Injury in plants is damage caused by other organisms or by the non-living (abiotic) environment to plants. Animals that commonly causeinjury to plants include insects, mites, nematodes, andherbivorous mammals; damage may also be caused byplant pathogens including fungi, bacteria, and viruses. Abiotic factors that can damage plants include heat, freezing, flooding, lightning, ozone gas, and pollutant chemicals.
Plants respond to injury by signalling that damage has occurred, by secreting materials to seal off the damaged area, by producingantimicrobial chemicals, and in woody plants by regrowing over wounds.
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Animals that commonly causeinjury to plants include pests such asinsects,mites, andnematodes. These variously bite or abrade plant parts such as leaves, stems, and roots, or as is common among thetrue bugs, pierce the plant's surface and suck plant juices. The resulting injuries may admitplant pathogens such as bacteria and fungi, which may extend the injury.[1] Caterpillar larvae ofagricultural pests such ascabbage white butterflies (Pieridae) can completely defoliateBrassica crops.[2]Molluscs such assnails graze on plants includinggrasses andforbs, abrading them with their rasp-likeradula; they can inflict substantial damage to crops.[3]Grazingmammals including livestock such ascattle, too, bite off or break parts of plants including grasses, forbs, and forest trees, causing injury, and again, potentially admitting pathogens.[4]
Abiotic factors that can damage plants include heat, freezing, flooding, lightning strikes, ozone gas, and pollutant chemicals.
Heat can kill any plant, given a sufficienttemperature.Alpine plants tend to die at around 47 Celsius; temperate plants at around 51 Celsius; and tropical plants at nearly 58 Celsius: but there is some overlap depending on species. Similarly amongcereal crops, temperatebarley andoat die at around 49 Celsius, but tropicalmaize at 55 Celsius.[5]
Freezing affects plants variously, according to each species' ability to resist frost damage. Many forbs, including many garden flowers, are tender with little tolerance to frost, and die or are seriously damaged when frozen. Many woody plants are able tosupercool, with tough buds and stems containing molecules that lower thefreezing point or help to prevent thenucleation of ice crystals, and cell walls that mechanically protect cells against freezing.[6]
Flooding of soil quickly kills or injures many plants. The leaves become yellow (chlorosis) and die, progressively up the stem, within about five days after the roots are flooded. The roots lose the ability to absorb water and nutrients.[7]
Lightning strikes kill or injure plants, from root crops likebeet andpotato, which are instantly cooked in the ground, to trees such ascoconut, through effects such as sudden heat and pressure shock waves created when water inside the plant flashes to steam. This can rupture stems and scorch any plant parts.[8]
Ozone, a gas, causes injury to leaves at concentrations from as little as 0.1part per million in the atmosphere, such as may be found in or near large cities.[9]It is one of manypollutant chemicals that can damage plants.[10]
Plants respond to injury by signalling that damage has occurred,[11] by secreting materials to seal off the damaged area,[12] by producing antimicrobials to limit the spread of pathogens,[13] and in some woody plants by regrowing over the wound.[14]
Plants produce chemicals at the injury site that signal the presence of damage and may help to reduce further damage. The chemicals involved depend to some extent on the plant species, though several of them are shared among species; and the signals given depend on the cause of the injury. Plants injured byspider mites release volatile chemicals that attractpredatory mites, serving to reduce the attack on the plants. As another example, maize plants damaged by the caterpillars ofnoctuid moths release a mixture ofterpenoid substances which attract theparasitoid waspCotesia marginiventris, which kills caterpillars.[11][15] Many plants give off such herbivory-induced signals.[16]
Plants secrete a variety of chemicals to help seal off damaged areas. For example, the grape vineVitis vinifera is able to block thexylem water-transport tubes in its stems using the chemicaltylose in summertime, andgels in wintertime when the plant is dormant. Tylose helps to prevent pathogens such as wood-rotting fungi and the bacteriumXylella fastidiosa from spreading through the plant: the chemical is produced as a response both to the bacterium and to mechanical damage such as viticulturalpruning.[12]
Many woody plants produceresins[17] andantimicrobial chemicals to limit the spread of pathogens after an injury.[13][18]
Manywoody plants regrow around injuries, such as those caused bypruning. In time, such regrowth often completely covers the damaged area as thecambium growth layer produces new tissues. Well-pruned trees with undamagedbranch collars often recover well, where poorly-pruned trees rot below the wound.[14][19][20]