Mechanism of water transport

Plants, unlike animals, do not possess any metabolically active pump, like the heart to carry the fluid in the vascular system. The movement of water in a passive manner occurs by pressure and by the gradient in the chemical potential. Another way of water movement that occurs in plants is called as a cohesion-tension mechanism. Here, the water movement is caused by the absorption and transportation of water bulk, driven by the negative pressure that is created by the transpiration or evaporation of water from the leaves. The forces created by hydrogen bonding are called as “cohesive” and the water movement is due to the cohesive nature of the water movement during transpiration.

The significant tension in the water columns of the plant is sustained by the hydrogen bonds. This tension is considered to be helpful in the movement of water to 100m above the soil surface. The cohesive-tension is generated by transpiration. The evaporation inside the leaves occurs from moist cell wall surfaces surrounded by the airspace network. At the interface of the air and water, menisci are formed. The apoplastic water present in the cell wall capillaries is connected with the air present in the sub-stomatal cavity. The sun’s energy used for breaking the hydrogen bonds between the molecules helps in the evaporation of water from the menisci. The surface tension in the water at menisci pulls away from the water molecules, to substitute the molecules that are lost due to the evaporation. This surface tension or force that is transmitted through the water columns into the roots will stimulate the water influx from the soil. The continuous water transport pathway is otherwise called as Soil Plant Atmosphere Continuum (SPAC) by the scientists.

The water movement in the plants is carried out by cohesive tension mechanism which is primarily suggested by Stephen Hales. The movement of a solute across the semi-permeable membrane is dependent on the water movement as per the chemical potential of water, by the process of osmosis. The water movement between the cells and plant compartments is governed mainly by the osmosis. In the transpiration deficiency, the movement of water into roots is dominated by osmotic forces. The osmotic forces are manifested as guttation and root pressure, which are usually observed in lawn grass. Guttation is the process where the water droplets are accumulated at the leaf margins when the evaporation is low. The root pressure occurs when the solutes are accumulated at higher concentrations in the root xylem than in the other tissues of the root. The root water influx is driven by chemical potential gradient across the root and into the xylem. The plants where the transpiration occurs very rapidly, do not contain root pressure. The root pressure is considered to be playing the main role in filling the non-functional xylem, especially after winter.