Why do flowers produce nectar?

Nectar is a sweet liquid produced by an organ called a nectary. A bit like the discounts at a grocery that incite us to buy, the role of the nectary is to attract animals. Once they land on the plant, these animals do it a favor in exchange for nectar.

There are two types of nectaries: floral nectaries and extrafloral nectaries.

Floral nectaries

Locate within the flowers, floral nectaries attract pollinators so that these will touch the flower’s reproductive organs, first by rubbing on the stamens (male organs producing the pollen), and then on the pistil (female organ). That happens sometimes when pollinators visit other flowers and sometimes right there inside the flower where foraging initially took place. This is how the fertilization of the majority of flowering plants takes place. Incidentally, flowers open to signal that their reproductive organs are mature.¹

Extrafloral nectaries

Unlike floral nectaries, extrafloral nectaries do not intervene in pollination. These structures can be found on the surface of different plant organs (leaves, stems, and so on). A good example is the peony, which produces nectar on its flower buds (before they open). This has the effect of attracting ants, which protect and clean the plant.

Nectar’s high sugar content makes it a preferred food source for nectarivores (insects, birds, bats). What has to be kept in mind is that the plant essentially produces nectar with the goal of establishing an exchange of services.

The brief history of nectar: who came first, the nectar or the pollinator?

Here we’re faced with a question similar to the famous paradox of the chicken and the egg. A few hypotheses have been put forward about the origin of nectar. In the second half of the Cretaceous period (about 130 million years ago), flowering plants, or angiosperms, experienced massive diversification. Simultaneously, a diversification of insects took place – in pollinators, among others.²

Nevertheless, pollen already existed. Insect fossils, much older, have been found covered with pollen. The earliest pollinators were possibly beetles or flies¹ that fed simply on the pollen of gymnosperms (a group that includes conifers).

On the other hand, some people suggest that the first nectars would have appeared outside flowers, on other structures, in the form of sap rich in sugar. That’s notably the hypothesis of the leaky phloem.¹

In brief, without going into the details of paleontology, it’s plausible that nectar and pollinators appeared independently before the important co-evolutions of 130 million years ago.

Do all flowers produce nectar?

No, not all of them produce it. Some of them don’t need to. Dispersal of their pollen can be achieved by something other than an animal: the wind, for example, or water. Flowers that produce nectar are called nectariferous, while the term melliferous is used for those that are visited by honeybees (Apis mellifera). We should mention that many insects consume pollen, which is an essential source of protein for their diet.

A favorable arrangement for pollinators

The appearance and diversification of flowers made it possible for many species to diversify and establish mutualist relationships (mutually beneficial symbiosis). Nectar, for its part, plays the role of reward in order to solicit the service of certain animals. The diversity of nectariferous plants is just as surprising as that of nectarivores. A good way to discover nectariferous plants is to analyze the shape of the flowers or to be attentive to when they flower. Some flowers open only at night in order to make sure that they’re pollinated by nocturnal animals, for instance. You may very well discover close links between your plants and nectarivores!

To encourage a variety of pollinators in your gardens, offer them a diversity of nectariferous plants that flower at different times of year. Bear in mind also that native plants are those that have co-evolved with our native pollinators; they should therefore be a priority if we wish to support them.

References

¹ Nectar: properties, floral aspects, and speculations on origin
a. https://www.sciencedirect.com/science/article/abs/pii/S1360138503003078
² Ecological aspects of the cretaceous flowering plant radiation
a. https://www.annualreviews.org/doi/abs/10.1146/annurev.earth.26.1.379