Not only Phalaenopsis

Orchids

With about 28.000 species around the globe, the orchid family (Orchidaceae) is one of the most diverse families of flowering plants.

Although they mainly occur in tropical areas, with species characterized by eye-catching flowers and epiphytic growing, such as those belonging to the Phalaenopsis, Dendrobium or Cattleya genera, many species can be found in the Mediterranean region as well.

In Italy there are about 230 species, all terrestrial (or geophytes), which develop following an annual cycle.

Thanks to their perennial organs (such as bulbs), these plants can survive during the summer months: the above-ground part of the plant gets dry and it reappears in the autumn months, to bloom later in the following spring-early summer.

Despite the high number of species and environments that these plants are able to colonize, orchids can be extremely rare and they are among the organisms most threatened by anthropic activities and environmental changes, due to very close relationships that they establish with other organisms in the ecosystem.

Mechanisms for attraction and pollination

Orchids typically establish exclusive relationships with their pollinators. These are usually bees, wasps and flies, but also moths, butterflies, midges or, for tropical species, birds.

The attraction of the pollinators that visit orchid flowers and carry their pollinia (pollen masses) from one flower to another often involve complex and deceptive strategies.

Orchids commonly use nectar to attract their pollinators, but they can also simulate its production by means of bright colors, shapes or fragrances and even by imitating the flowers of other plants.

Other orchids deceive their pollinators by imitating the appearance and scents produced by insects. In Ophrys species, the flowers have a labellum that mimics the body of a female bee or wasp, in the shape, the iridescent colors, or colored spots and hair.

They also release an odour that simulates the pheromones produced by receptive females. When the male lands on the flower, it grabs the labellum and tries to copulate. During this process, the flower deposits the pollinia onto the insect, so that they will be transported and deposited onto the next flower.

Once pollinated, the flowers produce a capsule that can contain tens of thousands of microscopic seeds, so small that they are called “dust seeds”.

The mycorrhizal symbiosis

Such small seeds, about half a millimeter in length, are almost lacking nutritional reserves, so they are unable to germinate on their own and to develop into seedlings.
This limitation is, however, overcome by the association with specific soil fungi (“orchid mycorrhizal fungi”) which establish a symbiosis with the orchid. Thanks to this symbiotic association, water, organic carbon and minerals are provided by the fungus, allowing the germination of the seeds and the development of the protocorms (the heterotrophic initial growth stage of all orchids).

The symbiosis is active in the adult plants as well, both in the achlorophyllous species (such as Neottia nidus-avis) and in the species able to perform photosynthesis. The orchids depend on the organic carbon provided by the symbiotic fungi, and this nutritional strategy is called mycoeterotrophy.

In vitro reproduction

Since habitat degradation can limit the presence of pollinators and/or mycorrhizal fungi, the reproduction of orchids by laboratory methods may be the only solution for increasing and strengthening natural populations.
Indeed, seeds can germinate under sterile and controlled laboratory conditions, even in the absence of the symbiotic fungus, by using specific culture media containing all the nutrients that would be naturally provided by the fungus.

Alternatively, mycorrhizal fungi, previously isolated from the roots of adult orchids and cultivated in the laboratory, can be used for the symbiotic germination of the seeds. The “in vitro” assisted seed germination and propagation technique allows to produce large numbers of orchids ready to be reintroduced into their natural habitat.