Summary Climate change could increase the frequency with which plants experience abiotic stresses, leading to changes in their metabolic pathways. These stresses may induce the production of compounds that are structurally and biologically different from constitutive compounds. We studied how warming and altered precipitation affected the composition, structure, and biological reactivity of leaf litter tannins in Acer rubrum at the Boston‐Area Climate Experiment, in Massachusetts, USA. Warmer and drier climatic conditions led to higher concentrations of protective compounds, including flavonoids and cutin. The abundance and structure of leaf tannins also responded consistently to climatic treatments. Drought and warming in combination doubled the concentration of total tannins, which reached 30% of leaf‐litter DW. This treatment also produced condensed tannins with lower polymerization and a greater proportion of procyanidin units, which in turn reduced sequestration of tannins by litter fiber. Furthermore, because of the structural flexibility of these tannins, litter from this treatment exhibited five times more enzyme (β‐glucosidase) complexation capacity on a per‐weight basis. Warmer and wetter conditions decreased the amount of foliar condensed tannins. Our finding that warming and drought result in the production of highly reactive tannins is novel, and highly relevant to climate change research as these tannins, by immobilizing microbial enzymes, could slow litter decomposition and thus carbon and nutrient cycling in a warmer, drier world.

SummaryBy inhibiting soil enzymes, tannins play an important role in soil carbon (C) and nitrogen (N) mineralization. The role of tannin chemistry in this inhibitory process, in conjunction with enzyme classes and isoforms, is less well understood.Here, we compared the inhibition efficiencies of mixed tannins (MTs, mostly limited to angiosperms) and condensed tannins (CTs, produced mostly by gymnosperms) against the potential activity of b-glucosidase (BG), N-acetyl-glucosaminidase (NAG), and peroxidase in two soils that differed in their vegetation histories.Compared with CTs, MTs exhibited 50% more inhibition of almond (Prunus dulcis) BG activity and greater inhibition of the potential NAG activity in the gymnosperm-acclimatized soils. CTs exhibited lower BG inhibition in the angiosperm-acclimated soils, whereas both types of tannins exhibited higher peroxidase inhibition in the angiosperm soils than in gymnosperm soils. At all of the tested tannin concentrations, irrespective of the tannin type and site history, the potential peroxidase activity was inhibited two-fold more than the hydrolase activity and was positively associated with the redox-buffering efficiency of tannins.Our finding that the inhibitory activities and mechanisms of MTs and CTs are dependent on the vegetative history and enzyme class is novel and furthers our understanding of the role of tannins and soil isoenzymes in decomposition.

The exudation of secondary metabolites at phytotoxic concentrations has been proposed as a mechanism of invasion for some exotic plant species. Catechin is a natural flavanoid implicated in the potential allelopathic interactions of Centaurea stoebe. However, recent studies have shown that catechin is highly unstable and not likely to accumulate in growing medium at phytotoxic concentrations. All previous studies that investigated the allelopathic potential of catechin assumed a continuous exudation of this compound by C. stoebe. Contrary to this, but similar to many other plant secondary metabolites, we hypothesized that catechin exudation may exhibit a pulsed pattern that could facilitate its transient accumulation. Further, we aimed at optimizing a more sensitive detection technique. We tested the hypothesis by quantifying the diurnal pattern of catechin release by C. stoebe in a hydroponic system. Using sample processing, based on a solid phase extraction technique, and more sensitive fluorescent detection parameters, we were able to quantify catechin in the picomolar range from the growing medium. Catechin exudation exhibited a possible diurnal rhythm with respect to light intensity, with the highest concentration at 6 h after exposure to sunlight. Catechin also was found to undergo a degradation reaction resulting in a transient abundance of pyrocatechol in our system.