Contact: Diane Relf, Extension Specialist, Environmental Horticulture
Posted April 1997
You've been reading about how stress affects you and how to deal with it for most of the last decade. But did you know stress affects your plants, too? Yes, our plants suffer stress, too, whether they grow in our homes, landscapes, or vegetable gardens. Don't panic -- there's no need to look up a local plant stress relief support group. Some of these stresses are not only beneficial, but critical to the plant's life cycle.
Of course, some plant stresses are not helpful. Put your African violets in a dark closet for two months, and you will see the decidedly detrimental effects of severe-light stress. But, research on coleus seedlings showed that exposing the seedlings to two days of low-light levels improved their cold-hardiness compared to seedlings grown at full-light levels. Mild-light stress promoted tolerance of another stress.
Whether a particular stress will be helpful or harmful depends on the degree of stress, when it occurs in a plant's life cycle, and the plant species. It is confusing to sort out the interactions of various stresses with each other as they influence plants, but plant scientists are probing this area to learn more.
Why would one stress reduce the effect another had on a plant? Much of the reason has to do with growth hormones such as auxins, cytokinins, giberellins, and abscisic acid. Shoot, root, and fruit growth are controlled by the presence of these hormones, and the relative concentrations of the hormones alters growth patterns. Stresses, such as low water availability, change the hormonal distribution in the plant. For instance, water stress triggers decreased auxin and increased abscisic acid. Auxins are involved in the growth of all parts of the plant, and abscisic acid is an inhibitor that may be involved in inducing dormancy. Hence, under low water conditions, growth slows so the plant requires less water and is less likely to suffer damage.
Pruning is a stress that can have a dramatic effect on a plant, again through altering hormonal balances. New growth from a shrub killed to the ground by winter freezes or pruned to the ground by a gardener is vigorous and extensive. Leaves on the new growth are unusually large. Hormones normally produced by buds are not present, and the relatively high cytokinin to auxin ratio resulting produces the unusual growth. Gardeners exploit this when they rejuvenate shrubs by pruning them at ground level.
Without some stresses, many perennial plants simply wouldn't survive. Trees, shrubs, and herbaceous perennials put forth a flush of growth in spring when temperature, light, and moisture levels are optimal. As summer progresses, rain is more infrequent, temperatures may rise above those conducive to growth, and light intensities may be too high for the photosynthetic system to handle. Hormone levels change and growth slows. The plant begins to enter dormancy, long before the first frost. If it happened differently, the plant might enter autumn still growing vigorously, with tender young growth susceptible to death from the first hard frost. Gardeners can reverse the process of dormancy induction by watering, fertilizing, and pruning late into the summer. Pruning is especially damaging to the proper initiation of dormancy as it drastically alters hormone levels normally controlled by the buds that pruning removes.
Initiation of dormancy is not on the agenda of annuals. Go ahead and water, fertilize, and prune your annuals to your heart's content without worrying about their survival to next year. However, I wouldn't suggest watering lushly right up until when you leave for a three-week vacation in August. Your plants will probably survive better if they have developed some tolerance to mild drought before being left to the vagaries of the weather.
(Originally published as "Plant Stress Management," by Ellen Silva, Extension Technician, Department of Horticulture, in The Virginia Gardener Newsletter, Volume 10, Number 4.)
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