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24 May 2020

Pollen a fast one: conifer pollination

Autumn is possibly the most spectacular time of year to visit the Blue Mountains, with a rainbow of changing foliage lining every street and garden. And while the display our avenues of deciduous trees puts on is a sight not to be missed, there is another arboreal event which is less commented on, but equally as breathtaking and perhaps more fascinating; the pollen drop of Cedrus deodara

Why is everything suddenly yellow? 

Seeing a C. deodara drop its pollen is a sight not easily forgotten, with even just a light breeze stirring what looks like a huge cloud of tennis ball yellow smoke from the trees’ branches, to settle on footpaths, cars, play equipment and children as far as the eye can see. Apart from staring into the night sky or admiring a beach full of sand, this is one of the few times in life you’ll see billions of something at once, with one study finding that in a slow year a C. deodara can produce 4.7 billion pollen grains and as many as 14 billion pollen grains in a good year.

Conifer pollination 

While angiosperms have long been lauded for their complex and beautiful pollination processes, gymnosperms are often ignored. All conifers are wind pollinated, and while this doesn’t involve any fancy mechanisms involving mimicry or insects, it’s quite a feat when you consider the mechanics of such large quantities of pollen making their way from the male cones of one tree to the female cones of another. C. deodara’s pollen drop is a weird and wonderful thing to see on a large scale, but when we look closely at what’s going on at a microscopic level, it’s even more intriguing.

A lot of resources are required to produce literally billions of pollen spores annually in the hope that that will find their way to the ovule, so how does a Cedrus make sure that bits of dust which land on its female cones, or pollen from the wrong species, don’t try and pollinate the female cones or clog them up, rendering them a wasted megaspore? Angiosperms have a series of tissues and organs (such as their stigma, style and ovary) that function as a filter for pollen selection, but how do the “naked” gymnosperms determine what is an ideal pollen grain and allow it to fertilise the ovule? 

The pollination droplet 

C. deodara (as well as all conifer families except Araucariaceae) have what is known as a pollination droplet, possibly the most important feature of conifer pollination. A pollination droplet is a small liquid drop, secreted at the top of the female cone upon which pollen grains land, are engulfed, and are then drawn into the ovule. This is how the female cone “catches” the pollen grain. Once the pollen grain has been drawn into the cone, the tube cell of the pollen forms a pollen tube, through which the male cells make their way towards the female cells to fertilise the cone and create a seed. It takes approximately one year for the pollen tube to grow and migrate towards the female gametophyte.

Studies have found that conifers will only draw the pollination droplet back into the ovule if the right type of pollen lands in the droplet, being the correct species, not dead pollen and not dust nor debris. One study suggests that the pollination droplet withdrawal is triggered by a biochemical mechanism resulting from an interaction between the pollen grain and the droplet constituents.

This means that the conifer basically speaks to the pollen grain through its pollination droplet to decide if it’s the right size and shape, comparing the weight of the grain to the viscosity of its pollination droplet. It is thought as well that the composition of the droplet (which is made up of sugars and amino acids) only provides an ideal environment for the living pollen of the appropriate conifer species to grow its pollen tube into the ovary, as the proteins and sugars in the liquid act as an energy source to help the pollen grain on its way.  

While conifer pollination isn’t as showy, or seemingly complex, as angiosperm reproduction it is incredibly clever and beautiful in it’s own way, and also provides us with a good reminder that intriguing things are happening all around us in the garden if we just look closely enough.

Learn more
The Blue Mountains Botanic Garden Mount Tomah has a selection of these magnificent trees in the Living Collection. See a snapshot of how the dedicated Horticulture team maintains over 48,500 plants in the video below.


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