Most growers know to keep an eye out for common plant stressors like insects, diseases, and water and nutrient levels. It takes a little more effort and some monitoring to recognize those abiotic stress factors that are harder to detect. These include, but are not limited to environmental conditions such as temperature fluctuation, relative humidity levels, and air pollutants, both indoor and out, as well as lighting conditions.
Temperature extremes can cause plant stress. Every plant has a range of temperatures that it will thrive in, a bigger range that it will survive in, and a range outside of which it will not perform well or even die if exposed to. Most commercially obtained plants will include information with it about its preferred hardiness zones.
Plants that have been exposed to too much heat will often show signs that are consistent with drought symptoms such as wilting. If the plant is subject to temperatures that are too high for extended periods of time, they will display additional symptoms. As an act of “self”- preservation, the plant may bolt, or flower prematurely in order to shed its seeds. The plant may also appear “leggy” with stretched growth. If a fruit-bearing plant is exposed to too much heat during a fruiting period, the fruits may be undersized and fall off easily. The leaves may be stunted and/or fall off easily as well. Control measures for high heat are usually satisfied with additional water during stress periods and also additional shading.
Humidity is another factor relative to temperature that may be stressing your plants out. When humidity levels are too low, especially coupled with high temperatures, plants may respond by rolling their leaves. This is done to reduce the amount of leaf surface exposed to light, thereby preserving moisture. This phenomenon can occur when the humidity is at 50 percent or below. Continued exposure to low humidity can cause stunted growth as well. High humidity is more forgiving, as most plants enjoy the additional moisture afforded by the abundance of water vapor.
Plants that have been exposed to too much cold show a whole different range of symptoms. If the cold doesn’t outright kill the plant, it may kill the foliage, turning it dark brown to black, or cause a portion of the plant to appear to collapse. Cold-sensitive plants upon exposure, may develop brown spots on the veins of their leaves, or brown to black margins on the leaf ends, especially the newest growth. Foliage may also appear to have wet spots on them, when in fact they are dry to the touch; this is due to cellular damage from the cold. For plants that are growing outside, a covering prior to predicted cold spells may help as can spraying with water before daybreak. If possible, plants can be brought inside where they will be protected. For indoor plants, temperature control is easier. One can increase the heat supply in the area around the sensitive plants or make sure that there are no obstructions around the heat source. IN the event that a portion of a plant is damaged by cold, but the plant still survives, the affected areas can be pruned out.
The best method of prevention is to select plants that are hardy for the zone in which they will be planted in. If growing plants indoors, this is less of a concern, but still a consideration for location suitability near heat sources, doors, windows or other areas that may be subject to temperature fluctuations.
Air Particulate Stress
Plants are excellent filters for improving air quality, especially indoors. Even they have their limits though. An excess amount of any particular pollutant, or long-term exposure to second hand smoke for some species of plants can cause undue stress.
Plant pores, called stomata, need to remain “unclogged” in order to perform the necessary plant function of photosynthesis. If the surfaces of leaves are covered with various pollutants, their growth and development will suffer. If plants are exposed to any pollutants containing the gas ethylene, such as in cigarette smoke, senescence can occur. This is the premature aging and subsequent decline of leaves, branches, flowers and fruits of plants. The tar contained in cigarette smoke is also responsible for clogging plant stomata. Plants that are located outdoors in regions with high levels of air pollutants may be difficult to remedy. Indoor air quality can of course be improved with air filtration systems, or even just the exchange of air with fans or open windows when appropriate.
Unless grow lights are employed, or other overhead lights are deliberately left on, growers may not always be cognizant of unwanted levels of lights that may be affecting their plants. This light pollution may be from street lights, other exterior lights or if living in a multi-unit building, coming in from under the door adjacent to commonly shared space. Plants need to have a period of darkness as well as a period of extended rest.
The presence of unwanted light in and of itself may not directly affect plants. The spectra and intensity of light will dictate if a plant starts to show symptoms of interference of its photoperiods. Symptoms of plants that are receiving too much light can manifest themselves differently for different species. Some may grow unevenly towards the light source, or be considerably fuller on one side. Plants exposed to unwanted light may prolong their flowering or fruiting period to the detriment of the plant’s health. Plants that are exposed to too much UV light will show signs of stress through scorching of their leaves. This is more of a concern to growers at higher elevations, but can be a problem for any grower whose grow lights do not emit light at appropriate wavelengths for good growth. Other than changing to a more compatible bulb, shade cloth may be needed to limit the amount of incoming UV light.
Compared to air pollution or extreme temperature variations, light pollution is a fairly easy environmental stressor to combat. Once the source of the light has been identified, any number of coverings may be used to reduce or eliminate its unwanted effects.
Plant stressors come in many known and even predictable forms. What is not known or well understood are the mechanisms that plants use to perceive and sometimes react to these stressors. While it is assumed that plants have within them the capabilities of stress perception, there remains much work in identifying these.