What is the window of opportunity for pollination of popular Japanese plum cultivars? By Anna Mouton.
Japanese plums account for about 10% of hectares planted to pome and stone fruit in South Africa. Despite setbacks ranging from severe droughts to broken logistics, exports grew to a record 15 million cartons in 2020–2021. But plum producers face yet another challenge — erratic fruit set.
“Plum yields have not lived up to expectations,” says Louisa Blomerus, senior researcher at the Agricultural Research Council. “Some years are outstanding, and others are poor, but no one can figure out why.”
Although successful pollination is the first step towards good harvests, studies on plum pollination are scarce, according to Blomerus. “I think it’s because the flowers are so small,” she jokes. “Plum flowers are not easy to work with.”
She should know — she and her team hand-pollinated nearly 10 000 plum blossoms for each of the past two seasons as part of a Hortgro-funded project. The aim is to compare the effective pollination period of the most planted Japanese plum cultivars.
“This study has never been done on plums in South Africa,” says Blomerus. “Our cultivars have never been evaluated for effective pollination period.”
The process of pollination
Fruit set in Japanese plums requires fertilisation of an ovule nucleus by a pollen nucleus from a compatible cross-pollinator. Ovules complete their development after flowers open, and their viability is measured in days — the duration depends on the cultivar and season.
In commercial orchards, honeybees transfer pollen grains to flower stigmas, where the grains germinate within a day. Each pollen grain produces a tube that grows from the stigma down the style to the ovary. The pollen nucleus travels to the ovule through the tube. If the tube fails to reach its destination while the ovule is viable, fertilisation will not occur.
It follows that the window for fertilisation — known as the effective pollination period — is limited by the ovule’s lifespan and the pollen tube’s growth tempo. The shorter-lived the ovule and the slower the pollen-tube growth, the less likely successful fertilisation becomes.
Poor fruit set in Japanese plums can occur due to incompatibility, lack of pollination, low-quality pollen, or short-lived or low-quality ovules. Flowers already begin to form in the season before bloom, so many factors can affect flower quality and potentially lead to poor fruit set.
Conditions during bloom can also impact pollination. For example, bees may down tools during cold weather, while flowers may dry out during heatwaves.
“If a flower only has an effective pollination period of two days, you need to have all your ducks in a row,” says Blomerus. “The bees must be working, and the environment must be right — a heatwave mustn’t come through on those days. But if a flower is receptive for longer, you may be able to plant that cultivar under less favourable conditions.”
How are cultivars assessed?
The nine open cultivars that contribute the highest export volumes were selected for the project: African Delight, Angeleno, Fortune, Laetitia, Ruby Crisp, Ruby Star, Ruby Sun, Sapphire, and Songold. Commercial orchards are assessed in Bonnievale, Robertson, and Worcester. Each cultivar is only evaluated in one area.
For each cultivar, Blomerus visits the orchard at bloom and identifies 100 flower clusters spread across at least ten trees. Each cluster must contain a minimum of ten flowers that will open within the next 48 hours. These clusters are covered with paper bags to prevent natural pollination.
Blomerus and her two assistants return to the orchard once the flowers open. Daily hand-pollination is done for ten days. “Where possible, we use the same cross-pollinator that the grower has in the orchard as a donor,” she says.
Every day, they pick one cluster per tree and hand-pollinate all the flowers in that cluster. “We choose those that look like they opened together,” she explains. “The ones that aren’t open are removed.”
The pollen is harvested and dried at room temperature for about a day. Blomerus inoculates a sample on an agar plate the afternoon preceding a pollination trip and checks germination before the team sets off in the morning.
After ten days of hand-pollination, the paper bags remain in place for a further two days. Fruit set is determined 12, 21 and 42 days after pollination has been completed.
“The first assessment might be a little too early, but it provides a good indication of the trend,” says Blomerus. “The second is probably the most accurate, and the third is when the stone has hardened, just as the growers start thinning.”
The results so far
Data have been collected for the 2021 and 2022 flowering seasons, with another round planned for the 2023 season.
Based on the 2021 results, Blomerus has calculated the effective pollination periods of eight of the nine cultivars — Sapphire was not included in that year. In 2021, Songold had the shortest effective pollination period, lasting until day three after full bloom.
African Delight and Ruby Sun had effective pollination periods lasting until day four after full bloom, and Angeleno and Fortune lasted until day 5.
The effective pollination period lasted until ten days after full bloom in Laetitia, Ruby Crisp, and Ruby Star.
“Something that we’ve already seen for the 2022 season is that the early cultivars flowered about two weeks earlier than usual,” notes Blomerus. Although she is still analysing the data, it seems the fruit set was lower in 2022 than in 2021.
Seasonal variability can also influence pollen quality. “The pollen was excellent in 2021,” says Blomerus, “but this past season, the pollen was not as good.” She has been collecting weather data as part of the project and hopes it will help explain the difference in pollen performance.
Although effective pollination period trends appear similar in 2021 and 2022, Blomerus stresses that the data from all three years of the project must be analysed before drawing any conclusions. In the long term, she hopes that knowing the effective pollination period of different cultivars will help growers make better planting decisions.
What are people saying about this research?
“This project will be very valuable if it gives us more accurate figures on the effective pollination period. We still have fruit-set problems in some years, even though we use as many as three cross-pollinators for certain cultivars. Many variables affect fruit set, and we don’t have all the answers, so every bit of research helps.”
Grant Smuts. Managing director, Smuts Brothers Agri.