John Skinner, University of Tennessee, opened his presentation at the Oregon State Beekeepers Association 2016 conference by telling us that the interaction of plants and pollinators is his favorite topic. It is indeed nothing short of amazing. Further, the ramifications of this interaction, as we know, extend to growers, beekeepers, and all the rest of us. As he noted last fall, we’d have to cancel Halloween for lack of pumpkins without it.
Advertising, Plant Style
With Power Pointed signs of Dunkin Donuts and golden arches alluding to the means used by our own food sources to attract us, Skinner reviewed the show that blooms put on in luring pollinators by way of scent, color, texture that changes the reflection of light, shape, and depth. The advertising strategies of plants also include the likes of things we cannot see, such as the ultraviolet invitations drawn on the petals of primroses. The tulip poplar, the state tree of Skinner’s base in Tennessee (and my home state of Kentucky as well), ranks among the top in the realm of advertising.
Once a pollinator is on site, the plant world continues to elaborate situations that help ensure a good outcome. The separate male and female flowers of squashes are well known to many a gardener. Yet, the work involved in getting pollen from the anthers of the plant’s male flowers to the stigmas of the plant’s female flowers does more than provide the plant an opportunity to reproduce. Plants that have separate male and female flower parts are able to avoid self-pollination and thereby avoid the inbreeding that can lead to reduced health and vigor. In the end, the “transfer agent” receives a nectar reward.
Another tactic plants use to increase cross-pollination is exhibited in flowers of the genus Salvia and their quirky design for ensuring that their pollen makes a direct hit on a potential pollinator. On entering the plant, the pollinator moves one of the anthers in such a way that the anther deposits a “dollop” of pollen on the pollinator’s back. Were both the male and the female plants parts to be mature at the same time, self-pollination could occur. The thing of it is, though, the male plant parts mature before the female plant parts; the blossom the pollinator has entered cannot be pollinated right then. As the pollinator moves on, it will soon carry the deposited pollen to a plant with mature female parts—and voilà!
Sunflowers and pollinators have a thing going on as well. In sunflowers, the anthers split (and release pollen) in the mornings, during which time we now know that the flowers receive the greatest number of visits from pollen collectors. Later, female portions of the plant move upward, notable to us as disc florets, where pollen may find them. This process occurs over a period of time, such that a plant may take 7–10 days to bloom out. As is true of other species, once the sunflower is pollinated, its nectar reward diminishes.
We also find blueberries, known for buzz pollination by bumble bees, in which blossoms with male parts mature before female. And buckwheat that has three different sizes of anthers and stigmas, which helps promote outcrossing as well. Some species have plants with blossoms that are either all male or all female, thus luring pollinators first by scent and encouraging visits to plants with male blossoms first through clever advertising. There are even species that able to take extreme measures when all else fails. Peter Wohlleben, in The Hidden Life of Trees, speaks of bird cherries pollinated by bees and other insects. In this tree, which contains flowers with mature male and female parts at the same time, a pollen grain may be transferred to a stigma belonging to the same blossom. The fertilization process will begin, yet the tree then blocks it. The tree is able to distinguish the genetic makeup of the pollen of the same blossom from that of other of its blossoms—yes, in some unknown way.
This is a lovely time of year to consider the ways of plants developed over eons of time as well as all the fabulous pollinators that have been lured to their doorsteps. It’s also a time to plant! As we gain in understanding what plants the bees visit, we might also consider that, as well known as it may be that honey bees require pollen from a diversity of plants for good nutrition, the same may be true of nectar—which too varies from blossom to blossom.