WATER ABSORBING ORGANS OF PLANTS
Water is absorbed in plant through roots but under certain conditions, water is absorbed by shoot also. Some plants also absorb water through their leaves. The leaves of coast redwood (Sequoia sempervirens) absorb water from the fog in which they are frequently bathed along the California coast. Certain epiphytes, for example, orchids, absorb moisture from the atmosphere by their special modified hanging roots. The maximum absorption occurs in the region of root hairs where xylem is not fully matured and the epiblema and endodermis are permeable.
Root hairs are the main water-absorbing organ of plants. They develop in the region of maturation and their number varies from plant to plant. A root hair is the unicellular tubular projection or prolongation of outer wall of epiblema. Cell wall of root hair is composed of two distinct layers. Outer layer is composed of pectin and the inner layer is made up of cellulose. Both the layers are hydrophilic in nature. Cell wall is permeable to both solute and solvent. The cell wall surrounds plasma membrane and thin layer of cytoplasm. Plasma membrane along with cytoplasm acts as selectively permeable membrane. The cytoplasm encloses a central vacuole that contains cell sap. Nucleus of the cell is generally present at the tip.
APOPLAST AND SYMPLAST
- Munch (1932), a German physiologist introduced the concept of apoplast and symplast which is helpful in understanding the pathway of water movement through the root. He suggested that the water may pass through the root cortex through two pathways. The interconnecting cell wall and intercellular spaces, including the water-filled nonliving xylem elements i.e., tracheids and vessels form one pathway, to which he called apoplast. The other pathway is the living protoplast of the cell lying within the boundary of the selectively permeable membrane. It is called symplast. Vacuoles, which are separated from the cytoplasm by another differentially permeable membrane, that is tonoplast belong to neither of these two systems.
The apoplast is discontinuous and is separated into two regions.
- The cortex and tissues external to the endodermis, and
- The tissues of the stele i.e., tracheids and vessels which lie inner to the endodermis. This discontinuity of apoplast is due to endodermis.
The Casparian stripes in the endodermal cell wall prevent the passage of water from outside to inside or back, unless it passes through the cell membrane and cytoplasm of the endodermal cells.
The symplast forms a continuous system because the protoplast of the cortex, endodermis and vascular tissue are connected from cell to cell through cytoplasmic strands, called plasmodesmata. Flow of water through cortex involves both the pathways.
MECHANISM OF WATER ABSORPTION
The uptake of water by roots is known as absorption of water. Terrestrial plant absorb water through root hairs, which are in contact with soil water. The initial step of water uptake is imbibition of water by the outer layer of cell wall which is hydrophilic in nature. Two distinct mechanisms which independently operate in the absorption of water in plant are active absorption and passive absorption.
When the water is absorbed due to the activity of root itself, particularly root hairs, it is called active absorption. It may be osmotic or non osmotic.
OSMOTIC THEORY OF ACTIVE WATER ABSORPTION
At first, water gets imbibed on the outer cell wall surface. As the cell wall is permeable, it allows both the solute and solvent to pass through it. Water enters through the cell wall and comes in contact with plasma membrane. Plasma membrane, a thin film of cytoplasm and tonoplast together act as selectively permeable.
Cell sap remains inside the vacuoles and is separated from external soil water by selectively permeable membrane. It has been experimentally found that osmotic pressure of soil water is always less than 1 atm and osmotic pressure of cell sap is usually 2-8 atm. If the osmotic pressure is more and turgor pressure is less in root hair cell, it will result in increased diffusion pressure deficit. High diffusion pressure deficit of cell sap causes endosmosis of water across the cytoplasm, plasma membrane and cell wall of root hair cell. Water enters into the root hairs as long as the diffusion pressure deficit of cell sap is greater.
Water moves from root hair cells to cortical cells along concentration gradient and finally reaches up to pericycle and endodermis. Since the xylem elements have no turgor pressure, their diffusion pressure deficit is higher than that of the surrounding cells, as a result of which water enters into the xylem. This theory involves simplest movement of water, i.e., the water enters into living protoplasm of cells and then moves into the living protoplasm of another cell.
NON OSMOTIC THEORY OF ACTIVE ABSORPTION
It has been observed that absorption of water takes place even if the concentration of cell sap in the root hair is lower than that of the soil water. Normally under such conditions, the water molecules should diffuse out from root hairs into external solution due to exosmosis. But still, water is absorbed against the concentration gradient, i.e., from higher diffusion pressure deficit to lower diffusion pressure deficit. This type of absorption takes place at the expense of energy. According to some physiologists, the energy is supplied by cellular respiration. Thus, non osmotic absorption requires metabolic energy, which comes from respiring cells of the root. Following are certain evidences which show a definite correlation between respiration and absorption:
- Respiration and rate of absorption: It has been observed that factors which affect respiration also affect the rate of absorption. For example, respiratory inhibitors like potassium cyanide also inhibit the rate of absorption.
- Wilting of plants in poorly aerated soil: Respiration of root is closely dependent on the soil aeration. A well-aerated soil supplies required quantity of oxygen to the cortex and therefore, roots respire normally. In poorly aerated or water-logged soils, the external supply of oxygen is insufficient for the root respiration. Hence, absorption of root is inhibited in such soils and plant shows signs of wilting.
- Effects of auxins: Like other metabolic activity, absorption also increases in the presence of auxins.