There are, I’m told, some people who prefer decaffeinated coffee to my normal beverage (triple espresso), and while I don’t personally understand that, there’s no doubt that the market for decaf is big. But removing the caffeine from coffee beans can be expensive, not to mention altering the taste of the delicious java, so maybe it’s better not to put it in there in the first place?
A recent paper in AoB PLANTS examines a new variety of naturally decaffinated coffee and describes how the flowers differ from the parent strain by lacking sticky colleters. Colleters are little hair-like organs which are thought to protect dormant coffee flower buds from dehydration during the dry season, and could also play a part in resistance to insect damage. In normal flowers, the viscosity of the colleter exudate seems to hold the petals together, acting as an adhesive, and does not allow them to open until they absorb water, swell and can then overcome the barrier imposed by the exudate. Furthermore, the exudate seems to protect against dehydration through the formation of a thick layer on the young flower buds, which have numerous stomata on the external petal surface. This is the first direct evidence for a functional role of colleters and their exudate.
The low caffeine Decaffito® plants have very low caffeine content in all tissues, and this characteristic is associated with precocious flower opening. Similar to natural mutants of Coffea arabica, Decaffito plants accumulate theobromine, indicating a metabolic blockade of the last step of caffeine biosynthesis. Although it is still not clear what controls caffeine biosynthesis in Decaffito coffee mutants, the associated and undesirable precocious flower opening characteristic provides the first functional proof of the role of colleters and their exudate in protecting flowers against exposure to dry atmospheres and acting as an adhesive to keep the petals united until anthesis.
A functional role for the colleters of coffee flowers. AoB PLANTS (2013) 5: plt029 doi: 10.1093/aobpla/plt029
Colleters are protuberances or trichomes that produce and release an exudate that overlays vegetative or reproductive buds. Colleters have a functional definition, as they are thought to protect young tissues against dehydration and pest attack. Decaffeinated coffee plants, named Decaffito®, have recently been obtained through chemical mutagenesis, and in addition to the absence of the alkaloid, the flowers of these plants open precociously. Decaffito mutants exhibit minimal production and secretion of the exudate by the colleters. We compared these mutants with normal coffee plants to infer the functional role of colleters and the secreted exudate covering flower buds. Decaffito mutants were obtained by sodium azide mutagenesis of Coffea arabica cv. Catuaí seeds. Wild-type plants were used as controls and are referred to as Catuaí. The flower colleters were analysed by scanning and transmission electron microscopy in addition to histochemical analysis. Histochemical analysis indicated the presence of heterogeneous exudate in the secretory cells of the colleters of both variants of coffee trees. Alkaloids were detected in Catuaí but not in Decaffito. Transmission electron microscopy revealed that the secretory cells in the Catuaí colleters possessed the normal and common characteristics found in secretory structures. In the secretory cells of the Decaffito colleters, it was not possible to identify any organelles or even the nucleus, but the cells had a darkened central cytoplasm, indicating that the secretion is produced in low amounts but not released. Our results offer a proof of concept of colleters in coffee, strongly indicating that the exudate covering the flower parts works as an adhesive to keep the petals together and the flower closed, which in part helps to avoid dehydration. Additionally, the exudate itself helps to prevent water loss from the epidermal cells of the petals.