Relative Importance of Active and Passive Absorption

Many workers opined that active absorption is the main mechanism of water absorption and gave

very little importance to passive absorption.

However, according to Kramer (1969), the active absorption of water is of negligible importance in the water economy of plants. He stated that the root pressure and the related phenomena involved in the active absorption of water are mere consequences of salt accumulation in the xylem of different kinds of roots.

Following are the reasons for treating the active absorption as unimportant :

1. Root pressure is not observed in tall plants like conifers and other gymnosperms.

2. Root pressure is not observed in fast transpiring plants.

3. Amount of water exuded by the cut end of stumps due to root pressure is not equal to that lost by transpiration.

4. The plant as a whole absorbs more water than an excised system.

5. In submerged plants, water is absorbed from the general surface, through roots are present.

It appears, therefore, that the passive absorption accounts for most of the water absorption, and if active absorption exists, it co-operates with passive absorption or has no importance.



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