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Transport in Plants 

Plant physiology (Physis = nature of life; logos = study) is the branch of botany which deals with the study of life activities of plants. It includes the functional aspects of life processes both at cellular as well as sub-cellular level.

Water is mainly absorbed by the roots of the plants from the soil, then it moves upward to different parts and is lost from the aerial parts, especially through the leaves. Before taking up the absorption and movement of water in plants, it is worthwhile to understand the phenomenon of imbibitions, diffusion and osmosis involved in the water uptake and its movement in the plants. 

Pathway of water movement in root:

Water moves through three pathways in the root of plants:-

 (i) Apoplast pathway: The apoplastic movement of water occurs exclusively through the cell wall without crossing any membrane.

                                           

(ii) Symplast pathway: The symplastic movement of water occurs from cell to cell through the plasmodesmata.

                                           

(iii) Transmembrane pathway: Water after passing through cortex is blocked by casparian strips present on endodermis. The casparian strips are formed due to deposition of wax like substance, suberin. In this pathway, water crosses at least two membranes from each cell in its path. These two plasma membranes are found on entering and exiting of water. Here, water may also enter through tonoplast surrounding the vacuole i.e., also called as vacuolar pathway

Mechanism of water absorption:

Two distinct mechanisms which are independently operate in the absorption of water in plants. These mechanisms are:

(i) Active absorption       (ii) Passive absorption

Renner coined the term active and passive water absorption. 

(i) Active absorption: Active absorption takes place by the activity of root itself, particularly root hairs. It utilizes metabolic energy. There are two theories of active absorption:

(a) Osmotic theory: It was proposed by Atkins (1916) and Priestly (1922). It is purely a physical process, which does not directly required expenditure of energy.

(b) Non-osmotic theory: It was proposed by Thimann (1951) and Kramer (1959). It has been observed that absorption of water still occurs, if the concentration of cell sap in the root hair is lower than that of the soil water, or water is absorbed against concentration gradient (i.e., from higher DPD to lower DPD). Such type of water absorption occurs on the expense of energy obtained from respiration. The exact mechanism of utilization of energy is not well understood. It may be used directly or indirectly.

(ii) Passive absorption: It is the most common and rapid method of water absorption. It account for about 98% of the total water uptake by plant.

According to this theory, the forces responsible for absorption of water originate not in the cell of roots but in the cells of transpiring shoots. In other words in this type of absorption of water, the roots remain passive.

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Due to transpiration, the DPD of mesophyll cells in the leaves increases which causes absorption of water by these cells from the xylem vessels of leaves. As the water column is continuous from leaves to roots, this deficit is transmitted to the xylem elements of roots and finally to root hairs through pericycle, endodermis and cortex. In this way water is continuously absorbed due to transpiration pull created in the leaves. This type of water transport occurs mainly through the apoplast in cortex but through the symplast in endodermis and pericycle.

The path of water from soil up to secondary xylem is :

Soil --> Root hair cell wall --> Cortex --> Endodermis --> Pericycle --> Protoxylem --> Metaxylem

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