Credit: ©2008 Wiley

Leaves and fruits such as bananas, apples or pears lose their green colour during the natural ageing process when chlorophyll is broken down into non-fluorescent products (known as non-fluorescent chlorophyll catabolites or NCCs). Fluorescent ones (FCCs) are also very briefly formed during the degradation, but only as intermediates — they undergo rapid isomerization to form non-fluorescent ones, and are hardly detectable.

Bernhard Kräutler and co-workers at the University of Innsbruck in Austria, however, have now identified1 an FCC in the leaves and peel of ripe bananas, after noticing that they were blue under UV light. Fluorescence increases with the accumulation of the FCC within the peel during the ripening of green bananas, reaches a maximum for yellow bananas, and then decreases on further ripening.

The team, in collaboration with a group at Columbia University in New York, showed that the temporary accumulation of the FCC is due to a unique structural characteristic. Unlike all natural plant catabolites identified so far, which have always included a carboxylic acid group, this one features an ester. This ester hinders the isomerization reaction, preventing the rapid transformation of FCCs into NCCs and thus enabling the fluorescent catabolite to accumulate during the ripening process. Kräutler and co-workers envisage that the FCC accumulation could act as a signal to animals with UV vision that a fruit is fit for consumption.