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Micrografting Sweet Cherry Trees May Provide Orchards a Faster, More Reliable Supply of Trees
Imagine ordering a piece of cherry pie at a restaurant, and being told that your pie would be delivered in two or three years. On your way out the door, you’d probably tell the waiter, “That’s no way to do business!” Orchard managers, however, have to place their orders for sweet cherry trees two to three years in advance of receiving and planting them. For Washington State University undergraduate Matthew Allan, this is not merely an abstraction, but a family problem, as his father is a cherry producer near Yakima. His father has not only suffered long delivery cycles but, given the risks in the nursery business (including heavy losses due to frost in the past few years), he hasn’t always taken delivery of the number of trees he’s ordered.
Cherry trees are normally started in open-air nurseries. To create the rootstock, a whip, or young branch, is planted horizontally in the ground, allowing it to generate multiple shoots. The young shoots may be assailed by frost, pests, and diseases. Costly pesticides are then applied. After a full year, the rootstock is old enough to be grafted to the scion, a woody shoot with buds that will become the fruit-producing part of the tree. Then the nursery waits at least another year for the graft to heal. The bottleneck in the process is the supply of rootstock: Anything choking rootstock production reduces the number of trees that can be grafted.
Allan, a food science major working in the lab of assistant professor and horticultural genomicist Amit Dhingra, realized that if he could bring the process into the lab, he might be able to better control environmental factors. Dhingra also hoped to speed the process. “We wanted to see if it was possible to get an orchard-ready tree within a year, instead of two to three years” he said.
To begin the process, Allan and graduate student Tyson Koepke established rootstock materials in tissue culture by carefully sterilizing the buds of dormant rootstock trees, killing pathogens while keeping the plant cells alive. They then placed the plant tissue into a growing medium, a gelatin-like mixture of vitamins, minerals, and carbohydrates. Small rootstock trees sprang up, and the trees were then transplanted into soil. About 16-20 weeks after first placing the plant material in tissue culture medium, the rootstock was ready to graft.
Allan also grew the scion trees in culture using the same method, without transplanting them into soil. He then snipped the top of a scion tree to graft onto the rootstock. “Since the rootstock was growing in soil, and the scion was growing in medium, they were on different pages developmentally and metabolically,” explained Allan. “In the future, we’ll try to transplant the scion at the same time as we transplant the rootstock, so both components are growing at the same rate.”
Dhingra also wants to incorporate mist beds into the process. Mist beds are a type of growth chamber allowing for continuously high humidity, enabling the grafted trees to heal and mature more quickly. Success would allow producers to grow three to four cycles of trees each year, rapidly responding to orchard demand for new trees, and in the quantity desired. Orchards could plant trees more quickly, manage their land more efficiently, and respond to market demand for cherries more promptly.
As Allan looks forward to graduate school, he is appreciative of his experience as an undergrad at WSU. “Working with Amit was a very positive experience,” he said. “I felt lucky having someone of his expertise I could call on.”
Dhingra likewise praised Allan. “Matthew is that great combination of a person who understands the field process of the crop and someone who can work well in the lab. Cherries are a very difficult system in tissue culture, but Matthew had very good results. He established basic protocols for cherry grafting in the lab, which we can continue to research and refine. ”
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