At higher concentrations, active oxygen species harm the plant by damaging membranes, proteins, chlorophyll, and nucleic acids. Environmental and biotic stresses, such as salt, light, drought, wounding, and pathogen attack, are known to cause oxidative stress in plant cells, and a high activity of antioxidant enzymes has been associated with stress tolerance.
Some proteins show high sequence homology to known antioxidant enzymes from plants.
Possible functions of these enzymes in mature sieve elements will be discussed.