Short notes on the passive absorption of water

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The roots do not play any role in the absorption of water in the sense, the force or forces responsible for water absorption are generated elsewhere in the plant body. It is widely believed that transpiration has a direct bearing on the rate of water absorp­tion.

That is why in rapidly transpiring plants rate of absorption of water is very high. In a rapidly transpiring plant, a con­tinuous gradient of DPD is built up due to evaporation of water. The mesophyll cells have a higher DPD, and as a result, water in the xylem elements are attracted towards them.

This creates a tension (DPD) in the xy­lem elements which is transmitted down due to the cohesion of water molecules. The ten­sion or DPD reaching the lower tip of the xylem is carried across the root cortical cells and ultimately to the root hair cell. As a re­sult of this continuous chain of DPD, water migrates radially across the root cortex and travelling through the xylem finally reaches the leaf mesophyll cells, from where it escapes.

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Hence transpiration force or pull or suction force generated away from the roots, is responsible for the absorption of water by the roots. It would be more appropriate to use the term absorption of water through the roots rather than by the roots because roots are passive and do not influence the water uptake in any way. No event taking place in the root will influence the absorption of water (either way).

When the plants are rapidly transpiring, it has been estimated, that the DPD in root hair cells increased by -3 to -5 bars, whereas the DPD of the soil solution is less than -1 bar. It is this higher DPD that forces large quantity of water into the roots, ultimately to be lifted up to the leaves.

Evidences for passive absorption are obtained from the fact that the rate of transpiration is approximately equal to absorption of water, and any inhibi­tion of transpiration inhibits water absorption also (Kramer 1937, Lachenmeir, 1932). To sum up, it may be said that in passive absorption roots provide merely a conduit for the entry of water into the plants.

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