Abiotic stress is a term used to describe nonliving factors that adversely affect living organisms. Animals can be affected by abiotic stress, but plants are more vulnerable as they are unable to move to a less stressful environment. Biotic stress factors would include insect pests and diseases, whereas plant abiotic stress results from environmental factors. These may be related to climate — drought, extremes of temperature and wind, for example — or to chemical factors in the soil or the atmosphere.
Plants show a range of responses and adaptations that help bring about abiotic stress tolerance. Some of these involve structural or chemical changes, while others involve restriction of the growing period according to conditions. In some cases, symbiotic relationships have developed as a response to stresses.
Drought is one of the commonest forms of stress encountered by plants, and those that live in arid or semi-arid areas have developed various strategies for dealing with it. One of these is succulence. Thick, fleshy stems and leaves can store large amounts of water and reduce the surface area to volume ratio of the plant, minimizing water loss by evaporation. A waxy coating on the stems and leaves also reduces evaporation. Some non-succulent plants may have long roots that extend downwards to the groundwater.
Plants can sometimes adapt to seasonal or periodic stress by simply having a short life cycle that is timed to coincide with the most favorable conditions. For example, a plant may germinate, mature, flower and produce seeds during a short rainy season within an otherwise dry year, or it may lie dormant in an underground tuber for long periods, emerging after rain to quickly complete its life cycle. In temperate areas, forest floors during the summer months may be lacking in light for photosynthesis, so low growing forest plants may quickly complete their life cycles during the spring, before the forest canopy has developed dense foliage.
Abiotic stress may result from soil factors. A high salt content can be lethal for many plants, but some — known as halophytes — have adapted to salty conditions, which are found in both in coastal areas, such as salt marshes, and inland arid areas where high evaporation tends to concentrate salts originating in the mineral content of the soil. These plants may excrete salt from their leaves or store it within their cells in bodies known as vacuoles so that it is kept separate from the cell cytoplasm. Deficiencies in nutrients or the presence of toxic substances such as heavy metals in the soil can also result in abiotic stress.
Atmospheric pollution can be another source of stress. Sulfur dioxide and nitrogen oxides from fossil fuel burning can result in acid rain, which can damage the foliage of sensitive plants. Acid rain can also reduce soil pH, harming or killing plants that are not adapted to acidic conditions.
Climate change is thought to be a major source of abiotic stress for crops. Changing patterns of temperature and rainfall are having an impact on the cultivation of food and other crops, with some formerly productive areas suffering from drought, flooding or extremes of temperature. In order to alleviate the economic impact of crop failures, research is being carried out into the development, through breeding or genetic engineering, of crop plants that are more resistant to these forms of stress.