FUNCTIONS OF WATER

1) solvent - dissolves solutes; 
                   the cytosol of the cytoplasm is a water solution of dissolved solutes.

2) reagent - water is used in biochemical reactions, such as photosynthesis.

3) translocation medium - water translocates solutes in xylem and phloem.

4) temperature relations - water is very important in regulating temperature.
    a) stabilizes plants and environment (due to high specific heat)
    b) evaporative cooling (due to heat of vaporization)
    c) releases heat when freezes (due to heat of fusion)
    d) constant temperature during phase change - freezing/melting water/ice water
          stays at 32 F.

5) turgor pressure and growth

    turgor pressure - the positive pressure inside of cells due to water uptake.
    a) turgor pressure is due to osmosis.
    b) turgor pressure keeps cells expanded 
    c) turgor pressure keeps herbaceous plants and plant parts erect
    d) turgor pressure is the driving force for growth in size by causing cell expansion.

growth - an irreversible increase in size or mass.

plasmolysis - shrinkage of individual cells due to loss of turgor pressure that causes
                           a cell to become flaccid.

wilting - excessive water loss that causes loss of plant rigidity; 
                caused by plasmolysis of enough  individual cells to cause the organ to be limp.

RELATIONSHIP BETWEEN HUMIDITY AND TEMPERATURE
dew point - the temperature where relative humidity equals 100%. 
condensation - conversion of water vapor to liquid or ice; condensation occurs when the 
                         temperature drops below the current dew point.
precipitation - loss of atmospheric water; occurs when condensation is extensive enough
                           that the water or ice coalesce to form droplets/crystals that fall from the
                           atmosphere due to the force of gravity.
Cold air can hold less water vapor than warm air, so as temperature decreases the absolute humidity or specific humidity that can be held at saturation decreases.  Relative humidity is a percent of saturation.  So as temperature decreases the absolute or specific humidity at saturation decreases, but the amount of water vapor present stays the same, thus relative humidity increases (conversely, as temperature increases relative humidity decreases). If the temperature continues to decrease (such as cooling over night or as air rises in the atmosphere),  a temperature is reached where the relative humidity reaches 100% - this temperature is the dew point.  If the temperature drops below the current dew point, the air becomes over saturated and condensation will occur.  If this occurs in the upper atmosphere, a cloud forms or  precipitation occurs.  If this occurs close to the earth's surface, fog or dew forms. 

TYPES OF CONDENSATION
1) dew - condensation of water onto solid surfaces
2) fog - condensation of water into small droplets that stay suspended in air close to the
            earth's surface.
3) cloud - condensation of water into small droplets that stay suspended in air high in the
               atmosphere  - 6,000 to 50,000 feet high

TYPES OF PRECIPITATION
I) condensation occurs above freezing (when dew point is above 32 ^oF)
    a) drizzle - water droplets less than 0.5 mm
    b) rain - water droplets greater than 0.5 mm.
2) condensation occurs below freezing (when dew point is below 32 ^oF)
    a) snow- water condenses below freezing directly into small, loose ice crystals.
3) condensation occurs above freezing, followed by freezing
    a) sleet - liquid rain droplets fall through a layer of freezing air and then freeze.
    b) hail- liquid rain droplets fall through a layer of freezing air and then freeze; air currents
                 carry the frozen droplets back up into the upper atmosphere; they pick-up more
                 water,  then freeze upon falling through the layer of freezing air; each time the
                 ice crystal  circulates through the warm and freezing layer it gets larger until it 
                 finally falls to earth - formation of hail. ; view of thunderstorm from airplane

translocation - movement of water through plants, mainly through xylem
.
transpiration - loss of water vapor from leaves and other above ground plant parts; 
                           mainly occurs through the  stomata.

guttation - loss of liquid water from leaves;
                     occurs through hydathodes (similar to stomata, but they do not close).
 

SITE OF WATER ABSORPTION
1) young roots - most absorption, mainly through root hairs due to:
    a) very numerous - 14 billion on a typical rye plant.
    b) large surface area -14,000 ft² (1310 m²) on a typical rye plant
    c) rapidly and constantly produced - 975 linear ft (300 m) per day on a squash plant

2) older roots - little absorption due to:
    a) suberization of endodermis
    b) periderm (bark) formation
 

COHESION THEORY OF TRANSLOCATION IN THE XYLEM
1) Transpiration occurs and is driving force

2) Causes negative pressure in leaves

3) Column of water is pulled up in the xylem and translocated due to:
     a) H-bonding (hydrogen-bonding)
     b) small size of xylem pores
     c) negative charges on xylem walls
 

FUNCTIONS OF TRANSPIRATION
1) driving force for translocation: transpiration causes a negative pressure in leaves, 
      which "pulls" the water up the xylem.

2) evaporative cooling of leaves:  540 cal of heat energy is dissipated for every gram 
     of water that evaporates from leaves, which is a major contributor to the cooling of
leaves.

Transpiration is usually much greater than is needed to satisfy these two functions. Thus, many horticultural practices attempt to minimize excessive transpiration.

2) leaf orientation - vertically orientated leaves decrease transpiration, yucca

3) leaf surface - waxy, hairy or shiny leaf surfaces decrease transpiration, Echeveria waxy, Kalanchoe hairy

4) stomata - when stomata are closed, transpiration decreases

ENVIRONMENTAL FACTORS
1) humidity - high humidity decreases transpiration

2) temperature
     a) low temperature decreases transpiration.
     b) high temperature increases transpiration, but when it gets too hot the stomata close, 
          then transpiration may decreases.

3) light intensity
     a) darkness decreases, because stomata close, (except for CAM plants open at night).
     b) high light intensity increases temperature which increases transpiration, until stomata
         close then transpiration may decrease; occurs midday during heat of summer.

4) wind - as wind increases, transpiration increases, but if the wind gets too high, then the 
         stomata close and transpiration decreases

5) soil water
     a) when soil is moist, transpiration occurs according to the above factors
     b) when soil is too dry, stomata close causing transpiration to decrease (overrides
         the above factors)

TECHNIQUES USED TO DECREASE TRANSPIRATION
1) mist or spray foliage 
     a) in propagation an intermittent mist system is used
     b) mid-afternoon sprinkler irrigate plants in greenhouses/nurseries

2) decrease light intensity - grow plants under shade

3) harden-off seedlings
     a) decrease watering,
     b) decrease temperature, or
     c) decrease fertilizer, especially N.

4) antitranspirants - chemicals that close or clog stomata.
     Two Types
     a) physiologically cause stomatal closure
     b) wax, resin or latex that clogs stomata, WiltPruf