Getting good crops of avocados in New Zealand is not always easy, and pollination is part of the problem. The avocado is a fruit tree introduced to New Zealand from Central America. It likes warm conditions, so only grows in the North Island from the Bay of Plenty northwards. Avocado growers find it hard to provide a regular supply of fruit because the trees only give a good crop every second year. Dr Mark Goodwin of Plant & Food Research is trying to find out why, and the main part of the problem he is looking at is pollination.
Avocados have an unusual system of flowering to prevent self-pollination. Each tree can be almost covered by hundreds of thousands of tiny flowers – look at the flowers one day and they may be female, but the next day the same flowers will be male. The timing of this change is different in different avocado cultivars. In some cultivars, a flower opens in the morning as a female with a stigma, then closes at about midday. It reopens in the afternoon of the next day, but this time as a male with pollen. The flower closes in the evening and stays closed. In other cultivars, flowering is the other way round – flowers open in the afternoon as female, close, then reopen the next morning as male.
Bees to blame?
Avocado flowers are pollinated by insects, so growers bring honey bee hives into the orchard in the flowering season. Most avocado orchards have two different cultivars so that, when some trees have flowers at the female stage, other trees have male flowers. This means the bees need to collect pollen from male flowers of one cultivar (called the polleniser) and transfer it to females of another cultivar. Mark has studied bee behaviour in avocado orchards and found that bees prefer to visit flowers on the same tree, rather than move between trees. That means they aren’t very good at transferring pollen from male to female flowers. This situation can’t be helped by artificial pollination, which is used for kiwifruit, because avocado pollen does not survive the process.
Avocado trees may have hundreds of thousands of flowers, but for some reason, not many of them produce fruit. Mark has carried out experiments to see if lack of pollination is the problem. He has found that, when bees are left to do the pollination, only about three flowers in every 1000 produce fruit. If he hand pollinates flowers using a paintbrush, about 70 flowers in 1000 produce fruit. The bees are obviously not being very successful as pollinators, but as yet, Mark hasn’t found out why. He has used video cameras to monitor bee behaviour on avocado trees and found that bees don’t visit all the flowers, so that seems to be part of the problem. Other work suggests that bees don’t find avocado nectar very attractive, so they are tempted to visit flowers of other species nearby.
Mark works with kiwifruit as well and sees a big difference in the pollination of those flowers. Kiwifruit have many hundreds of tiny seeds in each fruit – to get this many, each female flower needs to receive on its stigmas about 13,000 pollen grains from male flowers. Each avocado fruit has just one large seed – each female stigma only needs to receive about 20 pollen grains to produce this. So part of Mark’s problem is to find out why it is that, if bees can carry so much pollen between kiwifruit flowers, they don’t do it for avocados.
Nature of science
Avocado pollination research is an example of how science contributes to New Zealand’s horticulture and agriculture industries.