The movement of water in the plant is primarily determined by two factors
Water potential gradient
Resistance of the path that the water travels
Water always moves from high to low water potential (less negative to more negative water potential).
Water uptake in the roots is affected by:
Soil water content
Properties of the soil
Architecture of the root systems
Presence of root hairs and fungi
Conductance of the root system
Symbiotic relationships
Soil Water Content: the amount of water present in the soil. Depends on the soil type.
Clay < Silt < Sand
Porosity - spaces formed between irregularly shaped soil particles
Large pores (sand) - contain more water but easily drained (low water retention)
Small pores (clay) - contain less water but more challenging to drain
Gravitational water: water molecules that are retained in larger pores; loosely held; easily drained from the soil; not easily available to plant; found in larger pores; drains out of root zone
Capillary water: when gravitational water is all absorbed, this is the available water that plant roots can absorb; water held in micropores; capable of capillary action (water molecules held by each other: cohesion, and by the surface of the soil: adhesion)
Hygroscopic water: remaining water adheres to soil particles and is unavailable to plants (high energy requirement is needed for this: similar in towel)
Saturation - when all the pore spaces are occupied by water
Field Capacity - when all the gravitational water has drained and only the capillary water remains
Wilting point - when the soil water constant is so low that the plant can no longer extract water from the soil
Solute potential of soil water is generally negligible (except in saline soils).
Pressure potential of very wet (saturated soil) is very close to zero and decreases as the soil dries out because of adhesion and surface tension
Gravitational potential of soil water is higher at higher elevations and vice versa (easier to get soil water near the roots)
Matric potential of soil water decreases as the soil dries out.
Overall, the soil water potential decreases as the soil dries. This makes it harder for roots to absorb water.
Water moves through the soil by bulk flow driven by pressure gradients. Water flows from regions of higher soil water content to regions of lower soil water content.
Root system
Soil to Root hairs to cortical cells to vascular tissues
RadialConductance - Water can travel from the soil into root vascular bundle in three ways
Apoplastic - through the cell walls (will not enter the cells)
Symplastic - through the plasmodesmata of the cells
Transcellular - moves through the cell walls and cytoplasm; requires membrane passages
Axial Conductance - water encounters the Casparian strip in the endodermis which forces water to cross through the plasma membrane.
Casparian Strip - waxy, suberin-impregnated region that forms a hydrophobic belt.
Importance: generates pressure; prevent back flow; movement of nutrients; prevents movement of pathogens
Aquaporins - channel proteins that transport water molecules and also a range of other substrates (CO, minerals, etc.)
Xylem - complex tissue composed of the tracheary elements, xylem fibers, and ray cells
Tracheary elements
large diameter
dead at maturity
thickened secondary walls
Vessel members - has perforation plates; found in angiosperms
Tracheids - a tube with no openings on ends; found in gymnosperms
Transpiration
Loss of water vapor from plants through the surfaces of aboveground parts but mainly through the leaves
Happens mainly through the stomata
Cohesion-Tension Theory
The driving force for water movement in the xylem is provided by the evaporation of water from leaf surface which creates tension (pulling)
Main driver of transpiration of water in the form of water vapor from the interstitial cells through stomata is simple diffusion
Higher humidity = decrease in transpiration
Higher temperature = higher humidity
Stomatal pore open - turgor pressure in guard cells are high - high solute concentration - low water potential
Stomatal pore closes - low pressure in guard cells - low solute concentration - high water potential
CO2 - increased CO2 inside the plant, close stomata
Water status - low water, close stomata
Light - high light, close stomata
Air humidity - high humidity, stomata opens
Cavitation - Occurs when the tension of water within the xylem becomes so high that dissolved gases within water expands to fill the tracheary element
Embolism - the resulting large gas bubble that obstructs the pathway. Pits between vessel elements can be used by water to redirect its travel when specific vessel elements are blocked
Guttation - exudation of drops of xylem sap on the tips or edges of leaves of some vascular plants due to high root pressures
Hydathode - structures that secrete water through pores in the epidermis or leaf margin, typically at the tip of a marginal tooth or serration
Phloem: the vascular tissue in plants which conducts sugars and other metabolic products downwards from the leaves. Complex tissue composed of the sieve tubes, companion cells, fibers and parenchyma cells
Sieve elements - living protoplasts at maturity but lack many cellular contents; end walls have sieve plates