Long distance transport of water in plants happens in bulk. This is called translocation.
Absorption of water from the soil happens through root hairs. Root hairs are extensions of the root epidermis and have thin walls. Water enters the root hairs because of osmosis. Presence of numerous root hairs increases the surface area and hence enhances the absorption of water.
The further movement of water takes place by two distinct pathways, viz. apoplast pathway and symplast pathway.
Apoplast Pathway: The free diffusional space outside the plasma membrane is the apoplast. This is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant. The apoplast is formed by the continuum of cell walls of adjacent cells as well as the extracellular spaces. The apoplast pathway facilitates the transport of water and solutes across a tissue or organ.
Symplast Pathway: The inner side of the plasma membrane is the symplast. The symplast pathway is made continuous because of the presence of plasmodesmata across the cell walls of adjacent cells. Small molecules; such as sugars, amino acids and ions; flow through symplast pathway. Larger molecules are also transported through this route with the help of actin structures.
The symplast pathway allows direct cytoplasm to cytoplasm flow of water and other nutrients along concentration gradient.
Root Pressure: When various ions from the soil are actively transported into the vascular tissues of the roots, water also follows. This increases the pressure inside the xylem. This positive pressure is called root pressure. The root pressure can push water up to small heights in the stem.
Guttation: In some plants, under the conditions of low evaporation, water comes out from the tips of leaves. Such loss of water in its liquid phase is called guttation. Guttation takes place in smaller plants only.
Limitations of Root Pressure: Root pressure can only provide a modest push. Hence root pressure does not play a major role in water movement in tall plants. Root pressure contributes towards reestablishment of continuous chains of water molecules in the xylem; which often break under enormous tensions created by transpiration pull.
Transpiration Pull: The evaporative loss of water by plants is called transpiration. Transpiration mainly occurs through stomata. Stomata are usually open during daytime and remain close during the night.
Opening and Closing of Stomata: A change in the turgidity of guard cells results in closing or opening of stomata. The inner wall of the guard cell; towards the stomatal aperture; is thick and elastic. An increase in turgidity results in the thin outer walls to bulge out. This forces the inner wall into a crescent shape and results in opening of stoma. The orientation of the microfibrils in the cell walls of the guard cells also helps in opening of stomata. These microbifibrils are radially oriented and thus make it easy for the stoma to open up. A loss in turgidity of the guard cells, leads to resumption of shape of the elastic inner wall of the guard cell and the stoma closes.
Temperature, light, humidity, wind speed, number and distribution of stomata, number of stomatal aperture with guard cells open, water status of the plant, canopy structure, etc.
Transpiration creates a suction force inside the xylem. This suction force is called transpiration pull. This is powerful enough to pull the water column from beneath. Adhesion, cohesion and surface tension are the important physical properties of water which further help in the upward movement of water through xylem.
Cohesion: Mutual attraction between water molecules is called cohesion.
Adhesion: Attraction of water molecules to polar surfaces is called adhesion.
Surface Tension: Any liquid has a tendency to occupy the least possible surface area. This property is called surface tension.
The above mentioned properties impart high tensile strength to water. The high tensile strength imparts an ability to resist a pulling force and high capillarity. The ability to rise in tubes is called capillarity. The thin tubes of xylem work like capillary tubes.
Adhesion-cohesion and capillarity result in formation of a continuous column of water molecules inside the xylem. This water column is pulled up because of transpiration pull. Thus, the adhesion-cohesion-transpiration pull theory explains the rise of water in very tall trees.
Translocation of Mineral Ions: Minerals ions reach xylem through active or passive uptake, or a combination of both. Their further movement through the xylem is alongwith the transpiration stream. The growing regions of the plant are the main sinks for mineral elements. Mineral ions are frequently mobilized; especially from older, senescing parts. Phosphorus, sulphur, nitrogen and potassium are the most readily mobilized elements. However, elements which are structural components are not mobilized, e.g. calcium.
Some elements are also transported in organic forms in plants.
Food is transported through phloem; from source to sink. Leaf usually plays the role of source and storage organs are the sinks. But there can be role reversal when new leaves emerge during early spring. Thus, movement of substances through phloem is bi-directional.
The phloem sap is mainly composed of water and sucrose, but other sugars, hormones and amino acids may also be present.
When glucose is prepared at the source, it is converted to sucrose.
The sucrose moves into the companion cells and then into the living phloem sieve tube cells; through active transport. This process of loading at the source produces a hypertonic condition in the phloem.
Water; from the adjacent xylem; moves into the phloem, by osmosis. This results in an increase of osmotic pressure. It forces the phloem sap to areas of lower pressure, i.e. towards the sink. The osmotic pressure must be reduced at the sink.
Active transport moves the sucrose out of the phloem sap into the cells in the sink. Once the sugar is removed, the osmotic pressure decreases and water moves out of the phloem.
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