LONG DISTANCE TRANSPORT OF WATER_PART 01

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LONG DISTANCE TRANSPORT OF WATER: The movement of water through root layers is ultimately symplastic in the endodermis. This is the only way water & other solutes can enter the vascular cylinder. Once inside the xylem, water is again free to move between cells as well as through them.Some plants have additional structures associated with them that help in water absorption. A mycorrhiza is a symbiotic association of a fungus with root system. The fungal filaments form a net work around young root or penetrate root cells. The hyphae have very large surface area that absorb mineral ions & water from the soil from much larger volume of soil. The fungus provides minerals & water to the roots; in turn the roots provide sugars and Nitrogen containing compounds to Mycorrhiza. Some plants have obligate association with mycorrhizae. E.g. Pinus seeds cannot germinate & establish without the presence of mycorrhizae. Water Movement up a Plant:A number of theories have been put forward from time to time to explain how the water is transported to various parts of the plant. Root pressure theory: If a stem is cut near its base, or incisions are made into a plant, xylem sap is seen to flow out through them. The phenomenon is known as exudation or bleeding. Priestly was one of the first to explain the process of upward flow of water in exudation is due to the hydrostatic pressure which is developed in the root system. The hydrostatic pressure developed due to the accumulation of water absorbed by the roots is called root pressure. If a vertical tube is attached to a bleeding stump, a column of sap will rise in it. It is most convenient to measure the pressure by means of a mercury manometer attached to the stump. Stephan Hales coined the term root pressure, described this method & recorded pressures of over one atmosphere. Effects of root presence is observable as night & early morning when evaporation is low and excess water collects in the form of droplets around special openings of veins near the tip of grass blades, & leaves of many herbaceous plants. Such water loss in liquid phase is known as guttation. In Saxifera; guttation takes place rapidly during flowering. Colacasia loses about 10 to100 ml of water daily during guttation.