Background/Question/Methods Environmental factors such as soil moisture, temperature and nutrients influence the chemical composition of plant tissues. Resource limitation and stress could cause plants to invest more in secondary metabolites used for defense, including low molecular weight monomeric compounds (phenolics, flavanoids) and large polymeric compounds (lignin, tannins). Though all tannins are composed of polyphenols, their reactivity differs according to the proportions of different subunits, the nature of cross-linkages joining the subunits, and their degree of polymerization. Hence, the biological function of tannin could be structure-dependent. Since tannins are expensive to produce, plants might attain better protection efficiency by producing tannins of different quality, rather than increasing tannin 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 well understood, qualitative changes in the structural chemistry of these metabolites are not. We hypothesized that climatic stress induces the production of polyphenols that are structurally different from the constitutive polyphenols, and this difference in structural chemistry will confer a higher reactivity to these stress induced polyphenols. We tested our hypotheses in the Boston Area Climate Experiment (BACE), which exposes an old-field community to four levels of warming (up to 4^oC) and three levels of precipitation (ambient, -50%, and +50% of growing season precipitation) in a factorial design. The difference in structural chemistry of tannins of Acer rubrum were identified based on total content of polyphenols, proportion of hydrolysable to condensed tannins, degree of polymerization, and structural characterization using mass-spectrometry, carbon-13 nuclear magnetic resonance (13C NMR) spectroscopy and Fourier-Transform-Infrared-Spectrometry (FTIR). The structural variations were related to biological reactivity based on protein precipitation assays using various soil enzymes.

Results/Conclusions The tannin yield from stressed treatments (high temperature + drought) was twice as high as from ambient treatments. However, the total Folin polyphenols did not show any significant difference among the various climatic treatments. The condensed tannins (CT, acid-butanol) were high in wet treatments, where as the hydrolysable tannins (HT, methylation-HPLC) were significantly higher than ambient in drought stressed treatments. The abundance of HT was further confirmed by the 13C NMR, and the CT in stressed treatments was predominantly composed of procyanidins. The tannins from drought stress treatments were highly reactive and precipitated significantly higher amount of soil enzymes. The effect of the above structural alterations in soil nutrient cycling will be discussed.