Preserving leaves for tannin and phenolic glycoside analyses: A comparison of methods using three willow taxa

Members of the Salicaceae often produce phenolic glycosides and condensed tannins. There is much debate on the best method for the preservation of leaf material prior to chemical analysis. Published results indicate freeze-drying, a method commonly used for tannin analysis, may be inappropriate for phenolic glycosides, unless done in a manner to prevent thawing during the drying process. Another commonly employed method, air-drying, is appropriate for phenolic glycosides but inappropriate for condensed tannins. I present evidence using willow leaves that demonstrates that: (1) leaves freeze-dried in external flasks without temperature control contain lower concentrations of phenolic glycosides (salicortin and 2'-cinnamoylsalicortin), (2) air-dried leaves have reduced concentrations of condensed tannins, while (3) vacuum-dried fresh leaves have high concentrations of both phenolic glycosides and condensed tannins. Freeze-drying caused salicortin and 2'-cinnamoyl salicortin concentrations to drop by 20 mg/g and 4 mg/g, respectively. Salicin, a product of salicortin and 2'-cinnamoyl salicortin degradation, is absent in vacuum-dried leaves, present in air-dried leaves and very high in freeze-dried leaves. Thus, the presence of salicin in this system is an artifact of the preservation technique. Condensed tannin concentrations dropped nearly 20 mg/g when leaves were air-dried. Thus, vacuum-drying fresh leaves allows researchers to quantify phenolic glycosides and condensed tannins from the same leaf material.