Water needs of plants change

Published 8:01 pm Saturday, August 26, 2017

Here it is late August and the days are still long, temperatures still warm and plants have access to all the water they want this go round.  The amount of water a living, growing plant actually needs during the season varies. As a matter of fact, the amount of water a plants needs and/or will take in changes during each day.

Among the things affecting water needs of a plant are changing light, wind and temperature.

Although stems and leaves can and do take in small amounts of water during rain or irrigation, it is soil water absorbed by the root system that accounts for the overwhelming majority of water used by plants. The drinking straw-like upward pull from leaves causes water to move up to and eventually out of leaves. Leaves have tiny, unseen pores called stomata.

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Leaves pull in carbon dioxide from the air and transpire water out to the air through stomata. In general, the brighter the day, the wider stomata open to let in more carbon dioxide to fuel corresponding increases in photosynthesis.

And larger stomata openings mean more water is released into the air, requiring more pulled up from the roots. This is true up to a point. If the build-up of carbon dioxide is more than the plant can use and gets out of balance with water, chlorophyll and all the elements of photosynthesis, stomata will actually close up some to reduce carbon dioxide intake, thereby reducing the loss of water.

The effect of wind on the water needs of plants is fairly simple. There is a damp air boundary layer on the outer surface of leaves. Blowing wind drives water out of this layer and makes for an drier, quicker path for water to evaporate from leaves into the air; thus the need for replacement water.

Temperature certainly has a role in plant water. Warm air can hold more moisture than cool air. So water loss from plants to air by transpiration through leaves increases with temperature.

Also, different type of plants utilize water for cooling in hot weather to some degree. And based on the plant species, sometimes it is just too hot for the watercooler system to keep up.

Vegetable gardeners have witnessed some plant species originating in cooler climates wilt during the day every summer here. Pumpkin is a surefire wilter for us and okra, a double-first cousin to cotton, can take the heat with limited water.

It was in biology class most of us learned that water with diluted nutrients is transferred upwards through roots and stems to leaves by plant tissue known as xylem.

The same course taught us liquid sugars and other good stuff made in leaves goes back down to stems and roots in liquid form via tissue called phloem. Sometimes I forget xylem is dead cellulose and phloem is live tissue.  But I never forget how to recall which one does what because xylem, pronounced zy-lum, zips it up and phloem, pronounced flo-um, flows it down.
Terry Rector is a spokesman for the Warren County Soil and Water Conservation District.