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202403 Fresh Quarterly Issue 24 08 Preharvest Ethylene Inhibitors Final
Issue 24March 2024

Improve postharvest outcomes with preharvest ethylene inhibitors

Experts explain two agrochemical options that growers can use to manage harvest maturity. By Anna Mouton.

Ethylene is a plant hormone that promotes fruit ripening, flower opening, and leaf dropping. In mature pome and most stone fruit, ethylene drives the formation of more ethylene, so even small amounts of ethylene can initiate ripening — ethylene inhibition is one of the strategies used to extend postharvest quality.

One way to inhibit ethylene is to apply 1-methylcyclopropene (1-MCP) within seven days after harvest. However, there are also preharvest options for ethylene inhibition, as presented at the 2023 Hortgro Science Postharvest Symposium by Prof. Karen Theron of the Department of Horticultural Sciences at Stellenbosch University and Dr Ian Crouch of ExperiCo Agri-Research Solutions.

Prevent ethylene synthesis with AVG

Plants make ethylene from the amino acid methionine via a pathway that includes a precursor called 1-aminocyclopropane-1-carboxylic acid (ACC). The enzyme ACC synthase regulates ACC production and, thereby, the ethylene production rate.

Application of aminoethoxyvinylglycine (AVG) blocks ACC synthase. This compound is available as a commercial formulation registered for use in apples, plums, peaches, and nectarines.

“It’s very critical to understand that AVG blocks ethylene production — it doesn’t prevent the action of ethylene,” emphasised Theron. “Once ethylene has formed, you won’t get any response to using AVG.”

She explained that the plant’s response to AVG depends on the cultivar, the dose, and the timing.

Naturally high ethylene-producing apple cultivars, such as Royal Gala and Bigbucks, generally respond well to ethylene inhibitors. Low ethylene producers, such as Granny Smith and Fuji, respond less well. Most cultivars are somewhere in the middle.

Studies on doses by Michigan State University showed that AVG applications on Gala at rates below the recommended dose had a limited effect, while two applications at the recommended dose nearly entirely suppressed ethylene production — but the cost of two applications likely outweighs the added ethylene-suppression benefits.

Correct application timing is essential. “It must be before the start of ethylene production,” stressed Theron. “In stone fruit that’s 7–14 days before your first major pick.” Growers should keep in mind that the first major pick might be the second pick, in which case they would apply AVG immediately after the first pick.

The recommended timing and doses for apples depend on the cultivar and can be seen in Theron’s presentation on the Hortgro YouTube channel.

Good spray coverage matters, but environmental conditions matter more, according to Theron, as AVG is poorly absorbed. Windless days with high relative humidity are ideal — windless mornings between 04h00 and 06h00 are even better. Re-wetting the trees with pure water 24–48 hours after application increases absorption.

Block ethylene action with 1-MCP

Whereas AVG prevents ethylene synthesis, 1-MCP blocks ethylene receptors. “It attaches to the receptor and locks it down,” explained Crouch. “It prevents the fruit from being receptive to ethylene until new ethylene receptor sites are produced.”

Blocking ethylene receptors stops further ethylene production, so levels remain too low to initiate ripening. Crouch showed data on ethylene levels with and without preharvest 1-MCP treatment in different cultivars.

In low ethylene producers such as Abate Fetel pears, 1-MCP reduced internal ethylene from 8.4 ppm to 0.6 ppm, while in high producers such as Packham’s Triumph, ethylene was reduced from 156 ppm to 0.7 ppm. Apples respond similarly.

“It can be applied in two ways,” said Crouch. “As a plant management tool, two to three weeks before harvest, or, if you have a crisis, it can be an emergency action treatment — for example, if you haven’t got people to harvest the fruit or you can’t get to the orchard because of rain.”

In pears, preharvest 1-MCP treatments can avoid ripening problems associated with postharvest 1-MCP treatments.

For harvest management in apples, Crouch recommends that growers start tracking maturity at least four weeks before their expected harvest release dates and apply 1-MCP when the fruit reach 5%–10% starch breakdown. “You want to get to it early,” he cautions, “before that train runs away.”

Crouch points out that delaying the harvest can increase the risk of sunburn and insect damage. Aside from that, pack-outs from apples and pears treated with preharvest 1-MCP and harvested 7–14 days later were similar to untreated fruit harvested at their usual time.

In a trial with Brookfield Gala, untreated apples harvested at their usual time had a 95% Class 1 pack out, whereas 1-MCP-treated apples harvested 14 days later had a 93.5% Class 1 pack out. But none of the untreated apples harvested 14 days later made it into a Class 1 carton.

For more of Crouch’s results, view his presentation on the Hortgro YouTube channel.

The benefits summarised

Ethylene inhibition slows starch breakdown and loss of firmness. For example, Theron presented data on the effect of AVG on Bigbucks apples. Application 21 days before harvest significantly reduced starch breakdown compared with an untreated control.

“You can use it as a management tool to hold maturity back, to ensure you don’t run into logistical problems,” said Theron. She noted that AVG application 21 days before harvest will delay the entire harvest, whereas application seven days before harvest will target the second and third picks.

According to Crouch, a preharvest 1-MCP application will delay starch conversion and maintain firmness whether applied to less or more mature fruit. Preharvest 1-MCP application also increased the efficacy of postharvest 1-MCP application by reducing ethylene levels in application rooms.

He added that 1-MCP could even be applied if fruit start to drop in an orchard. “It will start to put on the brakes,” he commented.

Both Crouch and Theron reported that preharvest ethylene inhibitors will lead to bigger fruit, as the fruit have more time to grow. This can be good or bad, depending on the desired product. Larger Packham’s Triumph pears may become knobbly.

Preharvest ethylene inhibitors will generally improve green colour retention in cultivars such as Granny Smith and Golden Delicious but may inhibit red colour development in blushed cultivars.

“You must either apply it when the fruit still have enough time to develop a blush,” said Crouch, “or, if you are doing it as a risk management tool right at harvest, the fruit must be sufficiently blushed and ready to pick.”

On stone fruit specifically, AVG can reduce soft tips, and AVG and preharvest 1-MCP will help control greasiness and superficial scald in susceptible apple cultivars such as Granny Smith.

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