Many flowering plants have evolved to prevent self-pollination, which means that pollen must be transferred from the anthers of one flower to the stigma located on another. This promotes outcrossing and genetic variation.
The mechanism to prevent self-pollination in avocados is called ‘synchronous dichogamy’. This describes the pattern of flowering on the trees. In all avocado trees, a flower will either be functionally female or male. In other words, an open flower will either have its female reproductive organs functional, or its male organs.
In the female phase, the stigma is white and ready to receive pollen, and the anthers lie flat against the petals. The male phase on the other hand may have a brown-coloured stigma, and the anthers are standing upright, making the male flowers appear much larger compared to the females.
Every flower will open first in its female phase, close, then the same flower will reopen in the male phase. Therefore, the morphology of the avocado flower prevents self-pollination because a flower will never have all the reproductive parts functional at any given time. It is often the job of the honeybee to collect pollen from a flower on one tree and deposit it on a flower located on another.
Avocado cultivars are classified according to which of the sexual phases is active in the morning or afternoon.
‘A’ cultivars, like ‘Hass’, have female flowers open during the morning. These flowers close and reopen again the following afternoon in their male phase. ‘B’ cultivars, like ‘Fuerte’, have female flowers open in the afternoon, which close overnight and open the following morning in the male phase.
There are some exceptions to this rule however, as cold temperatures will desynchronize flowering, which can result in female flowers being open throughout the night and both phases being open on a tree during the day.
But why is this knowledge important for commercial avocado growers? Well, if a farmer only plants ‘A’ or ‘B’ flowering cultivars in their orchard, there will only be one phase of flower open at any time. So, there will either be no pollen available to fertilize the females, or no viable stigmas to receive pollen. Therefore it is common for farmers to plant a range of cultivars within their orchards to promote cross-pollination. Sometimes, fruit will develop if a flower is self-pollinated, but it is believed that fruit which has developed as a result of cross pollination is ‘stronger’ and will be better able to withstand stress and have a higher chance of reaching maturity.
So if you want to boost your yields, remember the difference between ‘A’ and ‘B’ cultivars and make sure your orchard has a good mix of both.