Most plants use flowers to coordinate their sex lives: flowers contain ovules (or egg cells), which are housed in the ovary at the base of the (female) pistil, and pollen grains, which are released from anthers at the tip of the (male) stamen. Depending on the particular plant species, a single flower may contain just female or male parts, or both, but for any plant to reproduce successfully it must, in general, ensure the transfer of pollen from the anther to the stigma at the top of the pistil. Ideally, this movement will be from one individual to another so as to maximise the genetic diversity of the resulting offspring. In the vast majority of cases, plants enlist the assistance of a pollinator, usually an insect, to bring about this union. 

Have you ever wondered how plants are able to influence the behaviour of an insect to achieve this routine miracle?

This may sound like a simple question in elementary school biology, but, actually, ecologists are only just beginning to comprehend the remarkable lengths that plants have gone to, in evolutionary terms, in order to successfully attract a pollinator and thus complete this crucial process: the production of seeds and thus the proliferation of plant life across the planet.

Surrounding the pistils and stamen of most flowers are a series of ornate accessory organs, the petals and sepals, which are typically large and colourful and act a visual attractant to potential pollinators. Take a look in a florist’s shop or a showy garden and it is obvious that we humans tend to think of these organs as perhaps the only part of the flower worth bothering about: over many centuries of cultivation, gardeners have selectively bred a range of plants for their ornamental or horticultural value, enlarging the petals even further, multiplying their number within each flower and converting them into often garish and brash appendages. We often overlook the invisible but highly influential ‘olfactory cues’ – that is, fragrances – released by flowers. 

Scents are volatile chemicals that stimulate the nerve cells of a receiver – a human nose or insect antenna, for example. Current best-estimates indicate that flowering plants emit well over 1,700 different kinds of scent molecules, but the human nose is unable to discriminate the majority of them. Tellingly, whereas the eye of a honeybee possesses three kinds of photoreceptors to perceive colour, its antenna harbours a staggering 160 distinct types of receptor to perceive smell. And different blends of volatile chemicals interact with these receptors in a variety of ways, vastly increasing both the intricacy of their perception and the complexity of the responses that they elicit. In other words, floral scents convey more information to pollinators than we might think.

There are more than 2,100 native plant species in Hong Kong, many of which emit a floral odour detectable by the human nose. Some are pleasant, while others smell foetid – the variety of floral odours in Hong Kong alone is quite extraordinary! Some plants adopt a scattergun approach, emitting a cocktail of common volatiles (organic compounds such as linalool, phenylethyl alcohol and benzaldehyde) that indicate the presence of a reward (such as nectar or oils) to foraging insects. Plants offer these rewards in exchange for the service provided by a pollinator. While this generalist strategy may ensure some level of visitation, it can make for rather unfaithful relations if pollen grains are carried off to the ‘wrong’ flower belonging to another generalist species. This type of unreliable communication also means that herbivores can easily ‘eavesdrop’ on the signal, thereby detecting the plant’s presence and homing in on it for a feast. In light of this, it is no wonder that some plants have developed more specialised communication channels in order to attract just one or a selected few pollinator species.

Artabotrys is a genus belonging to the soursop family (Annonaceae) that contains more than 100 species, including the heavenly scented Climbing Ylang-Ylang. In addition to their strong floral fragrance, Artabotrys species are characterised by another unmistakable feature: hooks that help them climb up to the canopy of the tropical forests in which they grow. Interestingly, these hooks are a part of the inflorescence, meaning that each one bears at least one flower. As for most Annonaceae species, these flowers utilise a specialist pollination strategy involving the attraction of weevils and sap beetles. The petals form an enclosed floral chamber, within which visiting beetles congregate and rollick, mating with one another as they consume pollen grains and sugary sap – the entomological equivalent of Amsterdam’s red-light district! Importantly, whilst all of this is going on, the flowers first produce functional female parts only, followed by an asexual phase before they then produce functional male parts, thereby reducing the chance of self-pollination.

The Hong Kong Eagle’s Claw (Artaborys blumei) is a rare plant that is known from only a handful of sites in Hong Kong. In addition to possessing many of the characteristics discussed above, its flowers emit a fruity scent reminiscent of ripe bananas. Indeed, the chief chemical responsible for this aroma (isobutyl acetate) is the very same released by bananas, pears and papayas. Despite dedicated field surveys over successive flowering seasons, a team of local researchers including KFBG botanist Ned Liu failed to observe any pollinators or fruit production at three populations across Hong Kong. This is especially worrying because the species’ specialised pollination strategy makes it all the more vulnerable to extinction – no pollination means no seeds and therefore no future generations. As a result, the plants are now only able to propagate by vegetative (non-sexual) means, as parts of the stem break off, fall down and take root in the ground. Because this is a rare occurrence in Hong Kong’s degraded forests, not only is human intervention crucial to ensure their continued propagation and survival, but more research is necessary to better characterise the scent profile of the flowers in the hope that a reliable pollinator might yet be identified. Furthermore, ambitious, multi-species forest restoration will be essential to ensure the recovery of native insect diversity, in turn enabling the rehabilitation of pollination networks and so support the Hong Kong Eagle’s Claw and numerous other plant species with similarly complex, fragile and often hidden ecological relationships.