AVOCADO

Avocados have an unusual flowering behaviour that is affected by cold conditions which can impact on the level of fruit set. A mature avocado tree may produce in excess of a million flowers during the flowering period, most of which fall without producing fruit. The purpose behind the mass flowering is to encourage visits by a range of flying and crawling insects, believed to be mainly the honey bee. The avocado has a ‘complete’ flower, but with an unusual behaviour known as ‘protogynous dichogamy’. The avocado flower has both functional male and female organs in the one flower, but opens and closes twice over a two-day period — the first day as functionally female and the next as functionally male. Each opening stage only lasts about half a day. In general, on a single tree all the open flowers will be synchronized. That means they will be all functionally male or all functionally female. The avocado pollen of one tree is compatible with itself and quite capable of pollinating its own flowers — known as self-pollination. The unusual flowering behaviour is to reduce the likelihood of this occurring. To further maximise the likelihood of cross-pollination some trees will open first in the morning as functionally female, close and then reopen the next afternoon as functionally male (type A flowering sequence). Other trees open first in the afternoon as functionally female, close and then reopen the next morning as functionally male (type B flowering sequence). The timing of these stages determines the classification of varieties into either type A or B flowering. This is an evolutionary development to increase genetic diversity. Because a single flower takes two days to complete its cycle, on any one day some flowers will be opening as female and others as male, just at different times of the day. The stigma of an avocado flower will normally have ceased to be receptive to pollen when it starts to release its own pollen. However, as a result of the mass flowering, that under ideal temperatures (maximum 25°C, minimum 20°C) there is theoretically some overlap from other flowers providing a small window of opportunity for what is termed close-pollination. They can also flower at slightly different periods during the year. While the opening and closing of flowers over the two-day cycle is synchronized, it is also affected by air temperature. Under stable ‘ideal’ flowering temperatures the flowers behave much like the typical pattern. But under cooler conditions (maximum 20°C, minimum 10°C), or under even colder conditions, type B flowers may be delayed so much that the female stage may not be detected, with the flower appearing to open only as a functional male. Also, the male phase of type A flowers may be so delayed that they remain open over-night and into the next morning. The temperature effect on flower opening is not necessarily equal on all varieties. The avocado flower provides both pollen and nectar which will attract the bees, but due to its size, unusual opening sequence and lower sugar concentration in its nectar, it is not preferred by honey bees. Therefore, while populations of wild bees will visit your avocado trees and assist pollination, they will often be attracted more to other plants flowering at the same time, thus limiting their effectiveness. The avocado tree is noted for having large numbers of flowers in some seasons and yet setting very light crops. There is a range of factors that affect fruit set. Initially there is the requirement for pollen to be transferred onto the stigma (female parts) of the flower while it is receptive. The number of pollen grains being deposited on the stigma is not the end of the story though, as temperature and humidity are also involved. Temperature affects the rate of growth of the pollen tube — the lower the temperature the slower the growth rate. Temperature and humidity also affect how long before the stigma shrivels. The higher the temperature and lower the humidity, the quicker the stigma will shrivel. If the stigma shrivels before the pollen tube has finished its growth to the ovules then fertilization will not occur. Dry windy days with cold nights can lead to this. Two to three consecutive days of minimum temperatures above 10°C, combined with day temperatures above 16°C, are required to achieve effective pollination. Rain and nutrition can also impact on pollination.

VARIETIES