Plant Physiology: Water Relations and Transport

This set of vocabulary flashcards covers the fundamental principles of plant physiology including water potential thermodynamics, root absorption mechanisms, pathways of water movement, and the molecular regulation of transpiration.

Water Potential ($\Psi_w$)

The chemical potential of water representing the measure of free energy per unit volume relative to pure water; it determines the direction of osmosis and is measured in Pascals ($Pa$) or Megapascals ($MPa$).

Water Potential Equation

$\Psi_w = \Psi_s + \Psi_p$ (under normal cellular conditions omitting matric and gravity potential).

Solute Potential ($\Psi_s$)

Also known as osmotic potential, it represents the effect of dissolved solutes on water potential; the addition of solutes lowers free energy, making this value always negative.

Pressure Potential ($\Psi_p$)

The hydrostatic pressure exerted on water; it is typically positive (turgor pressure) in plant cells but can be negative in xylem vessels under tension.

Matric Potential ($\Psi_m$)

The force resulting from the binding of water molecules to solid, hydrophilic surfaces via adsorption; critical in dry seeds or soils but negligible in living cells.

van 't Hoff's Equation

$\Psi_s = -miRT$, indicating solute potential is proportional to molar concentration ($m$), ionization constant ($i$), gas constant ($R$), and absolute temperature ($T$).

Turgid State

A cellular state in a hypotonic environment where water influx continues until $\Psi_w = 0$ and $\Psi_p = -\Psi_s$.

Passive Water Absorption

The dominant mechanism (over 90%) driven by transpirational pull and negative tension in the xylem without the consumption of metabolic energy.

Active Water Absorption

Uptake driven by the root's metabolic energy (ATP) where ions are pumped into the xylem to create an osmotic gradient, often generating Root Pressure.

Radial Pathway of Water

The horizontal sequence of water movement: $\text{Root Hair} \rightarrow \text{Epidermis} \rightarrow \text{Cortex} \rightarrow \text{Endodermis} \rightarrow \text{Pericycle} \rightarrow \text{Root Xylem}$.

Casparian Strip

A waterproof band of suberin and lignin in the endodermis that blocks the apoplastic pathway, forcing water to enter the symplast.

Apoplast Pathway

The non-living continuum of cell walls and intercellular spaces where water moves rapidly via mass flow and diffusion.

Symplast Pathway

The interconnected network of living protoplasts linked by Plasmodesmata through which water moves with metabolic resistance.

Plasmodesmata

Microscopic, membrane-lined cytoplasmic bridges that physically link adjacent cell cytoplasms.

Aquaporins (AQPs)

Specialized tetrameric transmembrane proteins of the Major Intrinsic Protein (MIP) family that facilitate rapid, passive water transport across lipid bilayers.

NPA Motif

The specific amino acid sequence (Asparagine-Proline-Alanine) that lines aquaporin pores to allow single-file water passage while blocking protons ($H^+$).

Root Pressure

Positive hydrostatic pressure ($0.1\text{ to }0.2\text{ MPa}$) generated in the xylem by active ion accumulation; it can cause guttation and repair xylem embolisms.

Guttation

The appearance of liquid water droplets on leaf margins or tips, occurring through Hydathodes due to high root pressure and low transpiration.

Cohesion-Tension Theory

Proposed by Dixon and Joly (1894), it explains the ascent of sap through the high cohesive strength of water ($15\text{ to }30\text{ MPa}$) and transpirational pull.

Cavitation and Embolism

The snapping of the water column under extreme tension and the resulting air bubble that blocks water transport in xylem vessels.

Cellulose Radial Micellation

The radial arrangement of cellulose microfibrils in guard cell walls that forces cells to expand lengthwise and bow outward when turgid.

Phototropins

Blue-light flavoprotein receptors (Phototropin 1 and 2) that initiate the signaling cascade for stomatal opening in the morning.

H+-ATPase Proton Pump

An enzyme in the guard cell plasma membrane that expels $H^+$ to create an inside-negative electrical potential below $-100\text{ mV}$.

Abscisic Acid (ABA)

A stress hormone synthesized during drought that triggers ${Ca}^{2+}$ release and ion efflux, causing guard cell flaccidity and stomatal closure.