The availability of water controls plant distribution on scales ranging from microhabitats to continents. Precipitation and temperature are the primary control of biomes of the world, and both are closely correlated with terrestrial net productivity.

Water in plant systems is best described using concepts of thermodynamics as exhibited in water potential (^). This value, derived from natural gas laws, expresses the energy contained in water in units of pressure as mega-pascals (MPa). The water potential of pure water is defined as 0 MPa. Numerous physical and chemical factors affect water potential including the effects of solute (^s), matric (^m), pressure (^p), and gravity (^g). The addition of solute to water lowers the free energy of water in that system by altering hydrogen bonding between water molecules. The solute potential (^s) of this system would be negative. For example, a 1.0 molal solution of a nondissociating solute creates a solute potential of — 2.5 MPa. If this solution is separated from pure water by a cell membrane, then a potential energy gradient exists from the water to the solution. Water will move from areas of high potential energy (pure water) to areas of low potential energy (solution) following basic principles of thermodynamics. The movement of water into the cell would increase the cell volume. This increase in volume would be constrained by the cell wall, which would create a positive pressure on the water in the cell. The result would be an increase in the pressure potential (^p) of the cell. In a short time the pressure potential of the cell would offset the solute potential, and the cell would reach equilibrium. Ecologically, solute potentials become important in soils that are salinized by evaporation. In those soils a water potential gradient cannot be established to enable absorption of water by roots. Pressure potentials within living cells are typically positive; however, the pressure potential of the stream of water moving up the plant through its xylem is typically negative, resulting from greater loss of water from the shoot than is absorbed by roots. This negative pressure in the xylem column is frequently referred to as xylem potential (^x), is easily measured, and serves as a proxy for plant water status. The potential energy of water in the xylem column is also affected by gravity (^g), and water potential decreases with height by —0.01 MPam—1. Maximum tree height (c. 120 m) may be related to the decreased water potential with height and its effect on turgor and photosynthesis. In the soil, matric potentials (^m) are the most important determinant of soil water availability. Here the surfaces of sand, silt, and clay provide a strong attraction to water and limit the ability of plants to absorb the water present in soil.

Plants have a physiological limitation for water absorption based upon their ability to lower osmotic potentials of root cells. This limitation is described as the permanent wilting point (PWP), which is an expression of water content of soil when a plant is no longer capable of extracting water. The water potential at PWP varies with species, but for mesophytic, agricultural plants is approximately —1.5 MPa. At PWP in sandy soil the remaining water may be less than 1% by dry weight, but in clay soil the water content may be nearly 20% by weight.

Water use by individual plants varies considerably depending on size of the plant, availability of soil water, and atmospheric temperature and humidity. Water consumption of plants may range from 10 to 200kgday—1. Transpiration ratio (TR) is a measure of water loss to carbon gain by plants, typically expressed in mass ofwater lost to mass of plant weight gained. This value will vary between plants and the photosynthetic pathways they possess. Plants with the C3 photosynthetic pathway have a TR of 450-950; those with the C4 pathway have a TR of 250-350; while those with the CAM pathway have a TR of 18-125. These differences account, in part, for the relatively higher proportion of C4 and CAM plants in hot, arid regions where water is frequently limiting to plant growth. The water status of plants varies both seasonally and diurnally. On a daily basis stomata may close in the afternoon resulting from an inability to match absorption to transpiration rates. This results in a decrease in photosynthetic capacity. Similarly, on a seasonal basis, when evapotranspiration rates have depleted soil water, the low water potential of the plant will result in stomatal closure during much of the day.

Vertical movement of water in plants is widely accepted as occurring by cohesive forces. Water lost by evaporation in leaves is replaced by water pulled up by the strong cohesive forces between water molecules. The velocity of water movement is proportional to the fourth power of the radius of the conduits, of xylem vessels and tracheids. Large vessels provide for rapid water conduction, a phenomenon observed in lianas. Cavitation results from the strong negative pressures developing within the xylem coupled with the formation of air bubbles in the xylem columns. This leads to a break in the water column, which is termed an embolism. Freezing of water and the release of gas from thawing ice is associated with embolisms and the effect is more pronounced when the xylem is under negative pressure. Small conduits of tracheids found in gymnosperms provide protection from frequent embolisms and may account for the northern distribution of gymnosperms and their abundance at higher altitudes. Mixtures of large, rapidly conducting vessels with small diameter, and slower conducting tracheids provide a measure of safety for plants.

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