Plant Water Relations and Soil Properties

Comprehensive vocabulary flashcards covering the physical properties of water, water potential components, soil water categories, and plant water transport pathways.

Bipolarity of Water

The arrangement of oxygen and hydrogen in a water molecule that leads to the formation of hydrogen bonds between adjacent molecules.

Dielectric Constant

A quantity used to measure the polarity of molecules; for water, this value is as high as $78.4$.

Adhesion

The attraction between water molecules and the molecules of different compounds.

Cohesion

The attraction between molecules of the same compound, which in water is due to hydrogen bonding.

Tensile Strength

The maximum force per unit area that a continuous column of water can withstand before breaking; water can resist pressures more negative than $-20\,MPa$.

Specific Heat Capacity

The amount of heat required to raise the temperature of a unit mass of a specific substance by $1^\circ\text{C}$.

Latent Heat of Vaporization

The energy required to separate molecules from the liquid phase and convert them into the gas phase; for water at $25^\circ\text{C}$, it is $44\,kJ\,mol^{-1}$.

Viscosity

The property of resistance of molecules against the flow of a liquid, which decreases as the temperature of water increases.

Water Potential ($\Psi_w$)

The free energy status or chemical potential of water per unit volume, measured in $joule\,mol^{-1}$ or $MPa$, with pure water at atmospheric pressure being zero.

Water Potential Equation

The sum of the components in a plant cell expressed as $\Psi_w = \Psi_s + \Psi_p + \Psi_g$, where $\Psi_s$ is solute potential, $\Psi_p$ is pressure potential, and $\Psi_g$ is gravity.

Osmosis

A special type of diffusion involving the net movement of solvent molecules through a semi-permeable membrane into a region of higher solute concentration.

Isotonic Solution

A solution that has the same osmotic pressure as the cell sap.

Turgor Pressure ($P$)

The hydrostatic pressure exerted by the cell sap or cytoplasm against the cell wall, also known as pressure potential ($\Psi_p$).

Wall Pressure

The counter-equal and inversely directed pressure exerted by the cell wall on the cell cytoplasm.

Plasmolysis

The shrinkage of the protoplasm away from the cell wall due to water loss in a hypertonic solution.

Diffusion Pressure Deficit (DPD)

A term coined by B. S. Meyer (1938) representing the amount by which the diffusion pressure of a solution is lower than that of its pure solvent; calculated as $DPD = OP - TP$.

Solute/Osmotic Potential ($\Psi_s$)

The effect of dissolved solutes which reduces the free energy of water; for non-dissociating substances, it is calculated as $\Psi_s = -RTc_s$.

Matric Potential

The adsorption affinity of water to colloidal substances and surfaces, significant in dehydrated cells like seeds or desert plants.

Holard

The total amount of water present in the soil.

Chesard

The portion of the total soil water that is available to plants.

Echard

The amount of soil water that cannot be absorbed by plants.

Gravitational Water

Water that percolates downward through large soil pores under the force of gravity and reaches the low water table.

Capillary Water

Water held by capillary forces in the spaces between soil particles; it is the most significant form of water absorbed by plants.

Hygroscopic Water

A thin layer of water vapor held tightly around soil particles by adhesive forces; it is not available to plants.

Field Capacity

The percentage of water retained by soil particles after gravitational water has drained away, consisting of capillary, hygroscopic, and chemically combined water.

Water Holding Capacity

The maximum quantity of capillary, hygroscopic, and chemically combined water held in a fully saturated soil as a thin film.

Permanent Wilting Point

The soil water content at which plants cannot regain turgor upon rehydration; occurs when soil $\Psi_w$ drops below $-1.5\,MPa$.

Root Hairs

Thin-walled, unicellular, filamentous outgrowths of root epidermal cells that increase the surface area for water and ion absorption.

Velamen Tissue

A hygroscopic layer of cells found in the arid roots of epiphytes like orchids used to absorb atmospheric moisture.

Apoplast

The continuous system of cell walls and intercellular air spaces through which water moves without crossing membranes.

Symplast

The network of cell cytoplasm interconnected by plasmodesmata through which water travels across the root cortex.

Transmembrane Pathway

The route where water enters a cell on one side and exits on the other, crossing the plasma membrane twice for each cell in the series.

Casparian Strip

A structure in the endodermis that breaks the continuity of the apoplast, forcing water and solutes to move symplastically through the plasma membrane.

Xylem

A complex tissue consisting of tracheids, vessel elements, fibers, and parenchyma responsible for the transport of water and dissolved minerals.

Anoxia

The total depletion of oxygen in soil, often caused by flooding, which inhibits cellular respiration in plant roots.

Critical Oxygen Pressure (COP)

The oxygen pressure below which respiration rates decrease; for corn root tips at $25^\circ\text{C}$, this is about $20\,kPa$.