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Though abscisic acid (ABA) is one of the five major classes of plant growth hormones, it has never really been considered as a horticultural crop management tool because of its high cost. But with recent technological manufacturing advances that have brought down the cost of production, ABA could have potential as a thinning agent for tree fruit growers in the future.

For years, uses of ABA were not seriously considered for commercial horticulture. At $670 per milligram (the purchase price from a chemical supplier of laboratories), ABA just didn’t make economic sense for use as a chemical thinner in orchards that would require many, many grams of material, said Gregory Clarke, field research and development scientist for Valent BioSciences. However, Valent recently invested in production technology that enables manufacture of the growth hormone at a more reasonable cost.

"Now, ABA is economically feasible for agriculture. It’s not outrageously expensive," Clarke said.

Abscisic acid is associated with many plant growth functions, but it is primarily recognized for its stomatal control, and water and photosynthesis relationships, he explained. It is also involved in plant functions like stress, growth promotion, fruit maturation and ripening, senescence of leaves and plant organs, abscission, and dormancy. Work by Valent shows that a drench of ABA to sweet basil reduces damage that the herb typically receives when exposed to cold temperatures.

Valent is working to develop commercial applications for the hormone. An experimental use permit has been granted to Valent by the U.S. Environmental Protection Agency for using ABA on ornamentals during shipment and display at retail centers to protect the plants from drought stress, along with another permit for Crimson seedless table grapes to improve color.

New thinner

Valent, along with university researchers, have begun studying the use of ABA as a chemical thinning agent for apples, looking at various rates and timing, and in combination with BA (benzyladenine or MaxCel). Several years of trials show impact on fruit set and improvement in fruit size.

"Results to date have been encouraging," Clarke said, but added there are still many challenges to bringing ABA to the fruit industry as a thinning agent. Valent would like to find ways to optimize the tree’s uptake of ABA, which would allow further rate adjustments and help keep the cost of application down. Though the manufacturing process has reduced its cost, the hormone is still expensive, especially when compared to other growth regulators.

The company also needs to evaluate the temperature sensitivity of ABA. "All thinners, especially hormones like BA, are very sensitive to temperatures after application. It’d be nice to have a thinner where temperature didn’t matter so much," he said.

Moreover, evaluation is needed on its effects on fruit size and whether side effects can be minimized or eliminated. "ABA, at the right dose, will cause the oldest leaves on a spur terminal to yellow and drop off, which is not particularly a good thing," Clarke said. "But when used with BA, the combination eliminates that effect."

New use

Valent is also assessing a new use for an old product.

ReTain (aminoethoxyvinylglycine), or AVG, is commonly used by orchardists to manage harvest and slow down fruit maturity. Early research showed that AVG also can improve fruit set the following year in apples and pears when applied at postharvest or early in the season, Clarke said.

ReTain-treated fruit trees have shown from 20 to 200 percent improvement in fruit set in trials. AVG is already registered for improving fruit set in walnuts, a crop that suffers from pistilate flower abortion. Additional work is needed in pome fruits.

Clarke sees potential for the product in pome fruit that chronically have poor fruit set, adding that Valent would have to determine if the market supported the additional research and registration work.

"There’s a clear need in walnuts, but only in some pockets have I heard about a persistent need to improve fruit set in apples in the United States," he said. "For pear, I’ve heard more about poor fruit set and the need from European producers."

While AVG shows promise, optimum rates would need to be determined and the interaction of ReTain with other thinners explored if the use were to be added to the label. Additionally, the effects of ReTain on final fruit quality would also need to be studied.

He emphasized that new compounds, while sometimes interesting on the bench top of a research laboratory or in the greenhouse, must be of value to commercial agriculture. The economics of product development must pencil out for the company.

Clarke shared Valent’s perspective on the two new potential crop load management tools during the Great Lakes Fruit, Vegetable, and Farm Market Expo in Grand Rapids, Michigan.