Uptake of water in plant

Uptake of water

Uptake of water :

Two distinct techniques may be used to describe how water enters the root hair and the precise process of water absorption:

(a)Active uptake: Water is absorbed as a result of root activity and has no bearing on any action in the shoot.

(b)Passive uptake of water: Water absorption is controlled by the cells of transpiring shoots rather than by the root itself.

Although root water absorption is thought to be a passive, pressure-driven process, it is nevertheless reliant on respiration. The hydraulic conductivity of most roots is reduced by respiratory inhibitors (such as cyanide) and anaerobic environments (waterlogged conditions). These are some arguments in favour of active water absorption. The precise involvement of respiration and active uptake, however, is unknown.

Water intake is currently thought to be a passive process, with a few exceptions. Tension or negative pressure generated at the actively transpiring leaf surface acts as a pulling force in the xylem (Dixon and Jolly’s Cohesion-tension hypothesis).

A loss in free energy drives water movement inside the plant, and water can move through diffusion, bulk flow, or a combination of these three transport processes. Water diffuses due to ongoing thermal agitation between molecules, which tries to balance out concentration disparities. When there is a sufficient channel for bulk water movement, water moves via bulk flow in response to a pressure differential. Thus, the differential in water potential (i.e., solute potential and pressure potential) between cells, from root hairs to xylem, plays a significant role in water intake and transport.

Uptake of water 

Reference: Hopkins WG & Huner NPA. 2004. Introduction to Plant Physiology. John
Wiley & sons.

Water uptake and movement (transpiration stream) :

When compared to plants or soils, air (atmosphere) has a very low water potential, hence a water gradient forms from the soil to the air. Water will flow from roots to leaves when it travels from a high to a low water level.

Water lost through transpiration must be replaced by root system absorption of an equivalent amount of water from the soil. The soil plant-atmosphere continuum (SPAC) establishes an integrated flow of water from the soil through the plant and into the atmosphere (transpiration stream).

Soil is a complex media that consists of a solid phase made up of inorganic rock particles and organic material, a soil solution made up of dissolvent solutes, and a gas phase that is normally in equilibrium with the atmosphere. The porosity of a soil, and hence its water retention and aeration, is influenced by its structure. When a soil is newly moistened, such as by rain or irrigation, the water percolates down via the pore space until most of the air has been displaced. After that, the dirt is soaked in water. Gravity will allow water to readily drain from the huge spore space.

Capillary pores hold the water that remains after free drainage. The water in the soil is reported to be at field capacity at this time. As the water content of the soil diminishes, either from evaporation from the soil surface or because it is taken up by the roots, the air water interface will retreat into capillary gaps between the soil particle, creating tension and a negative water potential.

Tensions in the soil will attract greater bulk water toward the root when a root removes water from the soil. The extent of the root system determines the efficacy of the roots as absorbing organs. Only when the water potential of the plant sap (cell sap) is lower than the water potential of the soil can the plant absorb water from the soil.

Uptake of water

Movement of water 

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