Friday, December 14, 2007

Plant Science - How Does Water Move In a Plant

Ever wonder how and why water moves in the plant. The following is a scientific explanation. It all starts with the leaves.

From physics we know that substances have both kinetic and potential energy. Potential energy is the ability of that substance to do work and work is done when a substance produces heat or moves. Therefore water potential is the ability of water to move. Differences in water potentials from leaves to the roots creates suction forces that pulls water through plants.

Transpiration drives water movement in plants.

The surface of green plant tissues (leaves and stems) in higher plants contain specialized cell pairs forming stomata which are biological apertures that open and close allowing gasses to enter and leave the plant. The action of water vapor leaving through stomata of plants is the driving mechanism for transpiration. This is a passive process in plants. Water from leaf cells evaporates into air spaces in the leaf and when stomata open, the water vapor inside of the leaf diffuses into the atmosphere. The energy for this comes from the sun. Changes in temperature, wind speed, and relative humidity will modify this process (increase or reduce).

When stomata are closed and there is limited or no transpiration, water potential in the plant will be near to equilibrium with soil water potential around active water absorbing roots. When stomata open, leaf cells lose water. This creates a kind of suction. A difference in water potential is created between leaf water vessels and surrounding leaf cells. The water potential of leaf cells is lower and water will move from the leaf water vessels to the leaf cells. Differences between water potentials in leaf cells; leaf, stem, and root water vessels; root cells; and ultimately the soil water then creates a water potential gradient or suction that results in water moving in the plant. Osmosis in the leaves is the process that creates the suction.

Water is literally sucked up the plant with the leaves doing the suction.

Another factor in water movement in plants relates to the adhesive and cohesive nature of water. Plant water vessels are small in diameter and the adhesion of water to the sides of vessels and the cohesion of water molecules to each other creates a tension. Water is essentially pulled up the plant as a continuous column and can reach the top of tall trees. In the roots, epidermal cells with root hairs draw water into the cells by osmosis. Roots are growing into areas of the soil with moisture and root uptake of water also changes water potentials so there is water movement toward roots (until air pockets are created). Water will move root into the root water vessels. Root pressures are created in this way.

A continuous column of water being pulled up the plant from the soil water surrounding root hairs; into root hairs or cell walls surrounding root hairs; across the root into root water vessel cells; up the water vessels in the roots, then stem, and into the leaf; out of the leaf water vessels to the leaf cells and spaces; evaporating into leaf air spaces; and diffusing into the atmosphere through stomates. This movement is driven by a water potential gradient from transpiring leaves to roots. The leaves create a suction just like when you use a straw.

Written by Gordon Johnson, Extension Horticulture Agent, UD, Kent County

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