Plant records: Can a plant be too trusting?

Name: Orchids

Scientific name: Family: Orchidaceae

Known for: Beauty, epiphytism, providing vanilla flavouring, tiny seeds, mycorrhizae, deceptive pollination, mycoheterotrophy, medicinal usage, vegetating Krakatoa…

Record broken: The world’s most trusting plant…

The orchid family, the Orchidaceae, is one of the biggest flowering plant families with an estimated number of species ranging from 26,460 [Maarten Christenhusz et al.,‎ Plants of the World: An Illustrated Encyclopedia of Vascular Plants; University of Chicago Press, 2017] to >27,000]. Whilst that arguably means that there can be almost 27,000 unique ways to be an orchid, this item will have to restrict itself to generalisations. In particular, I want to celebrate one trait that orchids show supremely well, optimism. That optimism isn’t just that things will turn out OK, it’s in large part down to the trust that many orchids have in other organisms doing the right thing so that the orchid can survive. This trusting nature is exemplified in three important aspects of the biology of orchids that embrace the full life-cycle.

Starting out: in fate (and fungi!) we trust…

Most seeds are released in to the world with abundant food reserves to establish themselves as new independent plants until they can photosynthesise and make their own food. Orchids hold the world record for having the smallest seeds known. Generally, their seeds are so small that they appear like ‘fine dust’, and can weigh as little as 0.3 µg (that’s less than one third of a millionth of a gram!) for Schomburgkia undulata (Carol Baskin & Jerry Baskin, Seeds: Ecology, Biogeography, and Evolution of Dormancy and Germination, Second Edition, Academic Press, 2014) *. This extremely low mass is largely down to the fact that the orchid seed has hardly any – usually, no – endosperm (Baskin & Baskin, 2014), the food reserve tissue that more typical seeds contain. Absence of this built-in food supply means that the germinating orchid, and developing seedling, is dependent upon an external source of ready-made food. And that source is, maybe somewhat surprisingly, provided by a fungus.

Now a fungus is an organism that cannot make its own food – e.g. via photosynthesis; it requires a ready-made organic food source and is therefore heterotrophic (not autotrophic, like most green plants). How can a heterotroph supply the food requirements of an orchid? In some instances the orchid-feeding fungus may be directly exploiting organic food sources in the soil (Sarah Smith, New Phytologist 65: 488-499, 1966; doi: 10.1111/j.1469-8137.1966.tb05972.x). In others, the fungus obtains these organics from another flowering plant with which it has a mutual symbiotic relationship (Baskin & Baskin, 2014). Thus, in some cases an orchid is acting as a parasite upon another flowering plant, albeit indirectly via a fungal intermediary! There is also evidence that some orchids require presence of the correct fungus to permit seed germination in the first place (Baskin & Baskin, 2014). We thus have the situation where the survival of our orchid is not only dependent upon finding the right fungus to develop the so-called orchid mycorrhiza, but its continued survival until it is an independent, ‘self-feeding’ [autotrophic], photosynthesing individual, is dependent upon food derived from a fungus ‘nurse-maid’ – and, in some instances, ultimately, another unwitting flowering plant ‘partner’.

Even when photosynthetic, some orchids continue to obtain some of their organic carbon needs from the fungal partner and therefore have a mixotrophic style of nutrition (Baskin & Baskin, 2014). Whilst many orchids only exploit the fungal partner for food at early stages of development, some never become green and photosynthetic (and therefore autotrophic…) and ‘parasitise’ the fungal partner throughout their life (Tikhonovich & Provorov, chapter 14 in Comprehensive and Molecular Phytopathology (eds Yuri Dyakov et al.), Elsevier, 2007). Such nutritionally atypical flowering plants are termed mycoheterotrophs [Baskin & Baskin, 2014]. Such are the complexities of orchid-fungus feeding relationships that they have been viewed as predation or even parasitism on the part of the orchid (Baskin & Baskin, 2014) **.

The high life: in (and on!) trees we trust…

Whilst an appreciable number of orchids are typical plants in that they are rooted in the soil, and live quite literally at ground level, many others have adopted a much more elevated lifestyle. Such members typically can be found many metres above the soil surface perched somewhat precariously on the branches or trunks of trees. Plants living in this way are termed epiphytes. There are about 28,000 epiphytic plant species (Gerhard Zotz, Plants on Plants – The Biology of Vascular Epiphytes, Springer 2016, p. 16), and “most epiphytes are orchids and most orchids are epiphytes” (Zotz, 2016, p. 37). Almost 19,000 epiphytic orchid species are known, which not only means that the Orchidaceae accounts for almost 70% of all known epiphytes – but the same % of orchid species are epiphytes.

This epiphytic lifestyle has many challenges [Adibah and Ainuddin, Asian Journal of Plant Sciences, 10: 97-107, 2011; doi: 10.3923/ajps.2011.97.107], not least is the separation of the plant from its more usual water source, the soil. Although far-removed from the soil, roots of epiphytic orchids are still used as water-absorbing organs. Here they get their water typically from rain water or the humidity in the air that surrounds the aerial-situated orchid, employing a specialised tissue, the velamen, for this purpose ***.

One aspect of the trusting nature of these plants is exemplified in the case of leafless epiphytic orchids. Rather, than exploit other hard-working life forms and ‘steal’ their organic-rich, ready-made food, these orchids have developed photosynthetic capability in their roots. Thereby exhibiting trust in the ability of organs that evolved for one purpose taking on an entirely novel role, and doing so at rates that can sustain the rest of the plant. Another one of the great aspects of trust that the orchid demonstrates in this lifestyle is that the perch will be a stable one and not come crashing down to the floor and probably damaging – maybe even killing – the orchid. All life forms need support to help them through life, epiphytic orchids maybe more than most…

Deceptive pollination: in lust we trust…

Arguably, one of the most fascinating ways that orchids demonstrate their trust in other species is during pollination. Many orchids rely on the pollination services of insects. Whilst that’s no different to thousands of other flowering plant species, some orchids have taken this dependency to extreme levels in exploiting the insect’s own reproductive urge to serve the plant’s reproductive requirements.

To achieve the desired effect, such orchids often release chemicals into the air that smell like insect sex pheromones. These compounds are detected by male insects who, mistaking them for the real thing, seek out the female of the insect species whose presence is indicated by this olfactory clue. Following the scent signal towards the orchid, the insects then have reinforcing visual cues from the shape and size of the flower that looks like the female insect, and thus a potential mate.

Not only do these flowers smell and look like female insects, they may also have the texture, hence ‘feel’, of the real thing. The deception is so good that the hopeful male will often attempt to mate with the ‘female’. Although no insect reproduction results from this amorous encounter, the orchid’s pollen-containing structures – pollinia – become attached to the male insect’s head. The insect flies off, with pollinia firmly attached, and repeats the unsatisfactory copulation process with another orchid flower. This time, transfer of the pollinia is made from the insect’s head to the female organs of the orchid flower. Hopefully [more orchid optimism!], the transferred pollen will germinate and fertilisation will follow resulting in the creation of orchid seed to begin the next generation of ever-optimistic orchids. The hapless insect will continue its false sexual encounters with other orchids and, presumably unwittingly, help the orchid produce more offspring. So, at least one of the ‘partners’ in this ‘ménage à trois’ achieves some reproductive success ****. Appropriately, this ‘insect false-sex’ is termed pseudo-copulation.

Again, orchid success is intimately bound up with the life of another organism. In this instance orchid reproduction is tied to the sexual inclinations of its pollinating insect. But, and let us not forget, the success of this practice relies on the plant’s trust that insects will continue to be duped in their sexual endeavours by the wily orchid!

To summarise: At all stages of the life cycle orchids rely on the help of other species, whether it is fungi, insects or trees. They ‘trust’ that co-operation will continue to be provided as and when required, and in that way the optimistic orchid will survive.

Maybe what we can learn from all of this is that success of one species is – often intimately! – bound up with the success of (an)other species. Perhaps we humans should take a lesson from the ever-trusting orchids and recognise that we are likely to survive better as a species as members of a biodiverse community on this planet..?

* Not only are orchid seeds light-weight, they may be as short as 0.18 mm (Oberonia iridifolia), and up to 4 million seeds may be present in a single capsule (Cycnoches ventricosum var. chlorochilon) (Baskin & Baskin, 2014). And, rejuvenatingly (and also optimistically…), tiny, wind-borne seed of four terrestrial orchid species were among the first plants to recolonize Krakatoa, after plant life on the island was destroyed by volcanic activity in 1883 (Baskin & Baskin, 2014).

** Which also goes to show that just because something’s called a mycorrhiza need not necessarily mean it’s a mutually-beneficial arrangement for fungus and plant partners…

*** The velamen, an innovation so important that that it multi-functions as an ultraviolet radiation protector for the photosynthetic tissues of the epiphytic orchid’s aerial roots, and is also to be found in roots of non-epiphytic orchids and other terrestrial monocot plants…

**** And, although this degree of pollinatory specialisation, in which the orchid’s future is intimately tied-up with the fortunes of an insect, may look like the orchid equivalent of putting all one’s eggs in one basket, this highly-specific pollination strategy is remarkably efficient (Giovanni Scopece et al., The American Naturalist 175: 98-105, 2010; doi: https://doi.org/10.1086/648555).

Finally, if you want more orchids in your life (and who wouldn’t..?), it’s not too late to catch up with Simon Pugh-Jones’ 365 days of orchids.