Many plant functions are directly dependent on the properties of water and the substances dissolved in it. A brief overview of the properties of water is therefore necessary to begin our investigation into the relationship between plants and water. The nature of water's atomic structure causes its molecules to have unique electrochemical properties.
When water is frozen, its molecules arrange themselves into a special highly organized rigid geometric pattern, which causes the mass of water to expand and decrease in density. In a thin, confined column of water, such as that in the xylem elements of a stem, this tensile strength can reach high values, allowing water to be drawn up to the tops of tall trees. The properties of water, such as cohesion, adhesion and surface tension, cause the phenomenon of capillarity, the movement of water over small distances through a capillary tube.
Osmosis is simply a special type of diffusion: diffusion of water molecules through a partially permeable membrane. In Figure 2.2.2, more water molecules move from the water to the dilute solution than return the other way around, because there is a higher concentration of water molecules in the pure water than in the solution. When the water concentration is the same on both sides of the membrane, the movement of water in both directions will be the same.
At this point the net exchange of water is zero and the system is in equilibrium.
Diffusion, osmosis and active transport compared
Calculating Half-Times of Diffusion
Alternative Conventions for Components of Water Potential
The Matric Potential
Measuring Water Potential
- Psychrometer (Ψ w measurement)
- Pressure chamber (Ψ w measurement)
- Cryoscopic osmometer (Ψ s measurement)
- Pressure probe (Ψ p measurement)
If the tissue has a lower water potential than that of the droplet, water evaporates from the droplet, diffuses through the air, and is absorbed by the tissue. If the standard solution has a lower water potential than that of the sample to be measured, water will diffuse from the tissue into the droplet, causing heating of the droplet. A relatively quick method of estimating the water potential of large pieces of tissue, such as leaves and small shoots, is by using the pressure chamber.
The diagrams on the right show the state of the xylem water columns at three time points: (A). This can be detected visually by observing when water returns to the open ends of the xylem vessels that can be seen on the surface of the cut. When there is no transpiration, the water potential of the leaf cells and the water potential in the xylem come into balance.
When the transpiring leaf or shoot is cut, the tension in the xylem is immediately released and water is drawn into the leaf cells until the water potentials of the xylem and leaf cells become equilibrated. Thus, any measurement of the equilibrium pressure on such a leaf or shoot will result in a value that is approximately the water potential of the leaf, rather than the tension of the xylem. To be precise, one would have to add the Ψs of the xylem sap to the negative value of the equilibrium pressure to obtain the leaf water potential.).
One can explore the differences between xylem water potential and the water potential of a transpiring leaf by comparing equilibrium pressures measured in covered (i.e., nontranspiring) versus uncovered (transpiring) leaves. One of the colligative properties of solutions is the lowering of the freezing point as the concentration of the solute increases. The temperature at which the last ice crystal melts provides a measure of the sample's melting point.
The temperature of the stage is quickly lowered to about –30°C, causing the sample to freeze. When the last ice crystal in the sample melts, the temperature of the stage is recorded (note that the melting and freezing points are the same). The high pressure in the cell compresses the trapped gas, and from the change in volume, you can easily calculate the cell's pressure based on the ideal gas law (pressure × volume = constant).
In this way, the boundary between oil and cellular fluid can be pushed back to the top of the microcapillary. When the boundary is returned to the tip and held in a constant position, the initial volume of the cell is restored and the pressure inside the cell is exactly balanced by the pressure in the capillary.
Understanding Hydraulic Conductivity
Wilting and Plasmolysis
Water Balance of Plants
- Irrigation
- Soil Hydraulic Conductivity and Water Potential
- Root Hydraulic Conductance
- Calculating Velocities of Water Movement in the Xylem and in Living Cells
- Leaf Transpiration and Water Vapor Gradients
In some cases, sprinkling at night can prevent much of the injury because the salt-laden droplets do not evaporate as quickly. In recent years, a technique known as drip irrigation (also called drip irrigation or micro irrigation) has come into use in some areas of the world. This approach is very efficient, but it is also very expensive and requires diligent maintenance of the hardware to keep the system running.
Cereal crops, for example, are most sensitive to drought when they are flowering, when abortion of the very young embryos can result in a barren plant. One way to simplify the analysis of the complex process of water movement across the root is to treat the entire multicellular pathway—from the root hairs to the root xylem—as if it were a single membrane with a single hydraulic conductivity. In this analog, resistors are connected to each part of the circuit; the most important are the resistance at the stomatal pore (rs) and the resistance due to the layer of untouched air at the leaf surface (rb) (the so-called boundary layer).
Water vapor pressure (pwv) is measured in kilopascals (kPa) and is proportional to water vapor concentration (see Table 3.5.1) where cwv (leaf) is the water vapor concentration inside the leaf and cwv (air) is the water vapor concentration of the air outside the leaf, both expressed in moles per cubic meter (mol m–3). In the leaf, the total resistance is mostly due to the diffusion limitation imposed by the stomatal pore, but other parts of the pathway for water vapor loss, such as the boundary layer, can contribute significantly to r. The water vapor concentration of bulk air (cwv (air)) can be easily measured, but that of the leaf (cwv (leaf)) is more difficult to determine.
The air space volume is approximately 5% of the total leaf volume for pine needles, 10% for maize leaves, 30% for barley and 40% for tobacco leaves. Assuming equilibrium, we can calculate the concentration of water vapor in the leaf air spaces if we know (1) the water potential of the leaf (which is equal to the water potential of the wall surfaces from which water evaporates) and (2) the temperature of the leaves. To achieve vapor equilibrium, water evaporates from the cell wall surfaces until the water potential of the air in the leaf equals the water potential of the leaf.
In the range from 10 to 35°C, an increase in air temperature of 12°C doubles the water vapor concentration of saturated air. This decrease in RH is due to the water-holding capacity of the air, cwv(sat.), doubling. As a result of the drop in RH, water will evaporate in the airspace until the RH returns to a value of 99.3% and the air is again in water potential equilibrium with the leaf.
Transport of water and Transpiration
- Measurement of Transpiration
- Stomatal Mechanics
- Stomatal Mechanisms
- Significance of Transpiration
CO2 concentration in the atmosphere is fairly constant, while transpiration studies must consider evaporation as well as diffusion. Water in the soil depends only on evaporation from the soil surface and transpiration from plants. The waxy cuticle on leaf surfaces limits diffusion so that most water vapor and other gases must pass through the openings between the guard cells (Figure 4.2A).
Technically these openings are called stomas, but often the term is applied to the entire stomatal apparatus, including the guard cells. The cellulose microfibrils, or micelles, that make up plant cell walls are arranged around the periphery of the elongated guard cells as if radiating from an area in the center of the stomata (Figure 4.2 E, F). But the holding cell can increase in length; Because two guard cells are attached to each other at both ends, they therefore bend outward when they swell, causing the stomata to open.
The guard cells of some species were slightly more thickened along the concave wall next to the stomatal openings. What causes guard cells to take up water so that the stomata open. Conversely, high CO2 concentration in the leaves causes the stomata to partially close, and this occurs in the light as well as in the dark.
Guard cell uptake of potassium ions: As stomata open because the guard cells take up water, and water uptake is caused by more solute and therefore a more negative osmotic potential. Accumulating experimental evidence has made it clear that potassium ions (K+) move from the surrounding cells to the guard cells when stomata open. The amounts of K+ that build up in the vacuoles of guard cells during stomatal opening are sufficient to account for the opening.
When leaves are transferred to darkness, K+ moves out of guard cells into surrounding cells and stomata close. When strips of epidermal tissue are removed from broad bean (Vicia faba) leaves, most of the epidermal cells are broken, but the guard cells remain intact. Stomata also close in response to application of abscisic acid, a plant growth regulator, which causes loss of K+ from guard cells.
Transport of K+ from accessory cells to guard cells causes more negative osmotic potentials and thus opening of stomata, while reverse transport causes closure. When light is effective, it is partly because photosynthesis in the mesophyll cells of leaves lowers CO2 in the intercellular spaces, and the fuse cells respond by taking up more K+ ions.
Study Questions
Suggested Readings
