Plant and Soil Sciences eLibrary
L Agricultural Crop Water Use. The basic soil, water, and plant relationships are important to agri- cultural producers, but especially to irrigation users that. These energy relations are governed by the said equations, understanding of it is very important. . The figure shows the pathway of soil water into root system. Different forms of Soil Moisture and factors responsible for soil water retention are discussed.
As water molecules diffuse into the cells, the concentration of it increases so also the volume. This increase in the concentration of water creates a pressure called Turgour pressure build up within the cell; cell volume increases continuously till the time when the elastic wall is stretched to the maximum.
At certain point the wall because of its elastic property, starts exerting counter pressure called Wall pressure. When wall pressure is equal to Turgour pressure the movement of water in and out of the cell becomes equal and this state of the cell is referred to as Turgid and the phenomenon is called Turgidity. Development of turgour pressure gives mechanical strength for the plant to stand erect. It also acts as the motive force for many functions like movement of stomata, nictinastic movement of leaves of Mimosa pudica, movement of bulliform cells of grass leaves etc.
Cellular metabolism is so regulated, in response to certain stimuli whether it is external or from withincells develop DPD gradient, as a result, water moves into such cells and brings about turgidity which results in the movement.
Hydrophilic substances like polysaccharides, proteins etc. Water molecules in turn bind to the charged surfaces. As a consequences the imbibant swells in volume; such a phenomenon is called imbibition and the pressure generated due to imbibition i. During this process some amount of energy is lost and it is called imbibitional energy.
In many cases the imbibition force developed due to the imbibition of water is very high ranges from to bars. The same can be used for breaking big boulders in queries. Even today this method is in practice. Water in the soil is mostly and abundantly, under normal conditions, is available in the form of Capillary water.
In the soil the space in between soil particle forms a network of spaces, which normally is filled with water. The water that is present in such spaces is called capillary water. Structures involved in Absorption: The root terminal region is made up various structures such as; from the tip towards base, apical meristem, zone of elongation, root hair zone and zone of maturation.
The root hair zone is studded with root hairs; they are the extensions of epidermal cells in the form of tubular structures. Most of the water is absorbed by the plants is through root hair zone. The figure shows the pathway of soil water into root system. Apo plastic movement of water, that is the water moves through the space found in the cell wall.
This movement of water is called Ascent of Sap or Translocation of Water. Structures involved in Ascent of Sap: Various experiments like girdling, staining and plugging, indicate that the xylem tissue is mainly responsible for the movement of water. As xylem consists of tracheae otherwise called vessels, they form a system of fine channels running from roots to all other regions of the plant body and form a beautifully branched supply system, which is almost similar to that of arteries in animals.
Rate of direction of transportation: The rate with which the water is transported along the length of the stem varies from plant of plant. External conditions also play a significant role in controlling the rate of ascent of sap. This is quite a rapid process. Generally most of the water is translocated upwards i.
These theories are mostly based on the assumption that living cells play a vital role in pushing or pumping the water upwards. WestermaierGodlewskiJanse and others thought that the xylem vessels through which water is transported would just act as vessels through which water is transported would just act as reservoir.
In this method both living and nonliving xylem forms an integrated system and act co-ordinatingly in the transport of water upwards.
Unfortunately these theories had little experimental evidence to substantiate their claims. His theory was based on the assumption that living cells all-round the xylem tissues are in a kind of rhythmic contraction and expansion, similar to that of heart in animals.
The oscillations found on the graph indicated the pulsation activity of the cells. A little later Molish provided the plant with some heart stimulant and demonstrated an increased pulsation activity.
On the contrary anesthetics brought down the pulsation rhythm. Recent investigations however, using respiratory inhibitors like DNP, KCN, showed that the synthesis of ATP is essential for the rapid movement of water in the xylem parenchyma and other islets of living cells intermixed with the longitudinal array of trachieds and tracheae.
These strongly support the view that vital activity, in the sense, the energy is required for the ascent of sap. Absorption of water by roots has been mainly a passive process. However, the involvement of an active process is not ruled out totally.
On a rainy day, when the atmospheric humidity is at its maximum, and transpiration is at its minimum, root system absorbs excess of water than it can normally absorb. As a result of it hydrostatic pressure is built up within the roots, and this is called root pressure. This is believed to act as the motive force to force the water into the xylem columns upwards. Under the above said environmental conditions, water is forced out of the water-stomata as guttated water.
Hence root pressure has been considered as an important phenomenon in ascent of sap. However, it has been noted that, some of the tallest trees found on this planet do not show any root pressure. Thus this theory fails to explain the transportation of water especially in tall trees.
Passive or Physical Force Theories: Physical forces like capillary force, collision force, atmospheric pressure, imbibitions, diffusion pressure, are found to operate in plants in one way or the other. Along with the development of science of plant physiology, people from time to time have come out with various theories involving one or to time have come out with various theories involving one or the other physical force as an explanation for ascent of sap.
When water escapes by transpiration from the surface of the leaves, it is believed that vacuum will be created within the plant body. As the root system is submerged in soil water, with the atmospheric action on the soil water, in order to fill up the vacuum created in the xylem vessels, water just enters passively; thus the water is translocated upwards. Unfortunately plants are not closed systems but they exhibit openness, for, the gases can diffuse into and out of the plant system with ease and facility.
Added to this, atmospheric pressure can support and facility. Added to this, atmospheric pressure can support the water to be lifted only to a height of 34 feet; but there are plants which are taller than this and still there is transport of water.
Hence it can be concluded that atmospheric pressure could not be the force for ascent of sap. When one end of the blotting paper or a chalk piece is dipped into ink, the ink slowly moves up. This movement through the paper is called capillary movement.
Blotting paper is made up of innumerable cellulose fibres interwoven into a close network. Between such fibres, extremely narrow spaces are found, which are connected with each other and form a fine net work of capillary canals.
If water is provided to such capillary system at one end, water is sucked in and it moves along the channels of capillary network by a force called capillary force. According to capillary network by a force theory, such capillary system exists within the plant body. Tracheids and tracheae which are found longitudinally oriented in the vasculature have lumen as empty space, roots to terminal regions of the stem as continuous capillary system. When water is absorbed by the root system, the capillary system of xylem elements take up the water by capillary force and the water is supported to move upwards slowly but steadily.
The combined effects of growing population densities, large-scale industrial logging, slash-and-burn agriculture and ranching, and other factors, have in some places depleted soils through rapid and almost total nutrient removal. The depletion of soil has affected the state of plant life and crops in agriculture in many countries.
In the middle east for example, many countries in that find it difficult to grow produce because of droughts, lack of soil, and lack of irrigation. In regions of dry climate like Sudan and the countries that make up the Sahara Desertdroughts and soil degradation is common. Cash crops such as teas, maize, and beans that require a variety of nutrients in order to grow healthy.
Soil fertility has decline in the farming regions of Africa and the use of artificial and natural fertilizers has been used to regain the nutrients of ground soil.
Since the beginning of agricultural production in the Great Plains of North America in the s, about one-half of its topsoil has disappeared. Irrigation water effects[ edit ] The quality of irrigation water is very important to maintain soil fertility and tilthand for using more soil depth by the plants. So plant roots can not penetrate deep into the soil for optimum growth in Alkali soils.
Saline water enhance the turgor pressure or osmotic pressure requirement which impedes the off take of water and nutrients by the plant roots. Top soil loss takes place in alkali soils due to erosion by rain water surface flows or drainage as they form colloids fine mud in contact with water.
Plants absorb water-soluble inorganic salts only from the soil for their growth. Soil as such does not lose fertility just by growing crops but it lose its fertility due to accumulation of unwanted and depletion of wanted inorganic salts from the soil by improper irrigation and acid rain water quantity and quality of water.
The fertility of many soils which are not suitable for plant growth can be enhanced many times gradually by providing adequate irrigation water of suitable quality and good drainage from the soil. Mollisolsshown here in dark green, are a good though not the only indicator of high soil fertility.
Weather factors Soil Factors: Any soil factor which affects root density or depth can be expected to influence the response of the crop to irrigation.
Mechanical impedance, slow water penetration and poor internal drainage, and deficient aeration frequently are responsible for sparse and shallow roots.
Soil structure, texture, and depth determine the total capacity of the soil for storing available water for plant growth.
The total available moisture capacity within the root zone and the moisture-release characteristics of the soil are both important factors determining the rate of change in soil moisture tension or stress.
Deep-rooted crops on deep soils usually show smaller responses to irrigations than shallower-rooted crops on the same soil.
The rate at which water can move to the absorbing root surface may play an important part in water-soil-plant relations. Several different aspects of plant growth-such as elongation of plant organs, increase in fresh or dry weight, and vegetative versus reproductive development are easily recognized.
These processes are resultants of intricate combinations of many physiological processes which are probably not all equally affected by increasing soil moisture stress and an accompanying change in the internal balance of cells and tissues. Thus, it is not surprising that various measurable aspects of growth do not respond in the same manner to moisture stress. Weather conditions particularly light and temperature may influence the growth characteristics of the shoot and root as to affect soil moisture-growth relations.
The length of the crop season before fall rains or frost may at least partially determine whether harvestable yields will be affected by imposing different soil moisture stress levels during the growing period. Meteorological factors like light, temperature, humidity and wind control the rate of water loss by transpiration from plant leaves and evaporation from the soil surface. Plant growth is probably dependent upon plant turgor pressure, whose relation to soil moisture stress for different rates of transpiration needs to be explored.