# Isoprene Emissions, Oxidation Chemistry and Environmental Impacts
## 2 Biological Function of Isoprene Emission  
In the literature review by [PERSON] and [PERSON] [27], isoprene biology is described as 'enigmatic'. Sixty years after its discovery, with many of its chemical reactions and biological synthetic pathways recognized, the scientific community still does not fully understand why plants evolved to emit isoprene in the first place, and indeed why they continue to do so. When categorizing isoprene-emitting species, a simple pattern is difficult to decipher. Many closely related species can differ in their emissions or capacity to emit altogether; European oaks, for example, produce insignificant levels of isoprene, whereas their closely related American counterparts are high emitters of the species [28]. One of the benefits of understanding the biological function of isoprene could be to help scientists better explain these interspecies differences. However, theories as to why plants emit isoprene have been emerging for years, with little definitive evidence for one argument over another [7]. The emission of isoprene is a metabolic cost for plants (starting from carbon dioxide (CO\({}_{2}\)), 20 ATP and 14 Nicotinamide adenine dinucleotide phosphate (NADPH) per isoprene molecule [4]), but benefits may outweigh the cost, especially under high temperature [29] and oxidative stress [30]. A popular hypothesis as to the biological function of isoprene is thermotolerance. Specifically, protection against heat specks that arise when sunlight strikes a leaf and causes the surface to rapidly warm. A correlation between light exposure and isoprene emission was first reported in 1966, although this evolved into attributing more specifically the effect of the thermal excitation from light 30 years later [31,32]. This is consistent with findings that trees at the top of rainforest canopies emit the highest concentrations of isoprene, as they have to tolerate the highest radiation levels from the sun [33]. It was through this connection between daylight and isoprene emission that the direct relationship to photosynthesis was also established [5]. Isoprene protection against reactive oxygen species is another popular theory. Ozone (O\({}_{3}\)) has been shown to damage the photosynthetic apparatus in plants, with an increase in external isoprene concentration counteracting the detrimental effect [34; 35]. Similarly, it was found that the presence of isoprene in combination with nitric oxide (NO) could also be used against oxidative stress in the form of ozone or singlet oxygen [36]. Although, similarly to thermotolerance theories, the exact mechanisms of protection are not known, and counterarguments to these conclusions are also reported [37]. During the use of isoprene as an insect repellent, it has been shown to interfere with the chemoreceptors in some species of parasitic wasps, hindering their ability to locate the plant they sought to attack [38]. In a species of tobacco plant, isoprene has been shown to act as a deterrent to its damaging inhabitant, the tobacco caterpillar [39]. The relationship between animal and plant will not be discussed further, but it is worth mentioning its significance, as plants may have evolved isoprene emission for protection against a variety of stressors. Through the dependence of isoprene on biogeochemical and biophysical properties, isoprene emissions are directly related to land cover [40], and so accurate land cover inputs are vital for biogenic emission models [1; 2; 41]. Naturally, the highest emissions of isoprene are found between the tropics over the landmass where most of the Earth's biomass is located. The isoprene fluxes from different types of vegetation are limited, and this imposes constraints on model development. The Model of Emissions of Gases and Aerosols from Nature (MEGAN) global database developed by [PERSON] et al. [42] with updates highlighted in [PERSON] et al. [2] and [PERSON] et al.