Why do bananas fluoresce?

Method

  1. Take a slightly overripe banana, showing necrotic black spots
  2. Put it in a dark environment, e.g. a darkened room, a box
  3. Shine a UV light on it (the quality of the UV light has a big effect on how visible the fluorescence is*). You will see fluorescent halos glowing around the black spots.

Background

Chloroplasts contain up to 75% of the nitrogen content in photosynthetic tissues. Carefully co-ordinated breakdown of the chlorophyll within chloroplasts allows an efficient remobilization of nutrients to growing organs. This demo offers an intriguing way to show chlorophyll breakdown in the lab.

Asleaves age, chlorophyll is converted to colourless chemicals called NCCs (Nonfluorescent Chlorophyll Catabolites). On their way to becoming NCCs, chlorophyll goes through a brief state where they fluoresce –Fluorescent Chlorophyll Catabolites (FCCs). (Reminder – fluorescence involves taking in light at a shorter wavelength, such as UV, and emitting it at a longer wavelength).

In the overwhelming majority of plants, chlorophyll will pass through the FCC stage very quickly. However, in bananas, the FCCs are much more stable, resulting in fruit and leaves thatfluoresce under UV light as chlorophyll is broken down in the cells. In particular, we can see rings of bright blue FCCs as halos around necrotic areas. This halo shows the chlorophyll being broken down before the cell dies and the black patch spreads.

Researchers are trying to fully understand the process of chlorophyll breakdown. However, no-one has an answer as to whether stable FCCs confer an advantage on banana plants, particularly as NCCs appear to have useful antioxidant capacities.

Currentunderstanding is that “During senescence and remobilization of chloroplast nutrients, Chlorophylls are uncoupled from Chlorophyll-binding proteins within the photosystem and become phototoxic. Therefore, coordinated Chlorophyll breakdown maintains cell viability during senescence and allows an efficient remobilization of nutrients to growing organs” (‘Mechanism and significance of chlorophyll breakdown’, Christ B. and Hortensteiner S., Journal of Plant Growth Regulation, published online 26 November 2013)

For more quick and simple plant science demos, see

*We find that UV lamps illuminating in the range 385nm work best for this practical.These are often used for illuminating Green Fluorescent Protein (rather than for looking at bank notes etc.). We’ve had good visibility of the FCCs with LED UV keyfob lamps; these are typically available for around £5 each from retailers. The National Centre for Biotechnology Education (NCBE) sell a pack of two UV key fobs for around £10 (see ).