A word on water potential

So it's time for the first of the physiology posts.

Water is essential for all life on earth. Without a plentiful supply of it, we certainly wouldn't be here. Plants originated in aquatic environments, and one of the many challenges they had to overcome when they made their move onto dry land was how to obtain water. They cannot actively drink it like we can, so a way to passively move water from their roots to the stems evolved.

In order to regulate the water within their tissues, plants use pressure gradients. Because water will always flow from an area of high pressure to low pressure, by creating a pressure gradient in the tissues of the leaves water will be drawn into them from the stem, and into the stem from the roots. In this way, the whole plant becomes a pressure gradient - the roots always have the highest water potential, followed by the stems and finally the leaves. By opening and closing their stomata (pores on the leaf surface), water can leave the plant, and the gradient can continually function (otherwise the water would build up in the leaves, the pressure would increase and the gradient would eventually stop).
During drought of course the plant conserves it's water by closing the stomata, and usually the water potential of the whole plant decreases. If it continues without water for too long, leaves wilt and the plant eventually dies.

So, the underlying question about all this is how does the plant create different pressures in its tissues? There are two basic ways to achieve this. Firstly, you can change the volume of the plant cell, without altering it's size (think of a bottle of fizzy drink - if you shake it and the gas leaves the drink but you've not opened the bottletop, the bottle becomes highly pressurised), or you can pack the interior of the cell with solutes such as salt, and water will be drawn into it. Typically plants use solute loading to decrease and increase their water potential, taking up salts such as potassium from the soil and ground water.

In a nutshell, plants can use water thanks to basic physics. They can regulate their water use through a variety of mechanisms, all of which are vitally important for the plants photosynthesis and survival. An important component of why plants water regulate the way they do is a need to balance water loss with the uptake of the carbon dioxide necessary for photosynthesis (which occurs when stomata are open). So this will be the subject of the next post :)