Using a greenhouse experiment to test the consistency in the chemical response of Pinus sylvestris and Pinus uncinata pollen to UV-B radiation

Abstract

Previous studies have suggested that the amount of UV-B absorbing compounds (UAC) found in pollen grains increases when the plants are exposed to UV-B radiation. If this response is consistent across and within populations, pollen grains may be used as a proxy for reconstructing past levels of UV-B radiation to Earth. However, this response may vary between individuals in a population, or between populations. These variations are important to detect, as it would have to be considered in the use of pollen grains as a proxy. The purpose of this project is to investigate the chemical response of Pinus spp. pollen to UV-B radiation using a controlled greenhouse experiment. Branches with premature pollen cones from two different populations, Pinus uncinata located in the Pyrenees (Spain) and Pinus sylvestris located in Bontveit (Norway), were cut and transferred to a greenhouse in Bergen (Norway). Inside the greenhouse the samples were exposed to two different treatments: UV-B radiation and no UV-B radiation. When matured, the pollen was harvested and analyzed with Fourier Transform Infrared Spectroscopy (FTIR) and pyrolysis Gas Chromatography Mass Spectrometry (py-GC-MS) to quantify the abundance of para-coumaric acid. In addition to investigating the effects of UV-B radiation on the pollen grain chemical composition, we tested whether the absolute amount of UACs measured by py-GC-MS could be quantitatively predicted by FTIR spectra, either full spectra in a Partial Least Squares Regression (PLS) model or through the use of single wavenumbers of interest. The results show that the PLS model based on both full spectral FTIR values and py-GC-MS measurements is better at predicting the abundance of UACs than the value of the 1515cm1 wavenumber alone. In Pinus uncinata, the difference between methods was especially clear. This suggests that the most common quantification method used on the field can in some cases be quite imprecise, and that a calibration should be more utilized in the future. Although a strong, positive relationship between UV-B radiation and the accumulation of UACs has been confirmed by multiple studies in the past, this study did surprisingly not find a significant effect of the UV-B treatment. There is a weak, positive trend in both species, however this is not clear enough to conclude with a consistent response across species. The insignificant trend is probably due to the high variation between individual trees found in both species. In this study, the chemical response to UV-B radiation is thus not consistent neither within nor across species. The method of branch cutting and transport used in this study includes several factors that potentially can influence the chemistry of the pollen grains, and cannot be rejected as a possible explanation for the difference in results from other studies.