Hort Phys - Plant water relations 2

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Long Distance Transport

Transport of water in the xylem vessels and tracheids (2nd step)

Short distance transport

Water uptake from the roots in the soil until it reaches the xylem (1st step)

Third step in transport…

Transport of water into the atmosphere through the stomata

Root Hairs…

Provide large surface area for water uptake

Apoplast Pathway

between epidermal and cortical root cells, i.e. through their cell walls

and intercellular spaces, via diffusion (does not enter the cell)

Symplast pathway

Water enters the root hair and epidermal cells, and once inside a cell, it travels to other cells through plasmodesmata

Trans-cellular pathway

water moving from cell to cell by crossing cell membranes

water flow through root cortex involves …

both symplast and apoplast pathways; the latter accounting for a much larger proportion

Endodermis

The inner most layer of the root cortex

Casparian Strips

Road block for apoplastic water; in the endodermis cell walls block apoplastic water movement. made up of suberin, a complex mixture of hydrophobic, long-chain fatty acids and alcohols

Water crosses membranes of root cells via …

special membrane proteins called aquaporins

Water must move up the plant …

at long distance at a much faster rate than what can be supplied via diffusion

Bulk/mass flow

1) positive water pressure in roots, forcing/pushing upward water movement - Root pressure

2) Pulled from above (where air wicks water vapor from the leaf surface - pull – tension)

Does Root Pressure exist in plants?

especially under cool (night, early morning), humid conditions– liquid appears at the cut

Guttation

water droplets at the vein endings of leaf blades (early morning)

Hyathodes

Modified stomata at vein endings

Root Pressure

Typically not responsible for supplying water to the plants, However, RP could serve for water rise in certain herbaceous (small statured) plants only under certain conditions

conditions

1) has not been detected in all species

2) does not exist during most of the day-time

3) Pressure generated by R.P. is not sufficiently high

Cohesion-tension theory

H2O evaporates from leaf surfaces due to VPD (WP gradient), creating transpiration pull (tension), which pulls in water along the ‘soil-plant-atmosphere continuum’

Movement of water as an intact column is facilitated by cohesion between water molecules

Vapor Pressure (VP)

In a transpiring plant, water in the xylem of stem/leaf is always under tension (pull). Larger the # of water molecules in vapor phase = higher the V.P.

Vapor pressure deficit (VPD)

Difference between V.P of sub-stomatal cavity and V.P. of outside, relatively drier, air. Determines the rate of water loss.

The greater the VPD …

greater / faster the loss of water vapor from leaf (transpiration); from higher V.P. to lower V.P. area

Temperature influences

1) Relative humidity

2) Vapor pressure of the stomatal cavity

3) The magnitude of the VPD

The higher the temperature…

The steeper the VPD, and greater/faster the loss of water vapor from leaves (transpiration)

Relative humidity

(actual mass of water vapor in an air mass/Mass of water vapor air can hold at saturation) x 100