Abiotic stress
Abiotic stress is the negative impact of non-living factors on the living organisms in a specific environment. The non-living variable must influence the environment beyond its normal range of variation to adversely affect the population performance or individual physiology of the organism in a significant way.
Whereas a biotic stress would include living disturbances such as fungi or harmful insects, abiotic stress factors, or stressors, are naturally occurring, often intangible and inanimate factors such as intense sunlight, temperature or wind that may cause harm to the plants and animals in the area affected. Abiotic stress is essentially unavoidable. Abiotic stress affects animals, but plants are especially dependent, if not solely dependent, on environmental factors, so it is particularly constraining. Abiotic stress is the most harmful factor concerning the growth and productivity of crops worldwide. Research has also shown that abiotic stressors are at their most harmful when they occur together, in combinations of abiotic stress factors.
Examples
Abiotic stress comes in many forms. The most common of the stressors are the easiest for people to identify, but there are many other, less recognizable abiotic stress factors which affect environments constantly.The most basic stressors include:
- High winds
- Extreme temperatures
- Drought
- Flood
- Other natural disasters, such as tornadoes and wildfires.
- Cold
- Heat
- Nutrient deficiency
Effects
Abiotic stress, as a natural part of every ecosystem, will affect organisms in a variety of ways. Although these effects may be either beneficial or detrimental, the location of the area is crucial in determining the extent of the impact that abiotic stress will have. The higher the latitude of the area affected, the greater the impact of abiotic stress will be on that area. So, a taiga or boreal forest is at the mercy of whatever abiotic stress factors may come along, while tropical zones are much less susceptible to such stressors.Benefits
While abiotic stress may have negative impacts on individual organisms, there are cases where abiotic stress plays an important role in maintaining a healthy ecosystem. Important ecosystem mechanisms and improved overall stress tolerance may rely on occasional low levels of abiotic stress.One example of a situation where abiotic stress plays a constructive role in an ecosystem is in natural wildfires. Smaller fires are useful in reducing the overall fuel load of an area of forest or prairie. By clearing out dead brush and other organic matter, the risk of catastrophic and widespread fire decreases, and the residual ash of smaller fires helps add nutrients back into the soil. The observed benefits of these smaller and more controlled fires on land usability and species populations have led to the use of prescribed burning by humans for centuries. Varying perspectives on the benefits and risks of fire to ecosystems have influenced official policy through history. The U.S. Forest Service, initially focused on fire control, changed its policy to one of fire management in 1974, recognizing these fires as a natural part of an ecosystem. There is also evidence that a diverse fire history between patches of land within an area has been shown to benefit transitional landscapes between savanna and forest. Even though it is healthy for an ecosystem, a wildfire can still be considered an abiotic stressor, because it puts stress on individual organisms within the area. On the larger scale, though, natural wildfires are positive manifestations of abiotic stress.
What also needs to be taken into account when looking for benefits of abiotic stress, is that one phenomenon may not affect an entire ecosystem in the same way. While a flood will kill most plants living low on the ground in a certain area, if there is rice there, it will thrive in the wet conditions. Another example of this is in phytoplankton and zooplankton. The same types of conditions are usually considered stressful for these two types of organisms. They act very similarly when exposed to ultraviolet light and most toxins, but at elevated temperatures the phytoplankton reacts negatively, while the thermophilic zooplankton reacts positively to the increase in temperature. The two may be living in the same environment, but an increase in temperature of the area would prove stressful only for one of the organisms.
Lastly, abiotic stress has enabled species to grow, develop, and evolve, through the process of natural selection. Heritable traits that improve an organism's resiliency under stressed conditions increase the likelihood that the organism will survive and reproduce, enabling it to pass these traits to the next generation. Both plants and animals have evolved mechanisms allowing them to survive extremes.
Detriments
One of the detriments concerning abiotic stress involves farming. It has been claimed by one study that abiotic stress causes the most crop loss of any other factor and that most major crops are reduced in their yield by more than 50% from their potential yield.Because abiotic stress is widely considered a detrimental effect, the research on this branch of the issue is extensive. For more information on the harmful effects of abiotic stress, see the sections below on plants and animals.
In plants
A plant's first line of defense against abiotic stress is in its roots. If the soil holding the plant is healthy and biologically diverse, the plant will have a higher chance of surviving stressful conditions.The plant responses to stress are dependent on the tissue or organ affected by the stress. For example, transcriptional responses to stress are tissue or cell specific in roots and are quite different depending on the stress involved.
One of the primary responses to abiotic stress such as high salinity is the disruption of the Na+/K+ ratio in the cytoplasm of the plant cell. High concentrations of Na+, for example, can decrease the capacity for the plant to take up water and also alter enzyme and transporter functions. Evolved adaptations to efficiently restore cellular ion homeostasis have led to a wide variety of stress tolerant plants.
Facilitation, or the positive interactions between different species of plants, is an intricate web of association in a natural environment. It is how plants work together. In areas of high stress, the level of facilitation is especially high as well. This could possibly be because the plants need a stronger network to survive in a harsher environment, so their interactions between species, such as cross-pollination or mutualistic actions, become more common to cope with the severity of their habitat.
Plants also adapt very differently from one another, even from a plant living in the same area. When a group of different plant species was prompted by a variety of different stress signals, such as drought or cold, each plant responded uniquely. Hardly any of the responses were similar, even though the plants had become accustomed to exactly the same home environment.
Serpentine soils can be a source of abiotic stress. Initially, the absorption of toxic metal ions is limited by cell membrane exclusion. Ions that are absorbed into tissues are sequestered in cell vacuoles. This sequestration mechanism is facilitated by proteins on the vacuole membrane. An example of plants that adapt to serpentine soil are Metallophytes, or hyperaccumulators, as they are known for their ability to absorbed heavy metals using the root-to-shoot translocation. They're also extinguished for their ability to absorb toxic substances from heavy metals.
Chemical priming has been proposed to increase tolerance to abiotic stresses in crop plants. In this method, which is analogous to vaccination, stress-inducing chemical agents are introduced to the plant in brief doses so that the plant begins preparing defense mechanisms. Thus, when the abiotic stress occurs, the plant has already prepared defense mechanisms that can be activated faster and increase tolerance. Prior exposure to tolerable doses of biotic stresses such as phloem-feeding insect infestation have also been shown to increase tolerance to abiotic stresses in plant
Impact on food production
Abiotic stress mostly affects plants used in agriculture. Some examples of adverse conditions are high or low temperatures, drought, salinity, and toxins.- Rice is a classic example. Rice is a staple food throughout the world, especially in China and India. Rice plants can undergo different types of abiotic stresses, like drought and high salinity. These stress conditions adversely affect rice production. Genetic diversity has been studied among several rice varieties with different genotypes, using molecular markers.
- Chickpea production is affected by drought. Chickpeas are one of the most important foods in the world.
- Wheat is another major crop that is affected by drought: lack of water affects the plant development, and can wither the leaves.
- Maize crops can be affected by high temperature and drought, leading to the loss of maize crops due to poor plant development.
- Soybean is a major source of protein, and its production is also affected by drought.
Salt stress in plants
Over generations, many plants have mutated and built different mechanisms to counter salinity effects. A good combatant of salinity in plants is the hormone ethylene. Ethylene is known for regulating plant growth and development and dealing with stress conditions. Many central membrane proteins in plants, such as ETO2, ERS1 and EIN2, are used for ethylene signaling in many plant growth processes. Mutations in these proteins can lead to heightened salt sensitivity and can limit plant growth. The effects of salinity has been studied on Arabidopsis plants that have mutated ERS1, ERS2, ETR1, ETR2 and EIN4 proteins. These proteins are used for ethylene signaling against certain stress conditions, such as salt and the ethylene precursor ACC is used to suppress any sensitivity to the salt stress.