What causes a plant to be stressed? As with humans, stresses can originate from the surrounding environment (called abiotic, or nonliving stresses); or, they can come from living organisms that can cause disease or damage (caused biotic stresses).
Water StressOne of the most important abiotic stresses affecting plants is water stress. A plant requires a certain amount of water for its optimal survival; too much water (flooding stress) can cause plant cells to swell and burst; whereas drought stress (too little water) can cause the plant to dry up, a condition called desiccation. Either condition can be deadly to the plant.
Temperature StressTemperature stresses can also wreak havoc on a plant. As with any living organism, a plant has an optimal temperature range at which is grows and performs best. If the temperature is too cold for the plant, it can lead to cold stress, also called chilling stress. Extreme forms of cold stress can lead to freezing stress. Cold temperatures can affect the amount and rate of uptake of water and nutrients, leading to cell desiccation and starvation. Under extremely cold conditions, the cell liquids can freeze outright, causing plant death.
Hot weather can affect plants adversely, too. Intense heat can cause plant cell proteins to break down, a process called denaturation. Cell walls and membranes can also "melt" under extremely high temperatures, and the permeability of the membranes is affected.
Other Abiotic StressesOther abiotic stresses are less obvious, but can be equally as lethal. In the end, most abiotic stresses affect the plant cells in the same manner as do water stress and temperature stress. Wind stress can either directly damage the plant through sheer force; or, the wind can affect the transpiration of water through the leaf stomata and cause desiccation. Direct burning of plants through wildfires will cause the cell structure to break down through melting or denaturation.
In farming systems, the addition of agrochemicals such as fertilizers and pesticides, either in excess or in deficit, can also cause abiotic stress to the plant. The plant is affected through an imbalance of nutrition, or via toxicity. High amounts of salt taken up by a plant can lead to cell desiccation, as elevated levels of salt outside a plant cell will cause water to leave the cell, a process called osmosis. Plant uptake of heavy metals can occur when plants grow in soils fertilized with improperly composted sewage sludge. High heavy metal content in plants can lead to complications with basic physiological and biochemical activities such as photosynthesis.
Biotic StressesBiotic stresses cause damage to plants via living organisms, including fungi, bacteria, insects, and weeds. Viruses, although they are not considered to be living organisms, also cause biotic stress to plants.
Fungi cause more diseases in plants than any other biotic stress factor. Over 8,000 fungal species are known to cause plant disease. On the other hand, only about 14 bacterial genera cause economically important diseases in plants, according to an Ohio State University Extension publication. Not many plant pathogenic viruses exist, but they are serious enough to cause nearly as much crop damage worldwide as fungi, according to published estimates. Microorganisms can cause plant wilt, leaf spots, root rot, or seed damage. Insects can cause severe physical damage to plants, including to the leaves, stem, bark, and flowers. Insects can also act as a vector of viruses and bacteria from infected plants to healthy plants.
The method by which weeds, considered as unwanted and unprofitable plants, inhibit the growth of desirable plants such as crops or flowers is not by direct damage, but by competing with the desirable plants for space and nutrients. Because weeds grow quickly and produce an abundance of viable seed, they are often able to dominate environments more quickly than some desirable plants.