Changes

<p>In a biological context, ROS are formed as a natural byproduct of the normal metabolism of&nbsp;<a href="https://en.wikipedia.org/wiki/Oxygen" title="Oxygen">oxygen</a>&nbsp;and have important roles in&nbsp;<a href="https://en.wikipedia.org/wiki/Cell_signaling" title="Cell signaling">cell signaling</a>&nbsp;and&nbsp;<a href="https://en.wikipedia.org/wiki/Homeostasis" title="Homeostasis">homeostasis</a>.^{<a href="https://en.wikipedia.org/wiki/Reactive_oxygen_species#cite_note-Devasagayam_2004_796-3">[3]</a>}&nbsp;However, during times of environmental stress (e.g.,&nbsp;<a href="https://en.wikipedia.org/wiki/Ultraviolet_light" title="Ultraviolet light">UV</a>&nbsp;or heat exposure), ROS levels can increase dramatically.^{<a href="https://en.wikipedia.org/wiki/Reactive_oxygen_species#cite_note-Devasagayam_2004_796-3">[3]</a>}&nbsp;This may result in significant damage to cell structures. Cumulatively, this is known as&nbsp;<a href="https://en.wikipedia.org/wiki/Oxidative_stress" title="Oxidative stress">oxidative stress</a>. The production of ROS is strongly influenced by stress factor responses in plants, these factors that increase ROS production include, drought, salinity, chilling, nutrient deficiency, metal toxicity and UV-B radiation. ROS are also generated by exogenous sources such as&nbsp;<a href="https://en.wikipedia.org/wiki/Ionizing_radiation" title="Ionizing radiation">ionizing radiation</a>.^{<a href="https://en.wikipedia.org/wiki/Reactive_oxygen_species#cite_note-4">[4]</a>}</p>

<h2>Formation and decomposition</h2>

<p>Singlet oxygen is highly reactive, especially with organic compounds that contain double bonds. The resulting damage caused by singlet oxygen reduces the photosynthetic efficiency of&nbsp;<a href="https://en.wikipedia.org/wiki/Chloroplast" title="Chloroplast">chloroplasts</a>. In plants exposed to excess light, the increased production of singlet oxygen can result in cell death.^{<a href="https://en.wikipedia.org/wiki/Reactive_oxygen_species#cite_note-Laloi_2015-11">[11]</a>}&nbsp;Various substances such as&nbsp;<a href="https://en.wikipedia.org/wiki/Carotenoid" title="Carotenoid">carotenoids</a>,&nbsp;<a href="https://en.wikipedia.org/wiki/Tocopherol" title="Tocopherol">tocopherols</a>&nbsp;and&nbsp;<a href="https://en.wikipedia.org/wiki/Plastoquinone" title="Plastoquinone">plastoquinones</a>&nbsp;contained in chloroplasts quench singlet oxygen and protect against its toxic effects. In addition to direct toxicity, singlet oxygen acts a&nbsp;<a href="https://en.wikipedia.org/wiki/Signal_transduction" title="Signal transduction">signaling</a>&nbsp;molecule.^{<a href="https://en.wikipedia.org/wiki/Reactive_oxygen_species#cite_note-Laloi_2015-11">[11]</a>}&nbsp;Oxidized products of&nbsp;<a href="https://en.wikipedia.org/wiki/%CE%92-carotene" title="Β-carotene">&beta;-carotene</a>&nbsp;arising from the presence of singlet oxygen act as&nbsp;<a href="https://en.wikipedia.org/wiki/Second_messenger_system" title="Second messenger system">second messengers</a>&nbsp;that can either protect against singlet oxygen induced toxicity or initiate programmed cell death. Levels of&nbsp;<a href="https://en.wikipedia.org/wiki/Jasmonate" title="Jasmonate">jasmonate</a>&nbsp;play a key role in the decision between cell acclimation or cell death in response to elevated levels of this reactive oxygen species.^{<a href="https://en.wikipedia.org/wiki/Reactive_oxygen_species#cite_note-Laloi_2015-11">[11]</a>}</p>

<h2>Damaging effects</h2>