Environmental factors such as soil moisture, temperature and nutrients influence the chemical composition of plant tissues. Since phenolic compounds account for 30% of carbon in the biosphere and regulate the microbial degradation of biomass, any changes in their composition has potential to influence soil C cycling. Lignins and tannins are hetero-polymeric phenolic compounds that defend plants from pests and pathogens. The protective functions of tannins and lignins change according to the localization, proportions of different subunits, the nature of cross-linkages joining the subunits, and their degree of polymerization. Hence, the biological function of these polymers could be structure-dependent. Since tannins and lignins are expensive to produce, plants might attain better protection efficiency by producing hetro-polymers of different composition, rather than increasing their quantity. Alterations in quantity and quality of polymeric compounds can affect various ecosystem processes, including litter decomposition and soil nutrient cycling. Although stress-induced changes in the quantity of secondary metabolite production are understood, changes in the structural chemistry of these metabolites are not. We studied the quantity, composition and organization of lignins and tannins in senesced leaves of Quercus rubra, exposed to different precipitation treatments (Drought, Ambient, and Wet) at the Boston-Area Climate Experiment (BACE) in Massachusetts, USA.
Results/Conclusions
Across the species, the lignin content was less affected by the precipitation treatments, whereas the content, composition and association of tannins varied with precipitation. In general the senesced leaves from the drought treatment had higher content of tannins. In contrast, under favorable conditions (Wet), tannins were produced in lower quantities and the CTs were highly polymerized. Further, even as the overall tissue tannin content declined, the content of hydrolysable tannins increased in the Wet treatments. The molecular composition of tannins influenced their overall quantity in senesced tissues, with treatments that produced condensed tannins with higher degree of polymerization resulting in greater tissue tannin content. Our results suggest that, though climate directly influenced the production of tannins in green tissues, the influence of climate on tannin content of senesced tissues was partly mediated by the climatic effect on the chemical composition of tannins. These different climatic impacts on senesced leaves may alter forest dynamics, not only in decomposition and nutrient cycling dynamics, but also in herbivory dynamics