The role of water in plant life activities
Physical and chemical properties of water related to plant life activities
plant water content
Inside the cell: 50% cell wall, 80-90% protoplasm, vacuoles mainly composed of water
Different types of plants: the same plant has different growth stages, shade plants > sun plants; different organs in the same growth period, young > aging organs
The state of water present in plants
Bound water: tightly bound to cellular components and unable to move freely
Free water: The adsorption force between it and cell components is weak and it can move freely
The role of water in plant life activities
The main component of protoplasm; the reactant substances of many metabolic processes; the solvent for biochemical reactions and plant absorption and transportation of substances; allows plants to maintain their inherent posture; maintains cell turgor pressure to promote growth; light transmittance allows the light-requiring reactions of aquatic plants to proceed normally; Regulate plant questions to reduce high temperature damage; regulate plant living environment
absorption of water by plant cells
The concept of water potential
Free energy: the energy used in physiological processes under constant temperature conditions
Chemical potential: free energy μw per mole of substance
Water potential: The chemical potential difference φw per partial molar volume of water. The water potential of pure water is 0. The water potential of the solution is negative.
Water potential components of aqueous systems: φw=φs φp φm φg
Solute potential: the value of the reduction in the water potential of the system due to the presence of solute particles φs = -iCRT
Substrate potential: The water potential decrease caused by the hydrophilicity of cell colloid material and the binding of capillaries to free water is always negative. The water potential of dry seeds φw=φm, and that of mature vacuole cells φw=φs φp
Pressure potential: The value of the water potential of the system that changes due to the presence of pressure. It is generally a positive value, is zero when the mass wall is separated, and is negative when there is severe transpiration.
Gravity potential: The influence of gravity on water potential can be ignored when the heights of the two regions in the system are not much different.
Several situations of water potential composition of plant cells
Typical plant cell water potential φw=φs φp φm φg, no vacuole cells are formed (meristem cells φw=φs φp φm, air-dried seed cells φw=φm), cells with vacuoles φw=φs=φp, initial plasmolytic cells φp =0φw=φs, fully saturated cellφw=0,φp=|φs|
movement of water
Convergence: The phenomenon in which groups of atoms and molecules in a liquid move together under the action of a pressure gradient. It is independent of the solute potential. It is affected by the diameter of the tubes (xylem vessels and phloem sieve tubes) and is proportional to the pressure.
Diffusion: The phenomenon that substance molecules transfer from a high chemical potential area to a low chemical potential area until they are evenly distributed. The driving force is the concentration difference (chemical potential difference)
Osmosis: a special form of diffusion, the phenomenon of water moving from a high water potential to a low water potential through a selectively permeable membrane; the selectively permeable membrane and the solutions on both sides separated by it are called an osmotic system; the conditions that constitute an osmotic system The selective permeability membrane is inconsistent with the solutions on both sides of the membrane.
water absorption by plant cells
How water enters cells
A single water molecule enters the cell through the gap in the membrane lipid bilayer
Water collects through water channels composed of aquaporins in the plasma membrane
Changes in water potential components during cell water absorption
Changes: When the cell initially plasmolytes (relative volume = 1.0) φp = 0, φw = φs; when the cell absorbs water, φs, φp, and φw all increase; when the cell absorbs water and reaches saturation, φw = 0 = φs φp; the cell becomes strongly During transpiration, the pressure potential is negative. The more water is lost, the more negative the pressure potential becomes, and the water potential is lower than the osmosis potential.
Judgment of water gain and loss: Put a cell in a solution, φw cell <φw solution absorbs water, > indicates water loss, = indicates balance
water movement between cells
Water movement between cells: parts with high water potential flow to parts with low water potential
water transport in plants
Pathway and direction: soil-root hair...stomata-atmosphere, high water potential to low water potential
Power (the fundamental power is water potential)
One is root pressure, and the other is transpiration pulling force.
Cohesion theory, cavitation phenomenon
The physiological basis of rational irrigation
Crop water requirements
Different crops have different water requirements
The same crop has different water requirements at different growth stages.
Crop moisture critical period
Reasonable irrigation indicators and irrigation methods
Field capacity is the upper limit of soil water available to most plants
Morphological indicators: whether the young stems and leaves are wilted, the color of the stems and leaves turns dark or red, and the plant growth rate decreases
Physiological indicators: dew point water potential meter, Abbe refractometer
Irrigation methods: New water-saving irrigation methods include precision irrigation, regulated deficit irrigation and regulated root zone alternating irrigation.
Reasons for rational irrigation to increase production
Physiological effects: Improve various physiological conditions of plants, improve crop photosynthetic performance, especially photosynthesis
Ecological effect: Improve the climate conditions on the irrigated ground, improve the cultivation environment, and indirectly affect plants
Water absorption by plant roots
Water absorption part (root hair area at root tip)
way
Symplastic pathway (high resistance and slow speed), apoplastic pathway (fast movement), transcellular pathway
mechanism
Actively absorb water (power is root pressure)
Passive water absorption (power is transpiration pressure)
Soil conditions that affect root water uptake
Soil moisture status: hygroscopic water (or bound water), gravity water, capillary water
Soil temperature conditions: Both low and high temperatures inhibit water absorption by roots
Soil aeration status: Well-aerated roots have stronger water absorption, while poor aeration can cause root poisoning.
Soil solution concentration: high concentration, low water potential, and difficulty in water absorption by roots; too high concentration, water backflow
Transpiration
physiological meaning and manner
physiological significance
stomatal regulation mechanism
The structure and characteristics of stomata and the characteristics of guard cells
Dicotyledons: crescent-shaped; Poaceae: dumbbell-shaped
Large number of stomata and wide distribution
Small stomata area and high transpiration rate
Guard cells are small in size, and their osmotic potential is easy to change and easy to adjust.
Guard cells are rich in mitochondria and chloroplasts
Guard cells have unevenly thickened cell walls
PEPC, energy metabolism enzyme system
Ion channels and receptors
Stomatal movement mechanism (osmotic regulation mechanism)
Regulating factors for stomatal movement: light (blue light, red light), CO2 (low, open), temperature, moisture, plant hormones
Internal and external factors affecting stomatal transpiration
index
Transpiration rate, transpiration efficiency, transpiration coefficient
lowering pathway
Reduce transpiration area: transplant and remove leaves
Reduce transpiration rate: transplant in the afternoon or on a cloudy day
Use anti-transpiration agents: metabolic type, film type, reflective type
Internal: stomata frequency, size, lower cavity, structure; External: light, temperature, humidity, wind
Starch-sugar conversion theory, photosynthesis promotes stomatal opening theory, inorganic ion pump theory, malic acid metabolism theory, cytoskeleton and stomatal movement
Two phenomena that confirm the existence of root pressure: wound flow and water spitting; two explanations for the mechanism of root pressure: osmotic theory and metabolic theory
Reasons: There are many root hairs, which increases the water absorption area; the outer wall of the root hairs is covered with pectin, which is highly viscous and has good hydrophilicity; the conductive tissue in the root hair area is developed, with low resistance and fast water movement.
Osmotic water absorption: after the formation of vacuoles, cells mainly rely on osmotic water absorption (mainly) φs; osmotic water absorption: water absorption caused by low matrix potential (dried seeds) φm; depressurization water absorption: caused by the reduction of pressure potential Water absorption (strong transpiration) φp
Water has high specific heat capacity; large heat of vaporization; large cohesion, adhesion and surface tension; good solvent; transparent liquid; incompressible
Lenticel transpiration, leaf transpiration (keratin transpiration, stomata transpiration)
Can reduce leaf temperature under hot weather conditions; can promote the absorption and transport of minerals and the transport of organic matter; promote the absorption and transport of water
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