chapter 30.5: transport of water and solutes in plants (plant physiology) - BIOL 2130

water potential

a measure of the potential energy in water so plants can send water to the great heights

• from roots to out the stomata

• difference between the PE of a water sample and PE of pure water

• greek letter psi

• measured in units of MPa

• PE of water = 0 as a baseline 

• influenced by: solute concentration, pressure within plant, gravity, and matrix effects (ions in plant)

  • water moves to equilibrate the system

  • high to low water: potential of system (total) = soil > root > stem > leaf > atmosphere

solute potential

solutes decrease water potential due to bonds between hydrogen molecules

• consume potential available in water = less energy available for movement

• negative in plants, zero in distilled water

• result in negative water potential

• hydrogen energy cant work when bonded to solutes

• decreases when increasing solute concentration, since less free energy available

• vant hoff equation: -MiRT

pressure potential

pressure exerted on or by water in plant cells, representing an expression of energy

• positive (compression): increases water potential

  • ex: turgid full plant

• negative (tension): decreases water potential

  • ex: wilting plant

• plants can manipulate through solute potential and osmosis

• opening stomata: releases water and decreases, decreasing water potential

• closing stomata: conserves water and increases, increasing water potential

pressure potential and solute concentration (pp and sc)

1) increase solute concentration → decreases solute potential (no free energy due to bonding)

2) this decreases total water potential (since solute potential decreases)

3) water moves into the cell by osmosis (lower concentration inside the plant)

4) influx of water increases pressure potential and restore turgidity 

gravity potential

always reduces water potential of the system because it pulls downward towards the soil

• more effective on taller plants than nonvascular, seedless plants (moss) → higher water column

• in a plant with no height: negative to 0

• plants cannot manipulate this

matric potential

always reduces water potential, measures how strong water binds to solid surfaces

• like cell walls or soil particles due to adhesion and cohesion

• always negative (dry soils/seeds) to 0 (saturated system when water freely available)

• binding reduces water’s free energy and movement

• plant cannot manipulate this

transpiration

loss of water vapor through evaporation at the leaf surface

• MAIN DRIVER of water movement through xylem

• causes negative pressure (tension) at leaf surface

  • dry air (not humid) makes bigger water potential difference than the leaf, increases this and decreases water potential

• tension in the leaves draw water thru plant!!!!

• stomata closes at night: moves upward and is maintained because of cohesion and adhesion in xylem vessels and tracheids

• NO ATP

transpiration control

atmosphere: drives transpiration rate and causes massive water lost

• up to 90% of water a plant absorbs being lost

• humidity, temperature, wind

cuticle: waxy covering on leaves to prevent some water loss

• desert plants or xerophytes

• thick covering of hair-like trichomes or stomata under leaf surface

  • reduces water loss by limiting air flow over stomata

stomata and guard cells: open and close to regulate gas exchange and water loss

photosynthate transporation

thru the phloem

• photosynthates: simple sugars

• sources: produce photosynthates (usually leaves)

• sinks: points for sugar delivery (roots, shoots, developing seeds)

• direction changes depending on development stage or season

• ex: potato plant – sink as leaves under certain conditions, and source as roots for stored sugar

  • after flowering when the weather gets cooler, uses sugars in potatoes instead of leaves to maintain

  • become dormant 

translocation

the movement of photosynthates from sources to growing parts of the plant, thru the phloem

• produced in mesophyll of leaves (connected by plasmodesmata)

• transported by STE in phloem, against concentration gradient and requires ATP 

  • STEs have reduced cytoplasmic material and allows pressure driven bulk flow

• moved to closest sink (phloem sap is aqueous and in sugar (30%), due to pressure differences driving bulk flow

  • bulk flow drives sugars to the sink with lower sugar content

  • high sugar content → decreases solute water potential → decreases total water potential → influx of water from xylem into phloem → increases pressure → bulk flow into sink