Agriculture for Engineers 4 (3+1)

Lesson 25. Conduction and Transpiration of Water and Plant Processes

25.1 WATER CONDUCTION

Water is conducted from the root surface to leaf surface through the plant body. The difference of yair and yroot surface results in the accent of water. The transpiration from leaf surface sets up imbibitational forces in the mesophyll cells that are transmitted through the hydrodynamic system in the plant to the root surface. Water moves in liquid form from the soil to leaf cells through root cells and the conductive system of xylem. It moves in vapour form from leaf cells to the air through intercellular spaces in the leaf and stomatal openings. Water moving into the xylem of roots from the root surface and then to leaves through vascular bundles finds its way along the path of least resistance. The xylem functions in water conduction. The water conduction is based on the cohesion theory. Water molecules have mutual attraction for each other, and water columns confined in small capillaries cohere with a tensile strength sufficient to pull them up to the evaporating leaf surface. It may block the lumina and offer resistance to water conduction. The xylem is however considered having an adequate conduction capacity to overcome the loss of conductivity owing to blocking by air bubbles (Kramer et al., 1967 and Kramer, 1969).

25.2 TRANSPIRATION

Transpiration is the process by which plants lose water in vapour form into the air through their aerial parts, mainly leaves. It involves nearly 99 per cent of the volume of water absorbed by young plants. Usually about 95 percent of the water absorbed is transpired and only about 5 per cent of that is used by the plant for metabolic purpose and making the body weight. About 90 to 95 per cent of the transpiration occurs during the day time and 5 to 10 per cent during the night time. Pineapple plant is the exception in which most of the stomata remain open during the night time and the major transpiration takes place at night. Transpiration is essentially a physiological process and can be considered primarily as a physical evaporation process from leaves. Plant structures and physiological behaviors of plants modify the rate of water loss in the process. The stomatal transpiration accounts for the greatest loss of water, while the cuticular or lenticular transpiration comprises relatively a very small loss. Transpiration rate is minimum in the morning. It increases with the increase in temperature during the daytime and reaches the maximum at around 2 PM local time.

Transpiration is usually expressed by transpiration ratio or transpiration coefficient that refers to the volume of water transpired by a plant to produce a unit quantity of dry matter. The value is dimensionless.

25.2.1 Transpiration Mechanism

Conversion of water from liquid vapour occurs at the moist surface of mesophyll cells. Mesophyll tissues of leaves are composed of loosely connected parenchyma cells with a large volume of intercellular spaces. The internally exposed surface area of cells is between 5 to 30 times the external leaf surface are (Kramer et al., 1967 and Kramer 1969). Cell walls are usually hydrophobic and are covered with a thin layer of water. When there is a water deficit, the conductivity of cell walls and the permeability of protoplasm decrease restricting vaporization of water. Water moves from the vascular system into masophyll tissues of leaf and evaporates from the cell surface into the intercellular spaces that are continuous to the substomatal cavities. Water vapour then diffuses into the atmosphere through stomata owing to the diffusion pressure deficit.

Transpiration depends on the number and extent of stomatal openings. Guard cells regulate the stomatal openings through which water vapour escapes into the atmosphere. With water deficit and water stress in plant, guard cells lose their turgidity and by that, reduce the size of stomatal opening. Under severe water stress condition, guard cells close the stomata and prevent further transpiration. Ting and Loomis (1963) stated that the diffusion of water vapour into the air from the substomatal cavity is not significantly reduced until the stoma is nearly close. The diffusive capacity of stoma is tremendous and the transpiration rate from a stoma is nearly 50 times the evaporation rate from the free water surface of similar area (Kramer et al., 1967 and Krmaer, 1969).

25.3 SOIL WATER AVAILABILITY AND WATER IN SOIL-PLANT-ATMOSPHERE SYSTEM

Plants absorb water from soil through roots, conduct it through their bodies and transpire the same into the atmosphere through leaves. This is continuous in the soil water-plant-atmosphere system all throughout as long the soil water is available and plants are actively growing. In this system the flow takes place along the water potential gradient from higher to lower potential energy and the flow rate is inversely proportional to resistance met in the path. The resistance is the greatest in the leaf-atmosphere interphase and least in the plant, the soil offering the intermediate. In the leaf-atmosphere interphase where the water transforms into vapour, the process is essentially diffusion influenced by aerodynamic factors. The greater is the diffusion pressure deficit (DPD), the greater is the magnitude of this transformation into vapour form. Therefore, the concept that the rate of water availability decreases with a decrease in soil water contents does not hold good always. The transpiration rate is low under low evaporative demand of the atmosphere even if the soil water availability or soil water content is very high. The growth measured in terms of height, leaf area, dry matter weight and so on which is dependent on the combination of a variety of conditions and physical processes, is affected differently by the soil water or plant water deficit and not entirely by the soil water availability.

25.4 WATER AND PLANT PROCESSES

Plant processes starting from germination to maturity of fruits and grains are affected by water supply. Some processes that have a great bearing in crop production are briefly discussed here:

25.4.1 Germination

Germination is the process of embryo growth with generation of sufficient force to break the embryo cover. In dry seeds all the tissues remain shrunken and the cell contents, plasmolysed. Seeds imbibe water and swell with cells within, becoming turgid. The amount of pressure developed by imbibition in the early stages of germination may be as high as 1000 atmospheres (Shull, 1914). The coleorhiza breaks the pericarn and extends length about 2 mm as in maize. The radical elongates to fill the extending coleorhiza and breaks through the sheath. The water content of soil or the soil water suction guides the extent of germination. When the soil water content approaches wilting point, germination processes get greatly reduced. The time for germination is considerably extended by increased soil water suction and this is probably due to a limited rate of water conduction to the embryo. Although the water availability is the dominant environmental factor, but temperature, light and supply of oxygen are also quite important in the process. With the start of biochemical changes and cell division in the germination process, the respiration rate increases with a rapid increase in oxygen requirement. Poor aeration of soil often restricts the process.

25.4.2 Seedling Emergence

Germination in common place in agriculture includes both the seed germination and the seedling emergence processes. Seedling emergence in comparison to seed germination is much more sensitive to soil water availability and other soil conditions. Soil water tension strongly influences both the per cent and the rate of emergence. The rate of seedling emergence sharply decreases with the decrease in soil water content after certain limit that varies with crops. Seed bed compaction in a relatively drier surface soil, encourages a greater movement of water from the surrounding areas and lower soil layers. By that seeds come in close contact with the soil and soil water. Thus, compaction of seedbeds mitigates to some extent the difficulties with reduced water content on seedling emergence (Triplett and Tesar, 1960).

25.4.3 Root Development

Root development is most important for a better plant growth and yield. It dictates the amount of water that could be explored by plants from different layers of soils. Water movement in the soil is usually too slow to make an adequate amount of water reach the roots. It is rather the expanding root system that meets water in the hitherto untouched regions of soil to maintain the water supply to the growing plants. An extensive and deeper root system allows the continuance of plant growth for a much longer period as compared to a restricted and shallow root system.

Soil water decides the depth of penetration of roots and lateral and relative growth of roots and shoots. Roots move towards the moist soil and follow the water when they are in direct contact or very close to it. The extent of root spread laterally or vertically downwards is governed by genetic factors and modified by soil environmental conditions. With greater availability of water, roots grow increasingly with shoots, the shoot growth being more than the root growth. With water stress the reverse situation occurs and the rate of shoot growth declines more than that of root growth. When excess water exists in soils, aeration becomes restricted and the root growth is inhibited. Only roots of rice crop and water weeds grow in water logged or wet soils.

25.4.4 Shoot Growth

A plant may be considered as a conduit for water from soil to air. Water is absorbed through roots and transpired through leaves. Plants for making their body weight and performing biochemical activity use less than 1 per cent of the water absorbed. Shoot growth is greatly influenced by the availability of water. When an adequate amount of water is available, plant cells remain turgid and plant structures retain their proper form. A continuous cell division and enlargement occur and shoots continue to grow in soils well supplied with water. The evaporative demand of atmosphere and soil water suction control the transpiration and water absorption. When there is a lag between the rate of transpiration and the rate of absorption, plant water stress occurs. So long the plant does not experience any water stress, the shoot growth continues at a potential rate. With occurrence of stress and its continual increase, the rate of shoot growth continually declines. The growth process stops as the soil water content approaches the wilting point. Growing tips of shoots receive water at the expense of the older parts of plants. When soil water is limiting and the lag between transpiration and absorption is high, growing tips and younger shoots are affected more than the older ones. Death of growing tips often occurs during the mid daytime in hot summer months although the soil water is not limiting. Certain crop plants such as cereals and tomato can make up the loss in growth following temporary stress by an increased growth subsequently with the availability of adequate water (Gates, 1955 and Kemper et al., 1961).

25.4.5 Root development and water use by crops

Roots provide the water absorbing surfaces and soils serve as the reservoir of water. The volume of water that can be explored by a plant depends on the extent of root development. A plant having an extensive root system with both downward and horizontal spread commands potentially a large reservoir of water and can stand drought conditions better. On the other hand, a plant with a shallow root system of limited horizontal spread soon exhaust the available water in the top soil layers and experiences a severe water deficit even when sufficient water is available in deeper layers. Obviously, deep root system with lateral extension should be encouraged to maximize the use of nutrients and water resources in the soil. This is particularly desirable for annual plants. Perennial plants have a grater effective rooting depth and need only to develop new roots to fully utilize the available water resources.

Root development is generally governed by genetical and environmental factors. When environmental factors are favourable, the inherent genetical characteristics have the full expression. Environmental conditions change frequently and cause variations in root development. A given plant may exhibit variations in rooting characteristics under different environmental conditions.