Dr Mark Goodwin of Plant & Food Research describes the strange system of flowering used by avocados to avoid self-pollination. It involves the flowers changing sex between one day and the next. He also explains the problems avocado growers have with low fruit set in alternate years. Mark’s research focuses on honey bees in an attempt to increase fruit production.
Jargon alert: Polliniser is the plant where the pollen is coming from.
DR MARK GOODWIN
Avocado flowers are very small compared to most other flowers, and they have this strange system that they use for breeding, in that the flower opens first in the morning as a female flower and then closes at lunchtime and opens the following afternoon as a male flower. So it actually changes sex in between, and it’s all so that it doesn’t pollinate itself.
An avocado is normally pollinated by honey bees or bumble bees, and growers bring in lots of honey bee colonies to ensure that they get lots of visitation. The real problem is very low fruit set. Avocados every second year have lots and lots of flowers, and fruit production’s not a problem. But in the middle years, there’s very few flowers. And where you’ve got really low fruit set, you just don’t get many fruit. So what we’re trying to do is improve the level of fruit set in the years when there’s not many flowers there to even out that production across years.
Honey bees aren’t doing a very good job at pollinating avocados. Normally only one flower in a thousand sets a fruit. We know that, when we hand pollinate – when we collect pollen with a paint brush and put it onto a flower, something like one flower in 10 sets a fruit, so obviously the bees are falling far short of what they could do, and as yet, we don’t know exactly why.
The thing we’re working at is to see how many pollen grains the bees are putting on flowers to see whether there’s actually a problem with the flowers not getting enough visits or the bees not moving between the polliniser and the flowers that we want to pollinate.
Mostly we do it with videos, and we can work out how many visits each flower gets, and after each bee visit, we can remove a stigma and count the number of pollen grains that the bees actually put on there.
So we can answer the question: “Is it getting enough visits?”, and then the next question is: “Are those visits putting enough pollen grains on?”, and we can look at stigmas in different locations and see if you’ve got more pollen grains when you get closer to the polliniser, and if that’s the case, it suggests we need more pollinisers.