A) forces created in the cells of the root
B) increased respiratory activity in root cells
C) tension on the cell sap due to transpiration
D) Osmotic force in the shoot system
Correct Answer: C
Solution :
In rapidly traspiring plants the force for \[{{H}_{2}}O\]absorption develops in shoot or aerial parts. It is 96% of the total water absorption. The roots play a passive role. In this type the cells of the root do not play any acdve part. So, in passive water absorption water is just pulled through the roots and not by the roots (as in active water absorption). The passive water absorption is most rapid type. Rapid evaporation of water from the leaves during transpiration creates a negative tension (lower water potential or high D.R D.) in water in the xylem of the leaves. This tension is transmitted to water of xylem of roots through the xylem of stem and water moves from soil solution to transpiring leaves. The rate of passive water absorption closely follows the rare of transpiration. It assumes that roots behave as osmometer in which water moves from a dilute soil across a differentially permeable membrane formed by the cortex to more concentrated solution in xylem because xylem has a higher osmotic pressure than that of soil solution. Xylem channels have the maximum D.R D. or lower water potential. The increase in D.R D. of xylem sap is brought about by the accumulation of salt in the xylem which require energy. D.E D. is 0.1 to 0.3 bars of soil solution while of cell sap of root hair is generally 2 bars and may be as high as 3 to 8 sometimes. So water travels from soil to xylem along the gradient of D.R D. (S.R) or water potential.
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