Editing Photorespiration (section)

===Altered substrate availability: lowered {{CO2}} or increased O<sub>2</sub>===
Factors which influence this include the atmospheric abundance of the two gases, the supply of the gases to the site of fixation (i.e. in land plants: whether the [[stomata]] are open or closed), the length of the liquid phase (how far these gases have to diffuse through water in order to reach the reaction site). For example, when the stomata are closed to prevent water loss during [[drought]]: this limits the {{CO2}} supply, while {{chem|O|2}} production within the leaf will continue. In algae (and plants which photosynthesise underwater); gases have to diffuse significant distances through water, which results in a decrease in the availability of {{CO2}} relative to {{chem|O|2}}. It has been predicted that the increase in [[wikt:ambient|ambient]] {{CO2}} concentrations predicted over the next 100 years may lower the rate of photorespiration in [[C3 carbon fixation|most plants]] by around 50%{{citation needed|date=March 2017}}. However, at temperatures higher than the photosynthetic thermal optimum, the increases in turnover rate are not translated into increased {{CO2}} assimilation because of the decreased affinity of Rubisco for {{CO2}}.<ref>{{cite journal |last1=Hermida-Carrera |first1=Carmen |last2=Kapralov |first2=Maxim V |last3=Galmés |first3=Jeroni |title=Rubisco catalytic properties and temperature response in crops |journal=Plant Physiology |volume=171 |issue=4 |date=21 June 2016 |pages=2549–61 |doi=10.1104/pp.16.01846 |pmid=27329223 |pmc=4972260 }}</ref>