Drooping Leaves are a Problem

Much of the USA has had a hot, dry summer this year. Plants burst forth in the spring with good amounts of rainfall, only to be tricked by hot dry weather later. If you are trying to cultivate special plants, be aware of dry spells and try to water at least once per week, deeply.

Wilted leaves on a Burning Bush, Euonymous alatus 'Compacta'

There are some interesting things about the response of plants to dry weather. In fact, they have developed a special hormone to deal with drought. It is called abscisic acid. ABA as it is known to botanists, is produced in the roots when the soil gets hot and dry. It travels up the plant stems in the xylem tubes and gets into the leaves. There it alters the osmotic potential of the cells which keep the stomata open. Stomata are like little mouths in the back of the leaves. The little mouths are normally held open by the plump guard cells so the leaves can breathe gases and perform photosynthesis. When the guard cells soften up in response to ABA, the stomata close and insure no water loss. This is a very clever thing for the stomata to do since most of the rest of the leaf is covered by waxy coverings and is quite waterproof. Anyway, the ABA is a signal that the weather is hot and dry and is a time of stress, so shut down everything and wilt. When water is again present, the ABA dissolves away and causes no harm.

A large Rodgersia spp. wilted down from drought

Another thing ABA does is prepare plants for dormancy. ABA is produced seasonally at the end of summer in response to the stimulation of shortening daylengths. Different species of plants are more or less sensitive to ABA, so it is one of many factors involved, but the bottom line is it makes the leaves fall off. That is why it is called abscisic acid, it makes leaves abscise. Some species of plants will shed a part of their leaves after a single wilting event. Some plants will not shed them until shortening days, cold weather, and other signals tell them it is time for winter dormancy. In many plants, ABA will stimulate the growth of corky cells located at the base of leaf stems. These corky cells sever the connection between the twig and the leaf. After no more water or nutrients are coming INTO the leaf, and no more carbohydrates from the leaf are going INTO the twig, they feel they can do without one another. The leaf begins to starve, and it loses first its chlorophyll, then its carotene, then its will to live and lets go. That is why leaves turn from green to orangy red (in some species) as they go into autumn. It is getting to that time of year, people…fall.


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