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  Plant-Water Relations, Biology tutorial

Plant-Water Relations:

Water being regarded as universal solvent, takes up 75% of our planet in form of oceans. Added to this water is also found in atmosphere in form of Hydrospheric mantle.  Evaporation of water from surface of ocean, creation clouds and raining, is the natural cycle developed during course of Evolution of planet. Approx 3.8 billion years ago, life took its origin as the speck of protoplasm in churning oceanic water that was not salty as it is today.  The birth of life has selected H2O as medium of biochemical activities.  Therefore water has turn out to be mother of life or Solvent of Life.

Cells of all organisms are composed of 90% or more of water.  And all other parts are either dissolved or suspended in water to create protoplasm that is frequently referred to as physical basis of life. Water is main part of living cells and comprises more than 90% of protoplasm by volume and weight.

It serves as medium for all biochemical reaction which occurs in cell, and also serves as medium of transportation from one region to another region.  Water is the extraordinary compounded composed of Hydrogen and oxygen (2:1) and it has high heat of vaporization, specific heat, heat of fusion and expansion (colligative properties).  Water due to its bipolar nature serves as universal solvent for it dissolves more substances than any other solvent.  Electrolytes and non-electrolytes such as sugars, and proteins suspend very well.  Even few hydrophobic lipid molecules illustrate some solubility in water.

Water serves as good buffer against changes in Hydrogen ion concentration (pH).  This is due to its ionization property.  Definite xerophytes utilize water as buffer system against high temperature.

Smallest water contents in living parts of plants take place generally in dormant structures, like mature seeds and spores. Great bulk of water in any plant comprises the unit system. This water is not in static condition. Rather it is part of hydrodynamic system, which in terrestrial plants engages absorption of water from soil, its translocation all through plant, and its loss to environment, mainly in process called as transpiration.

Cellular water relations:

The distinctive mature, vacuolate plant cell comprises a small osmotic system, and this thought is essential to any concept of cellular water dynamics. Though cell walls of most living plant cells are fairly freely permeable to water and solutes, cytoplasmic layer which lines cell wall is more permeable to few substances than to others.

If the plant cell in flaccid state-one in which cell sap applies no pressure against surrounding cytoplasm and cell wall-is immersed in pure water, inward osmosis of water in cell sap ensues. This gain of water yields in exertion of the turgor pressure against protoplasm that in turn is transmitted to cell wall. This pressure also exists all through the mass of solution inside cell. If cell wall is elastic, few expansions in volume of cell take place as the result of this pressure, though in several types of cells this is comparatively small.

If the turgid or partly turgid plant cell is immersed in the solution with the greater osmotic pressure than cell sap, a slow shrinkage in volume of cell ensues; Amount of shrinkage depends on type of cell and its early degree of turgidity. When lower limit of cell wall elasticity is achieved and there is constant loss of water from cell sap, protoplasmic layer starts to move away from inner surface of cell wall. Retreat of protoplasm from cell wall frequently carries on until it has shrunk toward center of cell, space between protoplasm and cell wall becoming occupied by bathing solution. This event is known as plasmolysis.

In some types of plant cells movement of water happens mainly by procedure of imbibition rather than osmosis. Swelling of dry seeds when immersed in water is familiar example of the process.

Stomatal mechanism:

Different gases diffuse in and out of physiologically active plants. Gases of greatest physiological importance are oxygen, carbon dioxide, and water vapor. Great bulk of gaseous exchanges between the plant and environment happens through minute pores in epidermis which are known as stomates. Though stomates occur on several aerial parts of plants, they are most characteristic of, and found in greatest abundance in, leaves.

Transpiration process:

The term transpiration is utilized to designate procedure whereby water vapor is lost from plants. Though essentially the evaporation procedure, transpiration is complicated by other physical and physiological situations prevailing in plant. While loss of water vapor can take place from any part of plant that is exposed to atmosphere, great bulk of all transpiration happens from leaves. There are two types of foliar transpiration: (i) stomatal transpiration, in which water vapor loss happens through stomates, and (ii) cuticular transpiration, that happens directly from outside surface of epidermal walls by cuticle. In most species 90% or more of all foliar transpiration is of stomatal type.

 Transpiration is the essential effect of relation of water to anatomy of plant, and particularly to anatomy of leaves. Terrestrial green plants are reliant on atmospheric carbon dioxide for their existence.

In terrestrial vascular plants principal carbon dioxide-absorbing surfaces are moist mesophyll cells walls that bound intercellular spaces in leaves. Entrance of carbon dioxide in these spaces happens generally by diffusion via open stomates. When stomates are open, outward diffusion of water vapor inescapably happens, and such stomatal transpiration accounts for the majority of water vapor loss from plants.

Water translocation:

In terrestrial rooted plants almost all of the water that enters plant is absorbed from soil by roots. Water therefore absorbed is translocated to every part of plant. Mechanism of ascent of sap in plants, particularly tall trees, was one of the initial procedures to excite interest of plant physiologists.

Upward movement of water in plants takes place in xylem, which, in larger roots, trunks, and branches of trees and shrubs, is identical with wood. In trunks or larger branches of most kinds of trees, though, sap movement is limited to few of the outermost annual layers of wood.

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