What drives cluster uniformity?

Gibberellins are plant hormones involved in many developmental processes. They promote individual flower survival and set, but treating the whole vine also stimulates vegetative growth that may compete with the developing berries and increase fruitlet drop.
In SA, gibberellins are registered to thin berries and improve bunch architecture, but poorly timed applications can increase shot berries in some cultivars.
Dr Or compared the response of inflorescences with different flower numbers to gibberellins. “We had clusters with 50, 100 or 155 flowers,” she says. “I made the poor students sit in the vineyard and count the exact number of flowers in the clusters.”
Each inflorescence was dipped in gibberellins – the rest of the plant was not exposed to gibberellins. Treated and untreated clusters were allowed to mature, and the number and weight of normal and shot berries were recorded.
The number of normal berries and their average weight did not differ in clusters of the same size, whether treated or untreated with gibberellins. The number of shot berries in clusters with 50 flowers also didn’t vary between treated and untreated clusters. But the story changed in clusters of 100 and 155 flowers.
Treated and untreated clusters with 100 and 155 flowers had more shot berries than clusters with 50. Clusters of 100 and 155 flowers treated with gibberellins also had many more and much smaller shot berries than comparable untreated clusters.
“Fruit set is regulated by the initial flower load,” says Dr Or. “Gibberellins bypass this regulation – they induce fruit set even without fertilisation.”
When left to manage themselves, vines will naturally regulate flower load and cluster size so that each berry can develop optimally. Gibberellin treatment forces the vine to hang on to more berries than it can support.
Each berry is a sink – it competes with other berries in drawing resources from the vine. “Within the cluster, there are many competitors, and it’s not fair,” explains Dr Or. “The initially smaller berries are the victims. So, the small berry becomes smaller, and the big one becomes bigger.”
In general, more flowers in a cluster mean more competition and less uniformity. But cultivars differ in their ability to support larger clusters and, therefore, their propensity to shot berries.

Abscisic acid induces drop

Abscisic acid gets its name from the mistaken notion that it’s directly responsible for the abscission – drop – of leaves and other plant organs.
This plant hormone has opposite effects to gibberellins in some, but not all biological processes. Dr Or showed that whereas gibberellins increase set, abscisic acid thins flowers and fruitlets. The remaining flowers and fruitlets tend to develop into larger berries that are more uniform in size than those in unthinned clusters.
Dr Or conducted experiments showing that abscisic acid reduced the survival of ovaries and very young fruitlets. It thins most effectively if applied between full bloom and anther loss. By two weeks after full bloom, abscisic acid no longer thins.
“It appears that, with time, the berry becomes a good enough sink to ignore the effect of abscisic acid treatment,” says Dr Or.
Her scientific curiosity compelled her to explore the mechanism of abscission in clusters. She discovered that abscisic acid turns on the genes coding for the enzymes that inactivate auxins and converts aminocyclopropane-1-carboxylate to ethylene.
“This modulates the balance between auxin and ethylene levels in a way that is known to induce the development of the abscission layer,” she notes.
But she still had questions. “I wanted to know why everything doesn’t drop when I apply abscisic acid. And that’s the same question for shot berries: why are some small and some big?”

When size matters

Dr Or hypothesised that variability in flower size within a cluster could explain the differential response to abscisic acid. To test this, she compared the weight of abscised to non-abscised flowers in clusters treated and untreated with abscisic acid.
“We sacrificed another pool of students,” she jokes. “They had to weigh hundreds of flowers, flower by flower, on an analytical balance.”
The students’ efforts revealed that the smaller flowers in a cluster tended to drop, regardless of whether abscisic acid was applied or not. Surviving flowers were 3.5 times larger, on average, than dropped flowers in untreated clusters.
However, on average, abscised flowers in the abscisic acid treatment were bigger than abscised flowers in the untreated clusters because abscisic acid pushes the medium-sized flowers to drop.
“The small flowers would have been dropped anyway,” says Dr Or. “The middle ones would have survived the natural competition, but when abscisic acid is involved, it creates extra stress, and they do not survive.”
Like fruitlets, flowers are sinks that compete for access to the resources provided by the vine. Bigger flowers are stronger competitors, but medium-sized ones can usually survive too. Add the stress of abscisic acid, and inter-flower competition intensifies so that medium is no longer good enough, and only the fittest survive.
Interestingly, this mainly applies to competition within clusters. Different clusters on a vine don’t compete that strongly with one another. “So, it seems like this isn’t an entire neighbourhood problem,” says Dr Or. “It’s a family problem in the same house.”
Abscisic acid inhibits cell division, so treated berries are smaller than untreated berries for the first few days. They soon catch up and are bigger than the untreated berries at harvest, probably due to less competition and reduced shading in clusters with fewer berries.
In SA, abscisic acid is registered to improve berry colour, but soon, growers will have registered products to thin table grapes.