the lower its solute potential is.
In an osmotic system, the solute potential represents the potential osmotic ability of water in the solution, so the solute potential can also be called the osmotic potential ψπ
The absolute value of osmotic potential and osmotic pressure is the same, but the sign is opposite.
The solute potential of a dilute solution can be calculated using van't Hoff's empirical formula: ψs=ψπ=-π=-icRT.i is the dissociation coefficient of the solute
Non-electrolyte dilute solution ψs = -CRT
Substrate potential ψm
Substances that can absorb water on their surface are called substrates
The value of the system water potential rising and falling due to the presence of the substrate is expressed as a negative value.
Pressure potential ψp
Refers to the value that changes the water potential of the system due to the presence of pressure.
If positive pressure is added, the water potential of the system increases; if negative pressure is added, the water potential of the system decreases.
Generally ψp>0 Initial recognition stage of plasmolysis ψp=0 ψp<0 during violent boiling
Gravitational potential ψg
Refers to the value of the increase in water potential in the system due to the presence of gravity.
Gravitational potential is related to the height of the water column and is often found at the base of plants
water potential of solution
ψw=ψs ψm ψp ψg
movement of water
Convergence
concept It refers to the phenomenon that groups of atoms or molecules in a liquid move together under the action of pressure gradient. (long distance)
diffusion
It is the phenomenon that substance molecules transfer from a high chemical potential area to a low chemical potential area until they are evenly distributed. (short distance)
The rate of diffusion is directly proportional to the concentration gradient of a substance and inversely proportional to the distance of diffusion
Osmosis
Refers to the phenomenon of diffusion of solvent molecules in a solution through a semipermeable membrane
The protoplasm layer is selectively permeable and approximates a semipermeable membrane
water absorption by plant cells
Components of plant cell water potential
cell solute potential
Cell pressure potential
cell matrix potential
Cell water potential composition
Water potential composition of cells without vacuoles
ψ cell = ψ cytoplasm = ψs ψm ψp
Vacuolar meristem and dry seeds
ψw=ψm
The water potential composition of vacuolar cells
ψ cell = ψ cytoplasm = ψ vacuole = ψ organelle
mature cells with vacuoles
ψ cell = ψ vacuole = ψs ψp
How plant cells absorb water
Penetration and water absorption
Cellular water uptake due to decrease in solute potential
Water absorption by cells containing vacuoles (such as water absorption by roots and water absorption by stomatal guard cells) is mainly osmotic water absorption.
Swell and absorb water
Refers to water absorption caused by low matrix potential
Reduce pressure and absorb water
Cellular water absorption due to reduced pressure potential
Changes in volume and water potential components of cells during water absorption
The solute potential, pressure potential and water potential of cells increase as the water content increases, and the cell's water absorption capacity decreases accordingly; conversely, the solute potential, pressure potential, and water potential decrease as the water content decreases, and the cell's water absorption capacity increases accordingly.
water movement between cells
Water always flows along the water potential gradient
Since the water potential of the soil > the water potential of the plant roots > the water potential of the stem xylem > the water potential of the leaves > the water potential of the atmosphere, the water absorbed by the roots can be continuously transported to the upper part of the ground.