The upcoming flowering stage of avocado production is arguably the most important phase that results in fruit set.
By now, the fat round flower buds that were sitting tightly in the leaf axil have extended and developed into panicles full of closed flower. This is called the cauliflower stage. In some orchards, there may even be the odd open flower.
A mature avocado tree can produce in excess of 1 million flowers with fruit set ranging from 0.3-0.5%.
The avocado flower is very unique in that each flower has a distinct male and female phase which is nature’s way of encouraging cross pollination between different cultivars and ensuring species diversity. This however does not preclude self and close pollination within the same cultivar as is demonstrated in orchards of only one cultivar or individual trees in home gardens.
Briefly explained, avocado cultivars can be divided into either A or B flowering cultivars. For A-flowering cultivars e.g. Hass, the flower will open for the first time (day 1) functioning in the female phase with only the female parts being receptive. The flower then closes after 2-6 hours and reopens in the afternoon of day 2 in the male phase, with the male flower parts now being functional and shedding pollen. The flower remains open and finally closes for the last time the next morning.
If pollination and fertilisation was successful, the flower will remain on the tree and begin to form a fruit. If fertilisation was not successful, ethylene will be produced at the base of the ovule and the flower will abscise (fall off).
For B-flowering cultivars e.g. Zutano, the flower opens on the afternoon of day 1 in the female phase, closes and reopens from the morning to afternoon of day 2 in the male phase. This synchronises with the A-cultivar flowers opening in the female phase and pollen transfer from the B cultivar to the A cultivar can then take place.
This synchronisation however seldom occurs perfectly and is mostly only synchronised at warmer temperatures of minimums of 15oC and maximums of 25oC.
Different B-flowering cultivars also flower at different stages of the season so need to be compatible with the flowering time of Hass if used as a polliniser eg. Fuerte flowers earlier in the season and would therefore not be a successful polliniser choice for Hass.
The cooler New Zealand climate and variable spring weather results in delayed and irregular opening in the female phase with a greater overlap occurring between the female and male phases. The pollen transfer within the same tree or neighbouring trees of the same cultivar during this overlap period result in close pollination.
Isozyme analysis of fruit has supported this and shown that most of the fruit (up to 85%) tested in a trial, was fertilised with pollen from the same varieties’ own pollen i.e. Hass pollen for Hass fruit.
In higher temperatures with lower humidity the stigma viability is short lived and it will appear brown or shrivelled on day 2. In climates with high humidity and cooler temperatures, the stigma is still receptive and viable on day 2 when the flower is in the functioning male phase making self-pollination possible.
Regardless of the flowering behaviour or rather misbehaviour, without pollen transfer, pollination and fertilisation cannot take place.
Pollen is transferred by a variety of >> insects, by wind and even by gravity. The honeybee however takes the lead role and is the principal pollinator. To ensure the best possible pollination and fruit set we need to ensure that there are enough bees within the orchard.
With heavy flowering and cooler climates, more hives per hectare are required and 8-12 hives in these instances are advised.
Bumblebees come in at a good second place as an effective pollinator for avocados and are less averse to cold, windy and wet weather conditions. A 1:4 ratio of bumble to honeybee hives is recommended for optimum pollination.
Once pollen is transferred many conditions have to be met before fruit set can occur.
To ensure optimum fertilisation, research has shown that the more pollen that is transferred to the stigma the better the chance of fertilisation. At least 20 golden pollen grains appear to be the magic number as some pollen doesn’t germinate, some pollen tubes don’t have the “vigour” to make it all the way to the ovule and some pollen tubes grow too slowly and the ovule loses its viability.
Once the pollen grain adheres to the sticky stigma, it starts to germinate, producing a pollen tube which grows down through the style, enters the ovary, finally reaching the ovule.
It can take anything from a few hours to 24 hours from germination to fertilisation. Fertilisation then initiates the development of the ovary into a mature avocado fruit and the ovule into the seed.
Fertilisation and fruitset are affected by a number of factors including temperature, humidity, nutrient status, abnormalities in the embryo, and water stress.
With so many influencing variables it is no wonder that the flower to fruit ratio is so low in avocados, but there are some steps we can follow to ensure optimal flowering, pollination, fertilisation and fruit set: