CHAPTER XII Soil and Plant Water Relations

WATER Makes up approximately 90 % of a plant's mass and performs many functions: 1.Required for seed germination. 2.Serves as part of the plant's structure. 3.Carries minerals into and through the plant. 4.Transports photosynthates and other biochemicals through the plant, 5.Cools the plant by evaporation 6.Involved in photosynthesis.

Characteristics: This phenomenon of polarity creates an attraction between water molecules. Water molecules can also attract or be attracted by Cations, such as Na+, K+, and Ca++, or Anions or clay colloids in the soil. Water is the universal solvent One of nature's most stable compounds, Water molecule is not symmetrical (creates a dipole)

SOIL WATER Availability Even though water is present in the soil, it sometimes is not available to the plant. The pore spaces are always filled with water, air, or a mixture of both. –When the pore spaces are filled with water, the soil is said to be saturated. Saturation is an unhealthy condition for plants if it lasts too long because the oxygen needed for respiration is missing. –When the pore spaces are filled mostly with air, the soil is too dry → -ve effect on plant growth.

The number and size of the soil pores vary with the soil's texture and structure. Clay soils have smaller but more numerous pores than sandy soils. Thus, an equal volume of clay soil holds more water than a sandy soil when the pores are filled (Fig. 12-2).

The ability of the soil to retain water is called its water-holding capacity. Fig

Water Movement and Retention in Soil Three forces are responsible for water movement within the soil. 1- Gravity causes water to move downward and is the principal force when a soil is saturated = percolation. 2- Adhesion is the force of attraction between unlike molecules (soil particles and water). 3- Cohesion is the force of attraction between like molecules (water and water) = capillary motion The latter two forces can cause water to move by capillarity in any direction—upward, downward, or laterally—and are the principal forces that move water in an unsaturated soil.

The upward movement of water, called capillary rise, is responsible for the loss of water from the soil surface by evaporation (Fig. 12-4). As soil dries, the water film surrounding each soil particle thins. Consequently, the adhesive and cohesive forces of attraction increase rapidly, making it more difficult for the plant to extract water that is held tightly in soil particles.

Salts are present in soil water. The salts create osmotic energy, and if the salts are present in a sufficiently high concentration, the osmotic energy prevents water movement into the plant (Fig. 12-6) = Plasmolysis. Remember that fertilizer is a salt.

After a prolonged rain or irrigation, the air in the soil pores is displaced with water. In this condition, the soil is saturated, When no more water is added, losses continue, first from the larger macropores=percolation and then from the smaller micropores=infiltration. Loss of water continues until the adhesive and cohesive forces equal gravity. At this moisture content, the soil is said to be at field capacity. The water has drained from the macropores but the micropores still contain water.

If no water is added, eventually the soil reaches a moisture content that does not sustain plant life and the plants permanently wilt. The soil moisture content at which a plant wilts and cannot recover when placed in an environment of 100 percent relative humidity is termed the permanent wilting point (PWP). Soil texture determines the relationship between soil water content and soil moisture tension (Fig. 12-8).

Available water (AW) is defined as the soil moisture between field capacity (FC) and the permanent wilting point (PWP): AW = FC - PWP Water between FC and saturation is not considered available because it is lost through drainage. Water at tensions greater than PWP is held too tightly by the soil for plants to remove.

PLANT WATER Absorption and Conduction of Water The energy to move the water comes from the water potential gradient that develops in the soil, plant, and air continuum: –In most cases, the water potential of moist soils is greater than that of the roots, so water flows into the roots (some anatomical features of the root aid or deter this process). –Once inside the xylem of the root, however, the flow of water is almost strictly pressure dependent, with the pressure decreasing from the root to the stem, to the leaves, to the air (Fig ).

The upward movement of water and dissolved minerals occur mainly in the xylem. The xylem extends from the root through the stem and other parts of the plant. The upward movement is sometimes called the transpiration stream because transpiration is the primary cause of this movement. The upward transpiration pull in the leaves is started by the evaporation of water molecules from the outer surfaces of the mesophyll cells.

Most of the water is lost from the leaves by transpiration. Water loss in the plant is regulated to a certain extent by the opening and closing of stomata on the leaf surfaces. Not all water leaves the leaf: 1.Some water is part of the plant's structures or held in the cytoplasm. 2.Some is used for biochemical processes, 3.Some is stored in the tonoplast. The pressure of water inside a cell creates turgor pressure, which gives plants rigidity. When there is insufficient water to create turgor, the plant wilts.

Absorption and Transport of Mineral Nutrients The sap in root cells has concentrations 500 to 10,000 times higher than those of the same element in the soil solution. –If simple diffusion were the only mechanism involved in taking up soil nutrients, the mineral nutrients could not move into the roots against such a high concentration gradient. –In addition, the plasma membranes of cells are largely impermeable to the movement of ions. Energy is required to move ions –Against the concentration gradient and –Through the impermeable membranes. This energy is obtained from the respiration of starches and sugars that originated in the photosynthetic processes.

Translocation of Sugars Sugars that are synthesized during photosynthesis move throughout the plant, primarily in the phloem tissues. The movement is –Mostly downward from leaves to roots, –Lateral or even upward movement from leaves to fruits or buds or other storage organs.

In woody perennials, when phloem tissue is severed by girdling: 1.The tissue above the cut proliferates =تتكاثر 2.The tissue below the cut is starved تعاني=for photosynthates (Fig ). This can severely stunt and even kill a tree The rate of translocation of sugars in the phloem is, in some instances, more than a thousand times faster than simple diffusion of sugar through water. The forces that create these high rates of movement are called active transport.