Fruit set
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, as discussed earlier.
Research has found that increasing the number of pollen grains deposited onto a receptive stigma will increase the likelihood of effective pollination — hence the benefit of increasing the number of pollinators and or pollinisers.
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 fertilisation will not occur. Dry windy days with cold nights can lead to this.
There is some suggestion that two to three consecutive days of minimum temperatures above 10°C, combined with day temperatures above 16°C, are required to achieve effective pollination. For more detailed information on temperature impacts on avocado fruit set see ‘Challenges growing Hass avocados in cool production regions’.
Rain and nutrition can also impact on pollination. Rain during the day will reduce the activity of bees, affecting pollen collection and transferral. This is similar for cold daytime temperatures as bees are reportedly much less active at temperatures below 16°C.
Having adequate levels of boron in the floral organs during flowering is reported to assist in improving fruit set. If low levels are monitored, then foliar application of boron before flowering may assist.
Additionally, research has shown that when an ovule is fertilised with pollen from a different variety the embryo that develops from that fertilisation has 'hybrid vigour' and therefore that fruit is more likely to remain on the tree and will be a slightly larger fruit than fruits derived from pollination from the same variety. While other research in California has shown that this effect does not occur in their conditions it is important to keep in mind as another reason to plant polliniser trees within the orchard.
Some basic pollination terms
- Pollen: The male fertilising agent of plants
- Anther: Part of the male organ of the flower from which the pollen is released
- Stamen: The collective term for the anther and filament (stalk)
- Stigma: Part of the female organ of the flower onto which the pollen is deposited
- Ovule: female part of the plant which develops into the seed
- Embryo: part of the seed which consists of the precursor tissues for the plant that germinates from the seed.
- Pollination: The transfer of pollen from the anther to the stigma
- Cross-pollination: Pollen deposited on the stigma is from a different variety
- Close-pollination: Pollen deposited on the stigma is from a different flower of the same variety
- Self-pollination: The pollen deposited on the stigma is from the same flower
- Pollinator: The agent which transfers pollen from the male to the female floral organ (e.g. honey bee)
- Polliniser: A cultivar that donates pollen to another cultivar
- Dehiscence: The stage when pollen is released from the anthers
- Effective pollination: Pollination which leads to fertilisation
- Non-effective pollination: Pollination which does not lead to fertilisation
- Fertilisation: The fusion of the male gamete with the female gamete forming the zygote (~seed).
Further reading:
Whiley AW, Schaffer B and Wolstenholme BN 2013, The Avocado – botany, production and uses. CABI Publishing. The Avocado – botany, production and uses provides a comprehensive description of all facets of avocados in great detail. While it does dispense agronomic information it is still useful for gaining background knowledge on avocados, which is beneficial to understanding the agronomy of avocados.
Avocados Australia Limited - Best Practice web resource, the best practice resource is the most comrehensive source of information for avocado producers and it is strongly recommended that growers register (for free).
www.avocadosource.com provides access to a range of research publications covering the issue of flowering, pollination and fruit set in avocados. Avocadosource is most suitable for those with a research mind, however anyone with an interest in avocados will find useful information here.