Microwave drying of plant material for herbarium specimens and genetic analysis

Abstract

AbstractMicrowave drying of plant specimens is a potentially useful alternative to chemical and traditional air drying preservation techniques because microwave ovens are widely available, and the method is quick and inexpensive and kills pests without leaving toxic residue. We tested the hypotheses that (1) there would be a difference in the concentration of genomic DNA that could be extracted from microwave–dried compared to silica–dried specimens, (2) the effects of microwave treatment would be observable even when the specimen had been first dried in silica gel crystals, (3) the effects of microwave drying would continue to be observable in PCR product concentration using chloroplast, nuclear ribosomal, and nuclear markers, and (4) microwave drying would cause contamination of the nucleic acids by cell material, and this contamination would be reflected in the spectral absorbance of the extracted genomic DNA. In a paired comparison of silica–dried, microwave–dried, and silica+microwave–treated leaves of Brosimum alicastrum (Moraceae) we found that microwave drying produced specimens with bright leaf colour, which to date have not suffered from insect or fungal attack. However microwave treatment significantly reduced the concentration of extractable genomic DNA, even when the leaves had been previously dried in silica gel, and reduced the concentration of PCR product produced using chloroplast and nuclear ribosomal but not nuclear markers. The genomic DNA extracted from both the microwave–dried and silica+microwave–treated material appears to contain some cellular contaminant which absorbs at 230 nm, indicated by low 260/230 nm absorbance ratios. We suggest that microwave drying is a useful alternative to chemical preservation techniques which limit the capacity for DNA extraction from preserved material. However microwave drying is less desirable than traditional air– or oven drying or the combination of traditionally dried herbarium specimens plus companion samples dried in silica gel crystals.