Assessment of age and greenness of herbarium specimens as predictors for successful extraction and amplification of DNA

Age and the greenness of leaves have been frequently used as indicators for selecting herbarium specimens for molecular studies. Although plant DNA extraction and amplification have been common lab procedures for the past 20 years, no studies specifically investigated the success of these indicators. Here the predictive value of age and the greenness for extraction and amplification success is assessed, using a large number of herbarium specimens from different plant groups. The investigation of these indicators is important because herbarium material is a precious commodity, and is often the only remaining floral record of now extinct ecosystems. In cases where little leaf material is available, most researchers still attempt to extract DNA. This study shows that age and greenness of leaves are unreliable indicators of extraction and amplification success, although together they can have limited usefulness. Furthermore, we found that the amount of extracted DNA from herbarium specimens decreases with c. 1% per year in age of the specimens. Therefore, researchers sometimes should refrain from using old rare specimens because chances of success are unpredictable and precious herbarium material might be wasted. The uncritical use of indicators such as age or leaf colour is therefore not recommendable. Furthermore, botanists should annotate how specimens were collected and dried because this information is essential for successful DNA extraction. Hopefully, similar studies will be reported in order to identify the best approaches to extract DNA from herbarium specimens.