Factors Affecting Absorption of Water In Plants – Agriculture Notes – For W.B.C.S. Examination.
1. Available Soil Water:
Sufficient amount of water should be present in the soil in such form which can easily be absorbed by the plants. Usually the plants absorb capillary water i.e., water present in films in between soil particles. Other forms of water in the soil e.g., hygroscopic water, combined-water, gravitational water etc. are not easily available to plants. Increased amount of water in the soil beyond a certain limit results in poor aeration of the soil which retards metabolic activities of root cells like respiration and hence, the rate of water absorption is also retarded.Continue Reading Factors Affecting Absorption of Water In Plants – Agriculture Notes – For W.B.C.S. Examination.
2. Concentration of the Soil Solution:
Increased conc. of soil solution (due to the presence of more salts in the soil) results in higher osmotic pressure. If the O.P. of soil solution will become higher than the O.P. of cell sap in root cells, the water absorption particularly the osmotic absorption of water will be greatly suppressed. Therefore, absorption of water is poor in alkaline soils and marshes.
3. Soil Air:
Absorption of water is retarded in poorly aerated soils because in such soils deficiency of O1 and consequently the accumulation of CO2 will retard the metabolic activities of the roots like respiration. This also inhibits rapid growth and elongation of the roots so that they are deprived of the fresh supply of water in the soil. Water logged soils are poorly aerated and hence, are physiologically dry. They are not good for absorption of water.
4. Soil Temperature:
Increase in soil temperature up to about 30°C favours water absorption. At higher temperatures water absorption is decreased. At low temp, also water absorption decreases so much so that at about 0°C it is almost checked.
This is probably because at low temp:
(i) The viscosity of water and protoplasm is increased,
(ii) Permeability of cell membranes is decreased,
(iii) Metabolic activities of root cells are decreased, and
(iv) Growth and elongation of roots are checked.
Relative Importance of Active and Passive Absorption of Water:
There are two views regarding the relative importance of active and passive absorption of water in the water economy of plants. Many workers in the past regarded the active absorption of water to be the main mechanism of water absorption and gave very little importance to the passive absorption. But according to Kramer (1969) the active absorption of water is of negligible importance in the water economy of most or perhaps all plants.
He regards the root pressure and the related phenomena involved in the active absorption of water as mere consequences of salt accumulation in the xylem of different kinds of roots. The salt accumulation produces a difference in water potential which brings about the inward movement of water (osmotic uptake) and development of a pressure in the xylem sap (root pressure).
There are many reasons for regarding the active absorption as unimportant:
(i) The volume of exudates from the cut stump is very small in comparison to the volume of water lost in transpiration by the similar intact plants under conditions favourable for transpiration.
(ii) Intact transpiring plants can absorb water from more concentrated and drier soil solutions more easily than the similar de-topped plants.
(iii) No root pressure can be demonstrated in rapidly transpiring plants. Such plants may show even a negative root pressure (i.e., if a little water is placed over the cut stump it is absorbed by the latter).
(iv) In conifers root pressure has rarely been observed.
It is held by certain workers that though the active absorption is not important quantitatively, it occurs all the time and supplements passive absorption. Two main arguments are against this view. Firstly, during periods of rapid transpiration the salts are removed from the root xylem so that their concentration becomes very low.
Under such conditions the osmotic uptake of water cannot be expected to occur. Secondly, even if we suppose that the salts are not removed during periods of rapid transpiration, the latter reduces the water potential of the cortical cells in roots to such a low level that the osmotic entry of water from cortex to xylem is not possible.
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