Plant Water Relations and Transport

Water Content in Herbaceous Plants

80-95%

Water Content in Woody Plants

35-75%

Water Content in Seeds

Very low, typically 5-15%.

Physiological Roles of Water

universal solvent; reactant in metabolism; medium for biochemical reactions; and maintains turgor pressure

Universal solvent role of water

Dissolves minerals, sugars, amino acids, and hormones for transport and metabolism

Reactant in metabolism

directly used in photosynthesis and in hydrolytic reactions

Medium for biochemical reactions

enzymes require a hydrated environment (e.g., cytoplasm) to remain active

Maintains turgor pressure

gives shape and rigidity, essential for cell growth, leaf orientation, and stomatal movement

Function of turgor pressure

Maintains shape, mechanical rigidity, and enables cell expansion.

Water potential (Ψw) of pure water

Assigned a value of 0 at standard conditions.

Direction of water movement

From higher to lower (more negative) water potential.

Function of root hairs

Vastly increase surface area for water absorption.

Apoplast pathway

Water moves through cell walls and intercellular spaces until the Casparian strip.

Symplast pathway

Water moves through the cytoplasm cell-to-cell via plasmodesmata.

Transmembrane pathway

Water repeatedly crosses cell membranes, often via aquaporins.

Cohesion in Cohesion-Tension Theory

Strong hydrogen bonding keeping the water column continuous.

Adhesion in Cohesion-Tension Theory

Attraction between water molecules and narrow xylem walls.

Energy source for transpiration pull

The sun, requiring no metabolic energy from the plant.

Guttation

Release of liquid water droplets through hydathodes at night or high humidity.

Stomatal transpiration percentage

Accounts for 90-95% of total water loss through stomata.

Factors increasing transpiration rate

High light intensity, low humidity, high temperature, and high wind speed.