Wilted plants with lots of water

Question:

With all the rain, I've seen something I didn't expect. While my garden was wet, my plants wilted! There was water standing in puddles around the plants, but they wilted! I thought maybe they had a disease, but as the ground dried, the plants began to look good again. Now they are fine! What's up?

Answer:

You have observed something that is not common in New Mexico (except in some valley locations). Plants will wilt if there is too much water in the soil. The roots of plants need oxygen to absorb water and minerals. If the soil is too wet, the small pores between soil particles will not have enough oxygen for the roots to absorb water and the plants will wilt.

This seems to be illogical, but plant roots have a protective mechanism called the Casparian strip. This is an impermeable cylinder inside roots. It provides the plant with a means of regulating what enters the plant. This allows the plants to exclude many substances that would be harmful, such as salts and some larger organic molecules.

This protection comes with a cost. The plant must expend energy to transport water and minerals across the Casparian strip. That means the plants must release energy stored in carbohydrates through the process of respiration. Respiration in plants requires oxygen just like it does in animals. (The word "respiration" means not only "breathing", but also refers to the biochemical conversion of sugars and oxygen into water, carbon dioxide, and energy.) The energy released by respiration is then available to allow plants to transport water and minerals across the Casparian strip. If oxygen is not available, respiration cannot occur and the plant cannot take up water.

The Casparian strip exists in roots only, not in stems. Cut flowers placed into water have no problem absorbing water because the water enters directly into the xylem vessels (water transport portion of the vascular system of plants) through the cut. Since stems are not usually the organs that perform the function of water and mineral uptake, there is no Casparian strip in stems. This also helps cut flowers absorb water.

It is good news that your plants are doing well now. If the water-logged conditions had persisted too long, diseases may have attacked the roots of the plant as the roots begin dying for lack of oxygen. (Yes, plants can drown.) However, apparently any damage that may have occurred to the roots of your plants was within the ability of the plant to repair, so disease has not become a problem. Keep watching the plants because disease may appear later. Don't overwater or underwater your plants now. After this high water stress, good garden care is important to prevent development of diseases.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

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