ACC

The newly registered 1-aminocyclopropane-1-carboxylic acid (ACC) increases the scope for late thinning of apples. By Anna Mouton.

“ACC became available during the past season for chemical thinning of apples in the 15–20-mm fruitlet-diameter window,” says Werner Truter, Product Manager at Philagro South Africa, AgroSolutions Division, EMEA Region.

Most chemical thinners perform poorly once fruitlets exceed 12 mm (the exception is metamitron, which is registered for fruitlets up to 18 mm). Therefore, ACC is a valuable addition to apple growers’ toolkits.

Street lights and fruit juice

With annual global production surpassing 200 million tonnes, ethylene is essential for producing plastics and other materials. Although chemists have been making ethylene since the mid-1600s, its interaction with plants was only recognised in the 1800s.

From the late 1700s, gas street lighting began appearing, and adverse effects on nearby plants, including the defoliation of street trees, followed. In 1896, a doctoral student at St. Petersburg University in Russia proved that ethylene released by illuminating gas was the cause.

For decades, no one imagined that ethylene was a natural plant hormone. Ethylene synthesis in plants was first demonstrated in 1934 by Richard Gane, working on Worcester Pearmain apples. Subsequent research continues to reveal ethylene’s far-reaching effects on plants’ growth, development, and stress response.

While Gane and fellow plant physiologists, Franklin Kidd and Cyril West, were investigating long-term fruit storage at the Low Temperature Research Station in Cambridge, another part of the ethylene puzzle was being assembled elsewhere in England.

Len Burroughs, a researcher at the Long Ashton Research Station, was studying the amino acids in apple and pear juices because he was interested in the influence of nitrogen-containing compounds on the fermentation and spoilage of ciders and perries (pear ciders).

During his analyses, Burroughs detected an unknown substance, which he named X. Resolving to identify it, he managed to extract a quarter of a milligram of X from 30 litres of pear juice and determined that it was 1-aminocyclopropane-1-carboxylic acid.

In his 1957 Nature paper, Burroughs commented that the significance of ACC was unknown, but that it appeared in apples and pears during the last month of ripening on the tree.

It took until 1979 for scientists to connect the dots between ACC and ethylene, when a team from the University of California, Davis, showed that ACC is an intermediate in the synthesis of ethylene from methionine. They did this work on Golden Delicious apples.

Ethylene, ACC and thinning

Elevated ethylene levels are associated with fruit abscission, so ethylene is an obvious candidate for chemical thinning. As ethylene is a gas, it doesn’t lend itself to orchard use, but ACC is available as a liquid formulation for spray applications.

Apple trees absorb ACC and convert it enzymatically to ethylene, achieving peak ethylene levels 2–3 days after application. Ethylene declines to base levels about 10 days after application.

In South Africa, ACC for fruit thinning was pioneered by Prof. Karen Theron, emeritus professor in the Department of Horticultural Science at Stellenbosch University. She evaluated ACC applications on apples during full bloom, 8–10-mm fruitlet diameter, and 15–20-mm fruitlet diameter. To her surprise, she consistently achieved the best results in the latest window.

“It doesn’t make sense, because the fruit’s greatest carbohydrate demand is at 8–12-mm fruit size. After that, the leaves begin exporting carbohydrates,” she says. Fruitlets are usually most vulnerable to abscission during periods of energy scarcity.

“We have also done work with ACC on one- or two-year-old apple trees where we don’t want any fruit,” says Theron. “When we spray at 15–20-mm fruit size, almost no fruit remain. Thinning is again more effective in this window than at earlier stages.”

Researchers worldwide have also observed that ACC works best in the later window, at least on apples. “It might be related to the availability of enzymes for converting ACC to ethylene, or the number of ethylene receptors,” speculates Theron.

In her trials, ACC, albeit at higher doses, even reduced fruit set at 20–25-mm and 25–30-mm fruitlet diameter relative to an untreated control.

“Thinning earlier is generally better than thinning later,” notes Theron. “When you thin late, you still get a positive effect on fruit size, but not on return bloom, especially in cultivars with early flower induction and initiation, like Fuji.”

Of course, Fuji is infamously wayward, difficult to thin, and prone to biennial bearing. “The best timing for ACC applications on the apple cultivars evaluated in our trials was 15–20 mm — except for Fuji,” says Truter. “Fuji is always unpredictable. We are still investigating alternative strategies to position ACC for Fuji.”

The outlook for ACC

The use of ACC for thinning pome and stone fruit is being studied extensively worldwide, including the effects of combining it with other chemical thinners. Some of the results are summarised in a recent SAFJ article.

Truter points out that ACC- (and s-abscisic acid-) containing products are under less regulatory pressure than older thinners such as carbaryl. “ACC and s-abscisic acid occur naturally in the plant,” he explains.

Besides ACC’s role as an ethylene precursor, evidence is mounting that ACC is a plant growth regulator in its own right. Among other functions, ACC is involved in regulating cellulose synthesis, pollen tube growth, root elongation, stomatal development, and stress responses.

Going forward, researchers may well find new uses for ACC in the orchard. In the meantime, it will doubtless be welcomed by apple growers suddenly faced with a too-heavy crop load after the optimal window for most thinning products has closed.