WATER: BASIC TERMS AND CONCEPTS

1. Vapor pressure = the partial pressure exerted by the water vapor in a given volume of air (can be measured in millibars, atmospheres, pascals, etc.).

2. Saturation vapor pressure = the maximum vapor pressure for a given temperature.

3. Vapor pressure deficit (or saturation deficit) = the difference between the actual vapor pressure and the saturation vapor pressure at the same temperature (an indicator of the evaporative power of the air).

Driving force of transpiration is the tendency for water vapor to move from areas of high vapor pressure to low vapor pressure. Steep vapor pressure gradients result in rapid rate of movement of water vapor.

Vapor pressure gradient is greatly influenced by:

i. temperature

ii. turbulence (mixing) versus still air

Rate of transpiration is determined by:

a. difference in availability of water in the atmosphere and soil

b. resistance to water movement into, through, and out of the plant

SOIL WATER

Three types of soil water defined by how tightly soil water is retained by the soil:

1. Hygroscopic water -- held with a pressure of 31 to 1000 atmospheres (1 atmosphere = 1000 millibars).

2. Capillary water -- held with a pressure of approximately .33 to 31 atmospheres.

3. Gravitational water = free water -- held with less than .33 atmosphere pressure.

FIELD CAPACITY AND AVAILABLE WATER

In a soil at field capacity (at approx. .33 atmosphere):

a. all the gravitational water has drained and only capillary water remains;

b. only capillary pores (micropores, < 0.05 mm diameter) are occupied by water;

c. macropores are occupied by air.

The permanent wilting point is point at which capillary water is held so tightly that it is unavailable to plants (arbitrarily set at 15 atmospheres).

Available soil moisture is the capillary water held at tensions between approximately .33 and 15 atmospheres

SOIL INFLUENCES ON MOISTURE AVAILABILITY

Moisture availability in the soil is determined by:

1. Moisture suction (i.e. force with which water is held by the soil):

a. Matric suction is the attraction of water to soil solids.

b. Osmotic suction is the attractive forces due to ions and other dissolved substances

2. Soil depth (volume).

3. Soil stratification

PLANT RESISTANCES TO WATER MOVEMENT

Resistances to water movement are classified as:

1. Root resistance

2. Stem resistance

3. Leaf resistance:

a. boundary layer resistance = any characteristic which increases the thickness of the "boundary layer" at the surface of the leaf (e.g. curling of leaf, increased hairiness, sunken stomates).

b. internal leaf resistance

i. intercellular (depends on distance water vapor must travel; thick leaves tend to reduce rate of transpiration)

ii. cuticular (usually very minor; thicker cuticle results in less cuticular transpiration)

iii. stomatal (this is the most signficant).

Generally stomates tend to open (and transpiration increases) as:

 intensity of solar radiation increases from low to moderate levels;

 water content of leaf increases;

 CO₂ content of leaf decreases to low levels.

Generally, stomates tend to close as:

 vapor pressure gradient between leaf and air increases;

 CO₂ content of leaf reaches high values;

 temperatures at the leaf surface reach very high values (e.g. mid-day closure is common).

ADAPTATIONS TO TOLERATE OR AVOID MOISTURE DEFICITS

1. True xerophytes can tolerate tissue dessication. Very rare for higher plants.

2. Most higher plants have mechanisms of avoiding excessive water loss:

a. xeromorphic leaves-- smaller cells in leaves, smaller leaf surface area, thicker leaves, fewer and smaller stomata,

sunken stomata,more hairs per surface area, thicker cuticles

b. highly sensitive stomates

c. deciduous habit during periods of increased moisture deficit

(e.g.'s ocotillo in Sonoran desert, tropical deciduous forests, temperate winter deciduous forests)

d. water storage (succulents)

e. phreatophytes (roots tap ground water): facultative versus obligate phreatophytes

f. desert ephemerals (herbaceous annuals)