A growing body of evidence suggests that plants alter their chemical composition in relation to the amount of incoming solar radiation (“insolation“) they are exposed to during life. Chemical changes are induced in order to provide protection against the deleterious effects of ultraviolet (UV) radiation; a relatively small, but important component of the total solar spectrum. UV radiation is linked with a range of detrimental biological effects, primarily stemming from damaged DNA. As sessile organisms, plants need to employ various mitigation mechanisms to prevent/reduce damage induced by UV radiation. Such mechanisms include effective DNA repair pathways, physiological adaptations, and UV-absorbing compounds. It is this last mechanism, UV-absorbing compounds (UACs), that is discussed here.
UV-absorbing compounds can be found in most plant tissues, but are concentrated in regions that are subject to the greatest exposure to insolation. In living tissues, ie leaves, UACs concentration varies over the course of a day, and can vary on the order of hours (Bornman 1999). However, tissues that form the outer layer of spores and pollen grains do not have the facility to change chemical configuration once detached from the parent plant. Any chemical protection incorporated into the tissues of spores/pollen must be sufficient to prevent damage to DNA in order to increase the chance of reproductive success. Such a passive chemical protection mechanism can be interrogated analytically. By analysing for the chemical signature indicative of UACs it is possible to understand the chemical response of plants to environmental stress, manifest as UV exposure. Demonstration of the viability of measuring UAC content of spores and pollen to understand UV, or more broadly Total Solar Input (TSI), has been achieved by investigating environmental situations where UV/TSI is known to vary. For instance across elevation gradients (Watson et al., 2007), under different shading conditions (Fraser et al., 2011), and through time (Lomax et al., 2008). Overall, we find that UAC concentration in spores varies proportionally with TSI; By applying our analytical technique to sedimentary records containing spores/pollen (“palynomorphs“) we can start to build a picture of past insolation. The need for empirical data on past insolation changes is important because until now reconstruction of past insolation has only been possible using mathematical modelling methods.
Work currently being conducted by researchers at The Open University is analysing the UAC content of plant spores and pollen locked within a tropical lake core, spanning the past 500,000 years. This analysis will provide the longest record of UACs through time. From this UAC record it will be possible to reconstruct TSI for the past 0.5 million years; something that has not been possible until the development of our analytical techniques.
Further updates on progress of this work will appear on these pages over the coming months. This work is being conducted by Drs Wesley Fraser, William Gosling, Barry Lomax (University of Nottingham) and Charlotte Miller.
Bornman, J.F. (1999) Localisation and functional significance of flavonoids and related compounds, in: Stratospheric ozone depletion; The effects of enhanced UV-B on terrestrial ecosystems (ed: Rozema, J.). Backhuys publishers, Leiden, The Netherlands. p.59-69.
Fraser, W.T., Sephton, M.A., Watson, J.S., Self, S., Lomax, B.H., James, D.I., Wellman, C.H., Callaghan, T.V. & Beerling, D.J. (2011) UV-B absorbing pigments in spores: Biochemical responses to shade in a high-latitude birch forest and implications for sporopollenin-based proxies of past environmental change. Polar Research 30, 8312.
Lomax, B.H., Fraser, W.T., Sephton, M.A., Callaghan, T.V., Self, S., Harfoot, M., Pyle, J.A., Wellman, C.H. & Beerling, D.J. (2008) Plant spore walls as a record of long-term changes in ultraviolet-B radiation. Nature Geoscience 1, 592-596.
Watson, J.S., Sephton, M.A., Sephton, S.V., Beerling, D.J., Self, S., Fraser, W.T., Lomax, B.H., Gilmour, I. & Wellman, C.H. (2007) Rapid determination of spore chemistry using thermochemolysis gas chromatography-mass spectrometry and micro-Fourier transform infrared spectroscopy. Photochemical & Photobiological Sciences 6, 689-694.