Developing a method to measure UV-B flux through time using aromatic compounds contained in fossil pollen and leaf cuticle (PARASOL)

Plants can get a suntan, too!

Increased exposure to ultra-violet radiation (UV-B) is known to have a host of effects on human health, crops, terrestrial ecosystems and biogeochemical cycles. It is also known that there will have been large variations in the amount of incoming UV-B flux throughout Earth's history in response to super-volcanic events and variations in solar input. Modelling estimates suggest that in some intervals in Earth's history (e.g. end of the Permian), incoming UV-B flux could have been up to 80% higher than present. If these estimates are correct then the impact on terrestrial ecosystems could have been profound, affecting all aspects of ecosystems from biomes through to genes, potentially altering the mode and tempo of evolution.

Up until now it has not been possible to reconstruct an accurate measure of UV-B flux through time, but recent research has shown that the measurement of aromatic compounds in the wall of pollen grains of Pinus spp can be used to produce reconstructions of incoming UV-B radiation over the past ca. 10,000 years. Production of these compounds is thought to act like a suntan - a protection mechanism for plants after exposure to increased UV. Crucially, these compounds are preserved in the pollen grains and cell cuticles buried in lake sediments. PARASOL aims to investigate whether this 'pollen grain suntan' can be used to reconstruct UV from the pine pollen in sediments over thousands of years. We have used pine pollen and pine needles because pine is a common species that produces much pollen, which is necessary for the analysis of fossil samples.

In PARASOL, we have spent a lot of time honing the analysis in the laboratory to achieve as accurate as possible estimates of how much coumaric acid there actually is in each pollen grain. This detailed work is important for developing a precise and repeatable method that can give us good quantitative estimates of past UV-B radiation.

We have also examined if the production of coumaric acid in pollen is genetically determined, or if it varies from year to year, or maybe even within years, depending on the incoming UV-B radiation. We did this by means of an experiment in which we wrapped some pollen cones in shade cloth in the month prior to pollen dispersal. We also compared pollen from the same trees in two different years under different UV-B irradiance. We found that coumaric acid levels in pollen are determined by the current year's UV-B radiation, and further that it is really only the last two weeks before pollen release that matters. This was confirmed in another study where we compared pine pollen from 19 botanical gardens across Europe. We chose the gardens to provide the greatest possible variation in temperature, precipitation, and UV-B radiation. It turned out that variation in UV-B radiation the last two weeks before pollen release could explain the variation in coumaric acid content of the pollen, while the average UV-B radiation through the past year or over many years, had no effect.

The methods developed in PARASOL provide an important basis for being able to reconstruct past UV-B radiation. The aim is to contribute to understanding how variation in UV-B radiation over millions of years has influenced and helped shape biodiversity and ecosystems on Earth.