Cutting-edge research in plant growth regulators

The summary below is based on the South African participants’ take on the most interesting and impactful presentations.

The hottest topic: ACC

What is ACC?

ACC occurs naturally in plants, where it serves as an intermediate in the synthesis of ethylene from methionine. It was first discovered in apples and pears in 1957, but scientists only figured out its role in ethylene biosynthesis in 1979. More recent research suggests that ACC is also a plant growth regulator in its own right. Ethylene participates in many processes, including seed germination, flower senescence, leaf abscission, and fruit ripening. The pre-harvest ethylene inhibitor AVG (aminoethoxyvinylglycine) reduces ethylene by inhibiting the conversion of methionine to ACC.

Researchers have long known that ethylene causes abscission of flowers and fruitlets in some fruit types. This is the basis for using ethephon as a fruit thinner – it’s converted to ethylene. However, the effect of ethephon is highly temperature-dependent, so misjudging the weather can lead to significant over- or under-thinning.

In contrast, ACC is enzymatically converted to ethylene inside the plant, so its behaviour may be relatively consistent. A presentation by Brian Sopcak and co-authors concluded that ACC applications are less likely to result in damaging ethylene concentrations than ethephon applications when temperatures are high.

ACC was recently registered in SA for thinning apples, hence the keen interest of our industry members attending the Plant Bioregulator Symposium.

Where does ACC fit in?

Luis Gonzalez and co-authors investigated early and late thinning of apples. They worked on Gala in New York State and Fuji in Spain. When applying plant growth regulators during flowering, the researchers found that ACC was ineffective on its own, but they achieved good results with a combination of ACC and 6-BA (6-benzyladenine). They concluded that increased ethylene and cytokinin levels during petal fall were effective at reducing crop load. The same team contrasted the window for applying ACC to other chemical thinners after fruit set.

Both NAA and 6-BA affect auxin transport, and 6-BA also enhances cell division. NAA can be used at up to 15 mm fruitlet size, but works best when applied at 6–8 mm fruitlet size. 6-BA can be used in the 6–16 mm range, but works best when applied at 8–12 mm fruitlet size. Metamitron inhibits photosynthesis. It can be used in the 6–22 mm range, but works best when applied at 10–16 mm fruitlet size. ACC can also be used in the 6–22 mm range, but is most effective on 18–22 mm fruitlets.

The researchers reported that combining chemical thinners was more effective than using only one.

ACC for late thinning

Another presentation on thinning apples concentrated on late thinning. When asked which work most caught their attention, this was the top choice for the majority of the South African technical advisers attending the Symposium.

Andrew Lopez Ferraz and colleagues tested two doses of ACC (300 and 400 ppm) applied when 50% of the central fruitlets were 20 mm. They compared this with metamitron (300 ppm), applied when 50% of the central fruitlets were 14 mm. They also included an untreated control.

ACC outperformed metamitron as a thinning agent at both doses, and ACC was also associated with a higher fruit mass at harvest. Neither ACC nor metamitron affected return bloom or shoot growth. Esmaeil Fallahi and Steven McArtney presented similar results with ACC applications at 20 mm fruitlet size. In Gala, ACC (150–350 ppm) was effective at reducing fruit set, but ethephon (300 ppm) was not. ACC was also effective in Fuji, with differing dose responses in different strains.

“I think this is what makes ACC unique – its later window of effectiveness,” says Graeme Krige, General Manager of Fruitmax Agri. Although there are already several chemical thinners registered for apples in SA, Krige sees ACC as a potentially valuable addition. “Any handyman is excited to have another tool in his toolbox,” he says. “That doesn’t mean we’re throwing out our other tools, but it gives us another option, should we need it.”

Thinning of stone fruit

“Despite the long history of ACC research in stone fruit in South Africa, it’s been registered on apples first,” comments Prof. Wiehann Steyn, General Manager of Hortgro Science. He added that Hortgro has funded work on ACC in stone fruit and that the results for Japanese plums were published in 2017. “Our growers wanted a thinning product because of the enormous cost of hand-thinning self-pollinating Japanese plums, especially,” says Steyn. ACC is currently registered on peaches and nectarines in SA, with registration for plums expected soon.

“The Hortgro-funded research was conducted with an earlier formulation of ACC, and the Registrar required more bridging work to register ACC on plums in South Africa,” clarifies Karen Theron, emeritus professor at Stellenbosch University, who was involved in this work. The presentations about ACC in stone fruit included a review of four trials on thinning peach blossoms by Michael Evans and co-authors.

They reported that higher spray volumes (1 870 per hectare) resulted in increased thinning compared with lower spray volumes (470–940 per hectare). The rate of application of the active ingredient per hectare was constant in the different treatments, but the authors speculated that greater carrier volumes enhance ACC efficacy. Another trial found no significant differences in fruit set when comparing ACC applications at the pink stage, 70% bloom, and 70% petal fall.

Evans and colleagues also noted that cultivars vary in their response to ACC, necessitating cultivar-specific application protocols.

Other work on peach thinning came from Everlan Fagundes and co-authors, based in Brazil. They evaluated ethephon, ACC, and vegetable oil, achieving promising results with ethephon and ACC. Vegetable oil did not cause thinning.

Fallahi and McArtney found that ACC effectively reduced fruit set in peaches and nectarines (450 ppm) and apricots (350 ppm) when applied at 75% of full bloom and sprayed at 935–1 870 per hectare.

Red colour development

“Good conditions for red colour development will never be guaranteed in the South African climate,” says Werner Truter, Product Manager at Philagro South Africa, AgroSolutions Division, EMEA Region. “We know colour development depends on genetics and canopy management, but climate plays a critical role, so growers need additional tools to improve colour development.” Truter presented the results of research performed by Philagro South Africa and Theron on Cripps Pink and Rosy Glow in Elgin. They found that ACC improved red colour development when applied 14 days before the estimated harvest date. Unsurprisingly, ACC also advanced fruit maturity, but this could be managed by postharvest application of 1-MCP (1-methylcyclopropene). Estanis Lezcano and Paula Regany conducted similar trials on Gala in Spain. They observed improved red colour at harvest when applying ACC, ethephon, or one of two synthetic auxins at 60 days after bloom.

In Washington State, Jozsef Racsko and Poliana Francescatto tested ACC on Cripps Pink, Gala, Honeycrisp, and WA 38 (marketed as Cosmic Crisp). They reported the best results with applications of 200–300 ppm at two to three weeks before harvest. Colour development was improved in all the cultivars, except Honeycrisp. “I found it interesting that South Africa is ahead of other countries in researching this use of ACC,” says Truter. “Many apple- production areas abroad don’t currently struggle with adequate red colour development, but as climate change advances, it will become more relevant to them. So, I think there will be a bigger focus on ACC to improve fruit colour at future symposiums.”

Bitter pit reduction

The second-most mentioned presentation when talking to the South African participants was by Chayce Griffith and co-authors. They examined the effect of auxins and ABA on bitter pit development in Honeycrisp.

“The highest concentration of calcium in a fruit is just after fruit set. Calcium dilutes as the fruit grows, and there’s a problem with calcium movement to the outer layers of the fruit,” explains Keith Bradley, General Manager of Fruitways Agri Services. “By extending the functionality of the xylem in the fruit, movement can continue.” Griffiths and colleagues hypothesised that they could promote and prolong xylem functionality by applying auxins and ABA. In contrast, an auxin transport inhibitor (TIBA or 2,3,5-triiodobenzoic acid) should reduce xylem functionality and increase bitter pit.

To test this, they applied a natural auxin (IAA or indole-3-acetic acid), a synthetic auxin (NAA), ABA, and TIBA at 30, 45 and 60 days after full bloom. In 2023, the untreated controls developed little bitter pit, and NAA and ABA reduced the severity of bitter pit but not the number of affected fruit. As expected, TIBA significantly increased bitter pit. The following season saw severe bitter pit, which could be mitigated by NAA and ABA.

The following season saw severe bitter pit, which could be mitigated by NAA and ABA.

The researchers also observed improved xylem functionality in 2024 in fruit treated with NAA and ABA. NAA raised calcium levels in the skin, but ABA did not. “We were visiting American growers the previous week,” says Marno van der Westhuizen, Research Implementation Manager at Hortgro Science. “A few of them told us that they spray NAA to control bitter pit on Honeycrisp. So, it was interesting to hear this presentation explain the mode of action.”

Vegetative growth

The problem of insufficient winter chill is not confined to the warmer South African apple-production regions. Leonardo Paim and co-authors from Brazil presented their work on various manipulations to improve breaks in young trees.

They tested the effect of repeated 6-BA and gibberellin (GA4+7) applications alone or in combination with girdling or ring incisions on one-year wood in Gala grafted onto various rootstocks. During two seasons, the researchers applied 6-BA and GA4+7 either three or six times at three-week intervals from the onset of flowering. Using 6-BA and GA4+7 in isolation didn’t result in more lateral breaks. However, combining the plant growth regulators with girdling and ring incisions was effective in increasing the number of breaks.

Systemic plant resistance

Not all plant growth regulators affect vegetative or reproductive growth. Melissa Muñoz and co-authors investigated the effect of ASM (acibenzolar-S-methyl) to reduce flood stress in apple trees. ASM was first developed in the 1980s to protect crops against fungal infections. The compound is not fungicidal. Instead, it activates the plant’s natural defences, helping the plant ward off new infections but not usually curing existing ones.

Muñoz and colleagues subjected potted Fuji trees on M.9 rootstocks to flood stress in glasshouse experiments. They found that 75 ppm ASM treatments before flooding improved tree survival by 50%. ASM also reduced the incidence of fire blight in non-flooded control trees.

In a separate study, Christopher Clavet and co-authors examined the effect of ASM and AVG on stem-end splitting and glomerella leaf spot in Gala. Stem-end splitting is a postharvest disorder, and glomerella leaf spot is a fungal infection.

AVG is already known to reduce stem-end splitting. The researchers wanted to see whether they could apply ASM and AVG close to harvest to control both problems. They found that 75 ppm ASM plus 66 ppm AVG significantly decreased the severity of glomerella leaf spot.

“I’m always excited about a non-toxic product that can protect the plant or the fruit,” says Nico Ferreira, technical adviser at Fruitmax Agri. “The researchers also referred to a scientific paper that showed that ASM induced resistance against an ambrosia beetle in dogwoods exposed to simulated flood stress,” adds Steyn. This led both Steyn and Ferreira to wonder whether this product might be effective against polyphagous shot-hole borer, which is also an ambrosia beetle. Steyn has already alerted Dr Minette Karsten, Crop Protection Programme Manager at Hortgro Science, to ASM. “This is why Marno and I attended, so we can keep an eye on what other people are researching,” says Steyn.

His take-home from the Symposium? “I think our research tracks what is happening internationally,” reflects Steyn. “I didn’t discover anything revolutionary that we need to add to our gap analysis, but it’s good to keep abreast of new developments and to maintain and develop new research and technical contacts.”

BOX

Plant growth regulators The 15th International Symposium on Plant Bioregulators in Fruit Production was held in Chicago in June 2025. Several South Africans who attended share the highlights and we also bring you insights on plant growth regulators from Em. Prof. Karen Theron and Dr Ian Crouch. For more on chemical thinners and other related content, visit the Fresh Quarterly website at www.freshquarterly.co.za.

Plantgroeireguleerders Die 15de Internasionale Simposium oor Plantbioreguleerders in Vrugteproduksie is in Junie 2025 in Chicago gehou. Ons hoor meer oor die hoogtepunte vanaf verskeie van die Suid-Afrikaners wat dit bygewoon het. Verdere insigte oor plantgroeireguleerders kom van em. prof. Karen Theron en dr. Ian Crouch. Vir meer oor chemiese uitdunmiddels en verwante onderwerpe, besoek die Fresh Quarterly webwerf by www.freshquarterly.co.za.