ABSORPTION OF WATER
Unlike aquatic plants, terrestrial plants have to absorb water form the soil all the time to maintain turgidity, metabolic activities and growth of the plant. It is essential to understand the structure of soil, its water content, and factors and also the structures involved in absorption of water.
Soil is made up of fine rock particulate of various sizes derived from the weathered igneous and sedimentary rocks. Environmental factors like, heat, wind, rain, cold, river streams ad oceanic waves act upon the rock particles, which break them down to smaller particles which on accumulation in a shallow or flat surfaces, constitute the soil. The process of soil formation on this planet is a continuous process and it is taking place for the past 4.0 billion years and will continue as long as this planet exists. The same process also operated on other planets.
Based on the size, structures of soil particles and the composition of organic and inorganic components, soils have been classified into rocky, coarse, sandy clay and loamy soils. Rock particles of big sizes do not hold any water between them and any such soil consisting of rock particles which do not hold water in between them is not good for the development of root system. Even the sandy soil of such small sized rocks is good for aeration but not for water retention. On the other hand, clay soils have colloidal particles that can hold water but very poor in aeration. However, the loam soil is good, because it has the mixture of clay, sand and decomposed organic material called humus. This soil provides good aeration and proper capillary spaces to hold water. Thus this soil is considered to be the best soil for the luxuriant growth of the root system.
Rain water is the main source of water for most of the land plants. When the rain water falls on the soil, some water percolates into and moves into inter spaces found between rock particulates.
At the same time a lot of water moves all along the slope of the landscape; this water is often called runway water, which is of no use to the root system. On the other hand, the rain water that enters into the soil moves downwards. On its way, it fills up all the capillary spaces and still moves downwards till it reaches the water table. Such water is called gravitational water. Again this water is not useful to the root system. Each particle holds a thin film of water around its surface, it is called hygroscopic water. This water is also not available for roots, because this water is tenaciously holding on to the rock surface. In spite of it, the water that is found in the capillary spaces, called capillary water, it is this water that is available to the roots. So the capillary water is the main source of useful water in the soil for the plants. Moreover, capillary system provides a network of spaces for aeration as well as movement of water from one region to another region within the soil.
The amount of water found within the soil as capillary water that is available for the root system is often referred to as useful water content of the soil. Hence the capacity of soil to hold maximum amount of utilizable or useful water is known as ‘Field capacity of the soil’, this again depends upon the nature of soil. Sandy and rocky soils are poor soils in terms of water holding potential. While clay soils have great water retention capacity but they have the worst aeration. However loam soil is the best for it retains good amount of water and also it has good aeration. However, the field capacity of the soil can be determined by finding out the difference between the weight of completely wet soil and that of the dry soil of a known quantity.
As roots with their numerous branches and millions of root hairs deplete water from capillary spaces, water from the other regions move into the depleted spaces; sometimes the water moves upwards from the water table and fills up the capillary spaces, but the refilling or replenishment process takes its own time. Thus plants experience deficiency of water for a short duration time, which is referred to a temporary wilting point. In some cases, the depleted water is not replaced for a long time, under such conditions, plants die, and such a state is called permanent wilting point.
Plants growing in aquatic habitat do not require any special structures for absorbing water, but plants growing in soil or growing as epiphytes, posses’ special structures for absorbing water. For example, epiphytic orchids contain velamen root specially adopted to absorb water directly from the atmosphere. But the terrestrial plants produce roots that grow into the soil.
Such roots not only fix the plant firmly in the soil but also help in absorbing water and mineral salts from the soil. Besides the above said functions, in different plants, roots also play other functions such as storage, climbing, pneumatophores, etc.
Roots, irrespective of their origin or kind, grow, branch and spread in the soil to a great extent. Some plants produce roots which grow deeper into the soil and reach the water table in some cases.
Such roots are called deep feeders. Most of them are tap root systems. But some plants produce branches which spread outwards and do not grow very deep and such roots are mostly adventitious or fibrous roots, they are called surface feeders. However, some fibrous roots also reach great depths.
The development of root system in plants is very extensive in the sense the total length of all the branches put together is amazingly great. For example, a four month old corn plant possess an adventitious root system, which if all its branches are put together it extends to about 45 kms. Trees with their perennial habitats posses’ deep feeder root system and their total length of the entire root system will be never less then 100 km. Added to this feature, roots continue to grow every minute and the extent of growth of root. System is in the range of 4-8 kilometers per day. The ramification of the root system and rapid growth of thousands of root tips is very essential, for the root tip are eternally seeking new areas for the absorption of water and minerals. In spite of its extensive branches and length, the actual region used for absorption of water is restricted to the terminal region of the roots.
The terminal region of every root and its branches consists of a region of meristem, region of elongation and region of differentiation. The region meristem is made up of actively dividing cells which by continuous cell divisions produce derivatives. The region of elongation is made up cells derived from the region of meristems and they are in the process of rapid elongation. Next to this, the region of differentiation or the region of root hairs is found. In this region, cells are in the process of differentiation, where some cells are developing into vascular tissues, certain cells specialize into cortical or medullary cells. But a large number of epidermal cells in this region undergo transformation into long tubular structures called root hairs. Beyond the root hair zone, i.e. towards the base of the root, the root exhibits dead root hairs, cortical cells with suberization and most of the cells are fully mature and some of them are even dead. Among the above mentioned regions root hair zone is mainly responsible for the absorption of water.
They are the tubular extensions of epidermal cells found in the region of differentiation. Each of these root hairs is 0.75 – 1.0 cm in length and 10 μ in diameter.
The total number of root hairs present in a four month old corn plant is about 14 billion. Taking the average length ad breadth of a single root hair, if the total surface area of all the root hairs is together computed and it turns out that this surface area absorbs about 128-150 liters of water per day, which is surprisingly 27 times the total amount of water lost from the transpiring surface are. Furthermore, the daily growth and extent of the growth of root hairs that develop, is more than adequate for the amount of water available in the soil. By chance, if there is no replenishment of water that is lost from the soil, the entire volume of soil water can be lost to the atmosphere in about 24-48 hours. This is done through the combined action of total water absorbing surface i.e. roots and root hairs, and transpiring surface area i.e. stomata. If the lost water is not replenished in the soil, plants experience water stress, and if this continues, plants, die, when they reach beyond permanent wilting point. Though more than 90% of the water is observed by the root hairs zone, the other regions also absorb the water but the amount of water absorbed by these regions is not much.
Living root system is very essential for the survival of plants, for it is involved in the absorption of water and minerals which is a must. The factors that regulate the health of the roots also control the process of absorption of water. But the most important of them are soil temperature, soil water, soil aeration and the root structure.
The efficiency of absorption of water depends upon the extent of root branching and the total surface area of the absorptive structures of root hairs. Though most of the water is absorbed by the root hair zone, other regions also contribute in absorption to some extent. Still the efficiency of absorption varies from one root system to the other.
In most of the cases, the soil temperature is little lower than that found in aerial regions. As temperature influences the viscosity mobility of water and also the metabolic activity of the plant cells, it affects the ability and the efficiency of absorption of water. The rate of absorption of water is lower, if the temperature is lowered this is because the mobility of water is decreases and the viscosity of liquid water increases. Thus low temperature resists the free movement of water which in turn affects the rate of absorption. However very high temperatures have adverse effect on the root’s efficiency.
Soil water is not pure water but it consists of a large number of minerals ad organic compounds in dissolved state, so it is a solution. The osmotic concentration of soil solution under full field capacity is always many fold lower the osmotic concentration of cell sap. This provides a kind of osmotic gradient between the soil solution and root cells. When conditions are favorable for rapid transpiration and shortage of water in the soil, plants
exhibit wilting features. If the water is not replaced within a particular period of time, plants experience permanent wilting stress and they may die. In fact, under normal conditions, the rate of absorption of water shows diurnal rhythm i.e. higher rate of absorption during day and low rate at nights. Thus soil water and its constituents determine not only the rate of absorption but also the amount of water absorbed.
Soil being made up of fine rock particles of different dimensions, clay particles and other components, possess plenty of lung space within which air is present. If the soil is water logged, most of the air is expelled from the capillary spaces and roots experience anaerobic conditions and their metabolism suffers. This affects the growth of the roots. Sometimes, excess of clay particles also clog the spaces and soil is rendered unsuitable for the normal growth of the root. Some plants are adapted to grow in water logged areas. In such situations the rate of absorption of water and mineral salts is greatly affected. Even greater accumulation of CO2 within the soil causes change in the pH of the soil solution. Such changes will be detrimental to the root system. But normally the soil CO2 is replaced by the atmospheric air.
Root is a highly organized multicultural structure containing a variety of cells, some are living and some are dead. Cell wall materials like cellulose, hemicellulose, lignin, etc., form the bulk of inert organic materials of the root cells. In between the micro fibrils and macro fibril part of the cell wall plenty of fine free spaces are found through which water and mineral salts can easily diffuse in and diffuse out. Similarly the intercellular spaces constitute 6-10% of the total volume of the root system. These spaces also act as spaces for free diffusion of water and other components. Such spaces which are free for diffusion of water and salts are called Apparent Free Spaces (AFS). More over, the network of such spaces provides a continuum from the outer surface of the roots up to the central vascular cylinder. Depending upon the water potential or osmotic gradient, if it is favorable, water from the soil easily diffuses into roots through AFS of cell wall and intercellular spaces. Such movement is called Apoplast movement and it is always very rapid.
Once the water diffuses into cell walls and fills up the inter cellular spaces then water also diffuses into protoplasm, and thus water can move in the protoplasm from cell to cell through protoplasmic strands provided the cells are living. The rate of movement through the protoplasm is extremely slow. Nevertheless protoplasm to protoplasm movement is referred to as symplastic movement. Rarely one finds the movement of water through the cellular vacuoles. The most rapid form of movement is Apoplastic and partially symplastic.
If conditions are favorable for absorption of water, one can visualize the pathway of movement of water from the soil solution into the vascular elements. First water enters into the AFS of root hair cell wall and then enters into the protoplasm. Bulk of the water absorbed by the root system enters through billions of root hairs which are continuously being produced by the growing roots. Once water finds its way into such epidermal cells, it moves rapidly along the AFS spaces found in the cortical cells towards central vascular cylinder. But endodermal cells with their radial casparian thickenings are known to resist the free movement of water along the gradient. However, some of the endodermal cells located opposite to protoxylem elements are found to be free from casparian thickening and they act as free passage cells. Thus water ultimately finds its way into xylem elements.
Aquaporin structural features
Various theories have been proposed to explain the mechanism of the absorption of water. Though a large number of plant physiologists are in the opinion that the process of absorption of water is by a passive mechanism, there are some, who still believe that active process also operates along with passive process, where passive process dominates over the active process.
Any physiological process that does not utilize metabolic energy is called passive process. The basic criteria for explaining the process of absorption of water in a passive mechanism, stems from the fact that there always exist a steep water potential gradient between the soil water and the cells including xylem elements found in the root system.
This gradient is always maintained because rapidly transpiring surfaces in the aerial regions develop a powerful transpiration pull which physically sucks the water upwards from the central vascular xylem elements. Thus xylem elements in the root always experience a negative pressure, which is transmitted across the cortical cells towards the peripheral root hairs, which actually act as the absorptive surfaces. As a consequence of negative pressure operating in the central vascular xylem elements, water from the soil just diffuses rapidly along the gradient into the roots. As the transpiration pull is continuously operating the negative pressure is constantly maintained at the xylem elements. This greatly felicitates the rapid absorption of water by the root system. So the forces that are responsible for absorption of water generate not within the root system per se but in the aerial transpiring structures. More than 95% of the water absorbed by the root system is operated by passive forces like DPD gradient created by the transpiration pull.
Any process that requires the input of metabolic energy is called active process. In the absence of ATP or any other energy rich compounds such processes does not operate. One of the main reasons as to why some physiologists believe that active process is also involved in the absorption of water is because under favorable conditions, the roots of certain plants develop hydrostatic pressure within them. Such pressure is called root pressure. For example, plants like potato, tomato ad colochasia develop root pressure when there is plant of water is the soil probably due to heavy rains and minimum transpiration, due to high RH in the atmosphere. Under such conditions roots absorb more water than it can hold. As a result of excess of intake of water a hydrostatic pressure builds up within the root. The development of root pressure in plants has been demonstrated in various species; where the bleeding of sap and even guttation has been attributed to root pressure. If respiratory poisons are provided root pressure does not develop. This is another evidence to show and that active absorption is responsible for the development of root pressure. It is very important to remember that active absorption need not lead to root pressure always. However, the mechanism of active absorption has been explained by two possible mechanisms.
1. Osmotically Active Mechanism: Under favorable conditions, as mentioned above, the special meristematic cells of the roots absorb and accumulate mineral nutrients from the soil. The same is actively transported across the cells towards xylem elements, into which nutrients are finally loaded. Absorption of minerals, transport and loading xylem is active processes, which require sufficient amount of ATP energy. As the xylem cells are loaded with more and more of minerals, a DPD gradient is created between the soil solution and xylem sap. This acts as the motive force for the movement of water into roots by passive osmosis. In this process, uptake of water does not require energy, but the accumulation of mineral salts which generates the force for the uptake of water, requires metabolic energy. Hence, this process is called osmotically active process. It is also believed that this process is mainly responsible for the development of root pressure. Interestingly, there is a good correlation between the conditions at which the root pressure develops and conditions at which nutrient uptake increases. It is very well known that roots absorb more ions and with a greater rapidity when a nutrient solution is very dilute. As respiratory poisons like DNP, inhibit ATP synthesis, they also inhibits active uptake of ions, thus inhibit osmotically active absorption and also the development of root pressure mechanism.
Protagonists of this theory believe that the root pressure develops due to the uptake of water against its own concentration gradient. It is speculated that water is actually pumped into root cells by certain energy dependent pumps located in plasma membranes. Experimentally this hypothesis cannot be tested, because use of respiratory poisons also affects most of the energy dependent metabolic process including the active uptake of mineral ions, which is mainly responsible for the uptake of water by osmotically active process. Hence this theory has remained as an untestable theory.