Isoprene is a naturally occurring volatile organic compound (VOC) with the chemical formula CH2=C(CH3)−CH=CH2. It is a branched-chain unsaturated hydrocarbon, meaning it contains double bonds between carbon atoms. Isoprene is produced by a wide variety of organisms, including many species of trees, where it is released into the atmosphere. This compound is a fundamental building block for a vast class of organic compounds known as isoprenoids, which are found in both plants and animals.
Why Trees Produce Isoprene
Trees produce isoprene primarily as a defense mechanism against environmental stresses. One significant role is in protecting against heat stress, a process known as thermotolerance. Isoprene emission increases substantially with rising temperatures, peaking around 40°C, helping stabilize cell membranes within leaves that can be disrupted by high temperatures.
Beyond heat protection, isoprene also acts as an antioxidant, combating oxidative stress. This stress arises from reactive oxygen species (ROS) like ozone, which damage plant cells and impair photosynthesis. By reacting with and quenching these harmful molecules, isoprene reduces lipid peroxidation and maintains photosynthetic capacity.
Isoprene biosynthesis occurs through the methyl-erythritol 4-phosphate (MEP) pathway within chloroplasts. Dimethylallyl pyrophosphate (DMAPP), one of the MEP pathway’s end-products, is converted into isoprene by the enzyme isoprene synthase. Inhibitors of the MEP pathway, like fosmidomycin, can block isoprene formation.
Isoprene’s Role in Atmospheric Chemistry
Once released by trees, isoprene undergoes rapid chemical reactions in the atmosphere, influencing air quality and climate. It is highly reactive, with a typical atmospheric lifetime of less than an hour, especially when reacting with hydroxyl radicals (OH).
Isoprene’s atmospheric chemistry contributes to ground-level ozone formation. In the presence of nitrogen oxides (NOx) and sunlight, isoprene’s oxidation products react to form ozone. This process degrades air quality, particularly downwind of major isoprene-emitting forests.
Isoprene also plays a role in secondary organic aerosol (SOA) formation. These tiny particles in the atmosphere affect visibility, influence cloud formation, and impact Earth’s radiative balance. Global production of isoprene-derived SOA is substantial, with contributions from compounds like IEPOX.
Factors Influencing Isoprene Release
Several environmental factors influence isoprene release. Temperature is a primary driver, with higher temperatures leading to increased emissions. Rates can increase exponentially, often maximizing around 40°C. This temperature dependence underscores its role in thermotolerance.
Light intensity also plays a substantial role, as emissions are light-dependent. Photosynthetically active radiation (PAR) impacts isoprene production. The tree’s physiological state, including leaf age and stress levels, also affects emission rates. Growing and mature leaves have the highest emissions, while very young or old leaves emit less.
Carbon dioxide (CO2) concentration can also influence emissions, though the relationship is complex. Seasonal variations, such as dry seasons with higher solar radiation and lower water availability, can lead to increased emissions from some tropical plants. A tree species’ genetic makeup is the fundamental controlling factor for its emission potential.
Common Isoprene-Emitting Tree Species
Not all tree species produce isoprene, and emission rates vary widely. Some tree genera are significant emitters. Prominent examples include oaks (Quercus), poplars (Populus), eucalyptus, willows (Salix nigra), and sweetgum (Liquidambar styraciflua).
Studies show some species within these genera, such as Populus hybrids and specific oak species, have high emission capacities. Their emission rates can be considerably higher than previously reported. The capacity for isoprene emission is a genetic trait, meaning it is inherent to certain tree genera and species.