Getting the Most Out of Herbaria: Systematics and Chemistry

2 Florid

Murder Most Florid by Mark Spencer, London: Quadrille, 2019

As mentioned in the last post, herbaria, both real and virtual, are most frequently visited by taxonomists, who are usually studying particular plant taxa or preparing flora of areas ranging in size from city parks to entire countries.  These are the traditional uses of plant collections and are still crucial.  However, several things have changed.  Now the “visit” is often to digital portals rather than onsite, making it much easier for researchers to look at specimens from far-flung institutions, IF the material has been digitized, and particularly if it is available through aggregators such as iDigBio, GBIF or JSTOR Global Plants with their links to massive numbers of specimens.  Still, coverage is uneven, with some collections more fully digitized than others.  Also changed is the way taxonomic information, once generated, is distributed.  Many flora are now published virtually, with or without an accompanying paper format.  The 2012 International Code of Nomenclature for Algae, Fungi, and Plants made it acceptable to publish descriptions of new species digitally as long as they were responsibly published and properly archived.

Plant taxonomy is also changing because of its increasing links with genetics.  Most treatments of species and genera now include DNA sequencing data.  While this has been going on for decades, the last ten years or so have seen greater use of DNA data derived from samples taken from herbarium specimens, with NGS, next-generation sequencing (NSG) making this possible. NGS techniques utilize small pieces of degraded DNA found in dried plant material easier to sequence and to determine how such sequences fit together to provide meaningful results.  That this work has revolutionized taxonomy is hardly news.  Still, it is interesting to look at how the information has solved various puzzles, such as the origin of European potatoes or the origin of the pathogenic Phytophthora strain responsible for the Irish potato famine of the 1840s.  In a study of the genetics of grapes, researchers used over 200-year-old specimens from the herbarium at the Royal Botanical Garden in Madrid.  These plants were collected by Simón de Rojas Clemente y Rubio, considered one of the founders of the botanical study of grape vines, especially varieties used in wine-making.

DNA is not the only chemical being extracted from specimens to glean useful information about plants and also about their ecological relationships.  For example, researchers in Copenhagen tested specimens of four species of Salvia used for medicinal purposes for levels of terpenoids, known to have medicinal applications.  These plants were collected over the past 150 years.  While the terpenoid levels did decrease with the specimen’s age, the “chemical composition of four Salvia species are predominantly defined by species, and there was a substantially smaller effect of year of sampling.  Given these results, herbarium collections may well represent a considerably underused resource for chemical analyses.”  Also being investigated are secondary metabolites that plants produce to control herbivore damage.  In one study researchers were able to extract pyrrolizidine alkaloids from plants in the Apocynaceae family that includes milkweed.  The specimens were as much as 150 years old, and even in those treated with alcohol or mercuric chloride, alkaloids were detectable.

There has also been work on the presence of heavy metal pollutants in collections as a way of tracking contamination.  A study at Brown University in Providence, Rhode Island analyzed samples from specimens collected around the city from 1846 to 1916, compared with newly collected ones.  Levels of copper and zinc remained relatively consistent, but lead levels were much lower in plants growing in Providence today.  It was impossible to test accurately for another toxic heavy metal, mercury, because mercuric chloride was so often used to prevent insect damage to specimens.  While toxic metals in plants might make them seem less palatable as food sources, there is an emerging field of agromining:  growing plants that are hyper-accumulators of metals like lead and mercury to eventually reduce soil contamination.  Herbarium specimens can be used to discover how long areas have been contaminated and also to identify species that are particularly good at extracting metals.  There are even some who think that growing plants in nickel-rich soil could be a way to extract this metal for sale.

Such studies suggest that the possible uses of specimens are only limited by the ingenuity of researchers in coming up with them.  It is fun to see what they can ferret out.   The British botanist Mark Spencer recently published a book on his work as a forensic botanist.  It has a great title:  Murder Most Florid (2019).  He was at the herbarium at the Natural History Museum, London curating the British and European collections when he was first asked by the police to aid in a murder investigation.  Human remains have been found in a forested area and had apparently been there for several years.  Would he be able to determine the time more precisely by studying plants at the site?  I don’t want to spoil this story or the other great ones in the book, but I will say that Spencer explains why a herbarium is essential for the work he does, now that he has become much more involved in forensics.

Reference

Spencer, M. (2019). Murder Most Florid: Inside the Mind of a Forensic Botanist. London, UK: Quadrille.