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In 2017, Cornell University physiologist Lailiang Cheng planted Honeycrisp on a variety of rootstocks in sand cultures as part of an experiment to understand how different rootstocks vary in their ability to uptake and deliver nutrients to the scion. (Kate Prengaman/Good Fruit Grower)

In 2017, Cornell University physiologist Lailiang Cheng planted Honeycrisp on a variety of rootstocks in sand cultures as part of an experiment to understand how different rootstocks vary in their ability to uptake and deliver nutrients to the scion. (Kate Prengaman/Good Fruit Grower)

Drip by drip, some 300 Honeycrisp trees in a Cornell University orchard soak up the perfect blend of nutrients as they grow in sand.

Removing the soil and its complex community of microorganisms gives researchers a clean slate to see how rootstocks differ in their ability to take up nutrients and transport them into the scion and, eventually, the fruit.

“We feed them all with adequate and balanced nutrients to see what moves into the trees,” said Lailiang Cheng, professor of tree fruit physiology at Cornell. The trees are growing well, but not for long. Cheng plans to destructively harvest them over the next few years to measure the biomass and nutrient composition of the leaves, fruit, branches and roots.

The findings will provide one piece of an industry-priority puzzle: how to use rootstocks to reduce bitter pit in Honeycrisp.

The project is part of a broad-ranging $4.3 million research effort to accelerate rootstock development for high-density apple orchards and to better understand the critical role rootstocks play in nutrition, horticulture and plant resilience to soil stresses and replant disease. Cheng leads the project, along with investigators in Washington, Idaho, Utah, Michigan and New York.

Growers and researchers across the country observed that bitter pit incidence significantly differs by rootstock; for example, during a particularly high bitter pit year in Michigan, one recent rootstock trial saw bitter pit range from none to over 50 percent. In one Washington study, incidence ranged from 9 percent to 37 percent.

“Bitter pit is a complex problem that requires a comprehensive approach,” Cheng said, including vigor management, crop load balance and nutrition. “Using rootstocks is a long-term solution.”

A better B.9

The first inkling that rootstocks play a role in bitter pit tolerance came from observing Honeycrisp on Budagovsky 9 roots. It imparts protection against bitter pit, but the mechanism for that is the rootstock’s overall low vigor, Cheng said.

Calcium deficiency causes bitter pit, specifically a lack of calcium in the cell membranes which weakens the structure and leads to pockets of leaky, dying cells. Work by Washington State University physiologist Lee Kalcsits shows that Honeycrisp is predisposed to this structural weakness on a cellular level. But the trees can’t move calcium freely to the area where it’s needed, either.

“Calcium follows the water stream. Leaves transpire a lot and fruit don’t,” Kalcsits said.

That’s why overirrigating puts Honeycrisp growers at risk of high culls, Kalcsits said at the Washington State Tree Fruit Association’s annual meeting in Yakima, Washington, in December during a session on vigor management. (See our related story, Fight vigor with water.)

Rootstocks vary in their efficiency at moving water into the scions, Kalcsits said. Less vigorous B.9 is less efficient at water transport, so more calcium can end up in the fruit than when Honeycrisp is grown on a more vigorous rootstock.

Understanding that mechanism shows that while B.9 offers bitter pit tolerance, it may still not be the ideal rootstock for most growers, since cumulative yield tends to be lower. From an economic perspective, looking at bitter pit-free yield is a better tool for comparing rootstocks to bitter pit incidence, Cheng said, citing work by his Cornell colleague, Terence Robinson, that shows higher returns on Geneva 11 or Geneva 41.

Within the Geneva series, the bitter pit incidence ranges significantly, with G.214 and G.969 appearing to impart much more tolerance than G.11 or G.41. Rootstock breeder Gennaro Fazio will also use insight from the nutrition studies by Cheng and Kalcsits to inform selection and evaluation going forward.

In 2018, the second-leaf Honeycrisp produced their first crop, which was analyzed for nutrient content to see which rootstocks can provide more nutrients to the fruit. (Courtesy Lailiang Cheng)

In 2018, the second-leaf Honeycrisp produced their first crop, which was analyzed for nutrient content to see which rootstocks can provide more nutrients to the fruit. (Courtesy Lailiang Cheng)

Physiology focus

Rootstocks’ ability to take up and transport other nutrients matters as well — specifically potassium, magnesium and phosphorous, Cheng said. That’s another aspect of his ongoing research.

“Our work shows that when you have a lot more potassium, a lot more phosphorous, you tend to have more bitter pit,” he said. “A lot of people think it’s a calcium issue, but it’s a balance issue.”

Comparing Gala and Honeycrisp, the latter has only about half as much calcium in the fruit, but significantly more potassium and phosphorus in the peel, Cheng said. Honeycrisp with bitter pit has significantly higher levels of those nutrients relative to calcium than does healthy fruit.

To understand why, it helps to know how the nutrients behave on a cellular level. When there is higher phosphorous, more calcium is precipitated, making it unavailable. Potassium, on the other hand, competes with calcium to be transported in the cell, and if there is more of it floating around, it’s even harder for calcium to get where it needs to go.

Think of the trickle-up effect, if you will, to getting the calcium from the soil into the right spot in the fruit to provide structural integrity to the cell membrane. Cheng calls this partitioning. Roots need to uptake calcium. Most of it goes to the leaves, even more so in Honeycrisp than Gala. What ultimately makes it to the fruit then has to compete with potassium and magnesium to get partitioned into various parts of the cells.

“You have less calcium partitioned into the fruit, and then, within the fruit, you have more potassium and less calcium and more phosphorous, so that makes the situation worse,” he said.

Some of the Geneva rootstocks also turn out to be more efficient at taking up potassium. If growers are applying the same amount of potassium as they did in the past with B.9 or Malling 9 orchards, that could be leading to the decrease in cell membrane calcium and higher bitter pit as well, Cheng said.

Bringing down potassium applications may be more cost-effective to control bitter pit than increasing calcium. That, along with vigor management, crop load management and appropriate irrigation can also work to reduce Honeycrisp’s genetic predisposition to bitter pit while researchers seek the best rootstocks to give the at-risk apple any possible advantage.

“Think of it as a cliff. Honeycrisp is teetering on the edge, so any factor — irrigate too much or drought stress or too much potassium — and you are going to push that Honeycrisp over the cliff,” he said. “Gala, on the other hand, it’s so far away from the cliff that no matter what you do you don’t push it over.” •

—by Kate Prengaman

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