Stress Is Good For Plants

Fig. 1 Petunias finished  in cooler temperatures in an unheated high tunnel (left) were more compact than plants finished in a heated greenhouse (right).

Do your greenhouse plants live a life of luxury? Soaking up the sun, bathed in nourishing nutrients, in a temperature-controlled climate — your greenhouse is like Club Med for plants. But then reality hits as the plant is shipped to the retail environment and then brought into the “real world,” when the consumer brings it into his or her home or landscape.

A plant that has been coddled in the greenhouse may not perform as well once taken to a more stressful environment. What’s a grower to do? One strategy is to purposely stress plants to make them more compact or enhance their ability to thrive once they hit a more stressful environment.

Plant Stress, Defined
Stress, as far as plants are concerned, can be generally split into two different groups: biotic and abiotic. Biotic stress is one caused by another living organism, such as disease or insect infestations. It is important to avoid biotic stresses as they can quickly spread from plant to plant and render a crop unmarketable.

Abiotic stress is one caused by non-living factors, including high- or low-temperature stress, drought stress, over- or under-fertilization, high salts and high or low light. While exposing a plant to abiotic stress may reduce productivity, when applied appropriately and in moderation, it is one way to keep a crop more compact, reduce its water requirements and improve its ability to withstand further stresses once it leaves your operation.

Let’s say you are growing a crop in flats or pot-to-pot and it is a crop you do not want to re-space. As stems and leaves grow, the plant canopy fills out the allotted space. As a plant’s leaves come into contact with its neighbors, the plant will undergo a “shade avoidance response.” This is an internal trigger causing stems to stretch more in attempt to reach the sun and avoid shade from its neighbors.
Plant growth regulators are one tool to control excessive growth, but another tool for keeping growth in check is exposure to a moderate temperature or water stress.

Temperature Stress Controls Height And
Delays Flowering

Most plants have an optimum temperature at which their development rate is the quickest. At the optimum temperature, unfolding of new leaves and progress toward flowering is the quickest. Relatively warm temperatures also promote elongation of stems and leaves.

Growing a crop cooler reduces its development rate, thereby reducing the number of nodes on a stem and limiting the stem elongation of each node. This leads to an overall reduction in stem length. A trade-off with low temperatures is that the plant will take longer to reach flowering, so using low temperatures to keep height in check may be a good strategy for flower crops once they have nearly reached flowering. One technique for bedding plant crops is to “tone” them by exposing them to lower-than-optimal night temperatures beginning at visible bud. For cold tolerant crops such as ageratum, dianthus, pansy, petunia and snapdragon night temperatures of 50°F to 55°F may be used. For cold sensitive crops such as begonia, celosia, coleus, impatiens and vinca, try night temperatures of 58°F to 62°F.

Another approach to using lower temperatures that we have collectively been researching at Cornell University and Purdue University, is using unheated high tunnels to finish bedding plant crops. We transplanted ‘Dreams Midnight’ petunias into 4-inch pots on April 1 and moved plants to either greenhouses heated to 65°F or high tunnels. Plants were grown until they were in flower and considered marketable, which was about mid-May.

Compared to the greenhouse, plants in the unheated high tunnel had an average temperature about 5 degrees cooler, but with much greater extremes varying from about 27°F on cold nights to more than 100°F on sunny days. Flowering of the high tunnel plants was delayed by about one week, compared to their greenhouse counterparts, but they were much more compact, at 4½ inches tall compared to 9 inches tall (Figure 1).

Drought Stress Manages Size And Improves Hardiness
Mild to moderate drought is another way to stress plants to help manage their size and improve their hardiness once purchased by the consumer. A plant grown with luxuriant water will grow bigger with softer stem and leaf tissue than one with mild drought stress.

Leaf wilting is a common symptom of mild drought stress. Stressing a plant to the point of wilting can be dangerous to a crop. If wilting is allowed to continue, the plant may become so dehydrated that it cannot recover. This is called the permanent wilting point (Figure 2). The intensity of water stress required to reach this point varies by crop. Severe water stress also causes the effective salt concentration on a root-zone to rise to dangerous levels, which can damage roots.

A more subtle way to apply water stress is to use chronic or low-level, but long-lasting, water stress. One strategy, long-used by growers, is to allow the root-zone to dry until a plant nearly wilts before watering it again. Instead of fully watering the container, a lower amount of water is used.
A more formal way to implement this is called Regulated Deficit Irrigation (RDI). In RDI, a grower purposely limits the amount of water given to the plant to some percentage of its actual needs. RDI has been shown to reduce the internode growth of some nursery crops, reducing the need for mid-season pruning. RDI can improve commercial crop quality, save water and yield plants better adapted to dry environments. A disadvantage of RDI is that it can limit ornamental crop marketability by reducing leaf and flower area.

At Cornell we used RDI to grow ‘Prestige Red’ and ‘Peterstar Red’ poinsettias. A control group of plants received 100 percent of water needs. Representative control plants were weighed to determine how much water they used each day and RDI treatment plants received either 60 percent or 80 percent of control plant needs. We implemented RDI using drip irrigation.

For example, if we estimated that control plants needed 5 minutes of water during an irrigation event, 80 percent plants received 4 minutes and 60 percent plants received 3 minutes of water. RDI was effective at reducing plant height; plants were 1 to 2 inches shorter than their control counterparts (Figure 3).

The overall size of the plant, including bract surface area, was also reduced by RDI. Bracts of 80 percent RDI plants were about 25 percent smaller and bracts of 60 percent RDI plants were about 50 percent smaller than control plants. However, a major benefit was that RDI plants used less water and therefore took longer to wilt in the postharvest environment.

Treatment plants were moved to a simulated retail environment. Plants were well-watered once and then we recorded the number of days to wilt. RDI plants lasted three to four days longer before wilting.

Our results suggest that a moderate RDI can restrain plant growth during production and result in a longer-lasting plant for the consumer.  GG

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