Pollination and fragrances in the tribe Maxillarieae


Günter Gerlach is a botanical curator at the Botanical Garden Munich in Germany. His research topics are broadly arranged: from systematics and taxonomy of the Stanhopeinae and Zygopetalinae going over to pollination biology, analysis and chemistry in the involved fragrances and floral oils of several neotropical plant groups and the ecology of the pollinating bees
   


ON: Dr. Günter Gerlach, how long have you been studied orchids and, particularly, fragrances and pollination?
GG: I started as orchid gardener in 1979 in the botanical garden of Heidelberg in Germany, worked there in this position 2 years and then studied biology at the same university. So I can say that I am studying orchids for more than 25 years. My fragrance investigations started with my doctoral thesis nearly 20 years ago. Looking for the pollination biology was the consequent next step immediately after having the first positive results in fragrance investigation.

ON: Your lecture concerned the "Pollination and fragrances in the tribe Maxillarieae". Could you develop a little this question? Which subtribes are circumscribed in the Tribe Maxillarieae?
GG: In the tribe Maxillarieae are included the subtribes (after Dressler 1993): Cryptarrheninae, Zygopetalinae, Lycastinae, Maxillariinae, Stanhopeinae, Telipogoninae, Ornithocephalinae, Oncidiinae, all in all more than 2500 neotropical orchids. The species within this tribe belong to different pollination syndromes which means that they use different classes of pollinators (female bees, male bees, flies) appealing at their respective behaviour. The communication between the flowers and their pollinators in many cases is effected by the floral fragrances.

ON: All species included have a scent?
GG: No, there are a lot of genera and species without perceptible floral scent. Floral scent always is an information for the respective pollinator, like there are the colour or form too. Floral scents could be very strong especially in the species belonging to the perfume flower syndrome, which means that the pollinators are attracted and rewarded by the fragrance compounds. Only here, male bees collect the fragrance compounds. Within Tribe Maxillarieae different pollination systems are involved, there exist perfume offering flowers, oil offering flowers, and a lot of deceptive pollination systems.

ON: When you said that the pollination systems in Tribe Maxillarieae are, some of them, rewarding and, some other deceptive ones, what exactly the means of this affirmative?
GG: Rewarding pollination systems offer different forms of reward to their pollinators. The most common rewards are nectar or pollen. In orchids pollen is not available because of the pollinaria it would result in a complete pollen loss and so a absolute breakdown of the pollination system. Orchids offer rewards as nectar, oil, sleeping places and perfumes. Deceptive ones do not offer anything. The pollinators are deceived by the flowers having the similar colours, forms or fragrances as non-deceptive ones. In sexual deceptive flowers male bees are deceived by flowers looking like their females. So the male begins to copulate with the flower effecting pollination.

ON: So the species of this Tribe can have a kind of nectar?
GG: Only a very few species within the Tribe Maxillarieae have nectar, most species here belong to other pollinating systems.

ON: Why the fragrances serve to select the bee species?
GG: Normally the fragrances serve as a guide to the flower which the pollinator remember from a previous visit. In perfume flower syndrome however, the Euglossine bees are very selective in their fragrance choice. So an effective isolation between co-occurring plants belonging to this syndrome exists.

ON: By the way, concerning the isolation, you also affirmed that bees are so special in their preferences that result an efective barrier against hybridisation. What exactly does it work?
GG: We do not understand until now how it works, because we do not know if the respective bees do not perceive or if they do not like the special aroma. This means that until now we do not know if the selectivity corresponds to the lack of receptors in the bees antenna or in inhibitory connections in the brain.

ON: When the species are artificially crossed, what is the result concerning the fragrance?
GG: We have only a very few artificial hybrids investigated. In a crossing of two Coryanthes species the fragrance composition was intermediary.

ON: The different fragrances of flowers are enough to distinguish species even when they are very similar, what kind of information the fragrance carries? they give very informative hints for taxonomy but fail in generic level?
GG: Speaking of fragrances of flowers belonging to perfume flower syndrome the fragrance composition for me is a very good character to distinguish the species within one genus. Unfortunately this information gets lost when the flowers are conserved (liquid or dried) in herbaria. So this information is of limited help for taxonomy. In generic level it fails because we found in some different genera the same fragrance pattern. It is very probable that in this cases and when the different plants occur together both species are pollinated by the same bee species. This does not result in a intergeneric hybrid because of the position of the respective pollinaria on the bees body.

ON: Inside this tribe, which are the most scented genera and species?
GG: Within the Tribe we have a lot of species of different genera in different subtribes which have an very intense odour. A lot of Stanhopea and Coryanthes species are found among those plants with intense smell, but we also find here Anguloas, Trevorias, Cirrhaeas, Houlletias. Some of them like Stanhopea platyceras, Houlletia lowiana and Houlletia sanderi have really disagreeable fragrances.


     
     



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