A besom is a bunch of twigs tied to a handle to make a broom. In fiction and children’s stories, a witch is often depicted as flying on such a broom. However, a witch’s broom* can also refer an abnormal growth of twigs in a tree. A number of trees and shrubs (evergreen or deciduous) can develop these ‘abnormal growths’, but in the UK they are more likely to be seen on Birch.  When high up in the canopy, they may be mistaken for a bird’s nest or a ball of mistletoe.  Mistletoe is a different organism, a partial parasite, growing on the tree, whereas a witch’s broom is part and parcel of the tree itself. Normally, the buds of a tree develop in a predetermined sequence that is governed / controlled by plant hormones.  On a twig or stem, the apical bud is dominant and the lateral buds are ‘held back’.  This is under the control of the growth regulator - auxin (indole acetic acid).  Auxin slows or inhibits the growth of the lateral buds so that the apical bud is favoured.  If auxin production is impaired the regulated development of the twigs is upset so that many buds open and develop, and a multitude of closely packed twigs is formed.   Many things can interfere with auxin production, for example, physical damage to a tree, which may allow the entry of micro-organisms - for example, infection with the fungus Taphrina betulina. This fungus can affect dwarf birch, silver birch and downy birch.  Different species of Taphrina can infect hornbeam and cherry resulting in witches brooms on these trees.  Sometimes, a witch’s broom can form as a result of a mutation occurring in a growing region (meristem).  Such brooms are more often seen in certain conifers, sometimes cuttings of these are propagated to make attractive dwarf or colourful new cultivars (for example Picea abies 'Clanbrassiliana Stricta’). * or hexenbesen in German. Thanks to Torquil and Helen for images.

Super fast poplar trees ? In theory, increasing the number of trees across the globe could help reduce the levels of atmospheric carbon dioxide that are so intimately associated with climate change.   As trees grow, they absorb carbon dioxide from the air turning into wood.  This effectively locks away the carbon for a period of time.  However, the growth of trees can be slow so if trees could be ‘persuaded’ to grow faster then more carbon could be ‘locked up’ A San Francisco based company called “Living Carbon” has genetically modified hybrid poplar trees to speed up photosynthesis so that they grow faster.  They picked on the poplar trees as their genome had already been sequenced. The work of Living Carbon is at early stage and further work is needed to see if the enhanced photosynthetic rates and growth are maintained over the life time of the modified trees.  Another possible benefit of these modified trees is that they might help restore damaged land as their roots also grow faster, helping to address soil erosion.  Further details are available on their website : https://www.livingcarbon.com/ A discussion about the value / usefulness of such trees in affecting climate change / carbon sequestration was featured in the BBC program Positive Thinking this week. Effects of climate change. Climate change is associated with extreme weather events, such as flooding, heat waves and prolonged periods of drought.  American scientists at Ohio State University have been looking at the effects of higher temperatures and lower water availability on fungal infection in Austrian pines. They found that within three days of infection (by certain fungi - Diplodia species),  the warmer and drier conditions resulted in  Reduced photosynthesis by the tree Enhanced removal of carbon resources of the tree to the fungus. This means that there are fewer sugars and other metabolites available to the tree for growth (and defence against infection).   It would seem that a warming climate will render many trees more susceptible to invasion by various pathogens.   Full details of this research here.    Stressed out plants. Thale cress or Arabidopsis thaliana is a small flowering plant, that is native to many parts of Europe and Asia.  It is generally viewed as a weed, found by roadsides and on disturbed land. It is an annual that has a short life cycle. Its genome was one of the first to be sequenced. It has become a popular plant in the researching of the biology of flowering, light sensing and understanding the molecular biology of many plant traits, including flower development and light sensing.   Recently it has been used to investigate the stress response to heat, intense sun and drought.  Such stresses result in the production of reactive oxygen species (ROS).  High levels of ROS are lethal but low level of ROS are a signal to the plant to protect itself.  This is done through the photosynetic machinery which generates an ‘alarm molecule’ (MEcPP).  As this molecules accumulates in the plant, it in turn triggers the production of salicylic acid (essentially a form of aspirin).  This helps protect the chloroplast from damage.  Salicyclic acid is involved in many plant systems, notably defence against infection but also germination and stomatal physiology.

Elms were once an important part of Britain’s landscape but in the 1970’s, the Elm population was savaged by a fungal infection known as Dutch Elm disease.    Read more...