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202409 Fresh Quarterly Issue 26 12 Lee Kalcsits
Issue 26September 2024

Roots to fruit

Apple rootstocks directly and indirectly impact fruit’s mineral content and the risk of nutrient-related disorders. By Anna Mouton.

“It all starts in the soil, but the soil is often not the main factor driving nutrient-related disorders — it’s the plant’s physiology that’s affecting imbalances of calcium or other nutrients in the fruit,” said Prof. Lee Kalcsits, Endowed Chair of Environmental Physiology for Tree Fruit in the Department of Horticulture at Washington State University.

Growers tend to choose rootstocks based on vigour and productivity, but Kalcsits’ collaborative research with the Root2Fruit SCRI project team demonstrates the importance of considering how rootstocks directly affect nutrient uptake and indirectly influence nutrient distribution in the tree.

Direct and indirect rootstock effects

Rootstocks differ in their ability to absorb water and nutrients from soil. Kalcsits illustrated this with results of rootstock trials conducted by then PhD student Dr Nadia Valverdi that included G.41 and M.9. They are in the same vigour class and produce similar-sized fruit, but fruit potassium levels are significantly higher on G.41 than on M.9.

“That translates into increased bitter pit for G.41,” said Kalcsits. “It’s a direct effect of G.41’s ability to take up more potassium than M.9.” High fruit potassium levels are associated with bitter pit independently of fruit calcium levels.

Differential mineral uptake could be due to rootstock architecture or physiology. “We’re not sure of the mechanisms, but we know the differences exist,” observed Kalcsits. “It’s something we’re continuing to investigate.”

The indirect effects of rootstocks result from interactions with scion behaviour, including changes in vegetative vigour, biennial bearing, crop load, and fruit size. These are all responsive to horticultural manipulation — they are levers a grower can pull to mitigate the risk of nutrient-related disorders.

Kalcsits shared survey results for bitter pit in a large sample of commercial orchards across Washington State. “The main drivers of bitter pit risk were very high vegetative vigour and low crop load,” he said.

Much of his work has been on Honeycrisp, widely grown in North America and particularly susceptible to bitter pit. Honeycrisp is also prone to biennial bearing. Growers can largely mitigate the risk of bitter pit by careful attention to crop load to avoid alternation.

“It’s really important not just to manage crop load year to year or within a year but also to manage variability in the orchard,” advised Kalcsits. During any given year, even if 80% of the trees in an orchard are cropping heavily, the fruit from the 20% that aren’t will likely suffer from bitter pit, affecting the overall orchard pack-out.

Nutrient delivery into fruit

To clarify how tree growth and crop dynamics affect nutrient-related disorders, Kalcsits explained the physiology of nutrient delivery into fruit. “Many nutrients follow the xylem stream when taken up from the soil,” he said. “They follow the transpiration stream — they go where water is being used.”

Raquel Gomez, an MSc student in Kalcsits’ group, compared the xylem activity of fruit at different times during the season. Xylem activity and nutrient uptake are far greater in fruitlets early in the season than in fruit in the middle of or later in the season. One reason for reduced xylem activity is that apples form a waxy layer and transpire far less later in the season.

Another is that developing apples increasingly demand carbohydrates, which are transported from the leaves in the phloem. In the process, the fruit’s primary water source shifts from the xylem to the phloem.

“Other nutrients — potassium, magnesium, and nitrogen — come with those carbohydrates and water from the leaves,” said Kalcsits, “but not calcium because it’s not phloem mobile.”

The flush of root growth in spring is associated with active nutrient uptake. In overly vigorous trees, those nutrients mostly go to the leaves, and the fruitlets miss out on their window for calcium loading.

Later in the season, vigorous trees with low crop loads and large leaf areas pump out carbohydrates that boost fruit growth. As the apples are no longer taking up much calcium, whatever they have is diluted as their volume expands.

For her PhD with Kalcsits, Dr Erica Casagrande Biasuz examined bitter pit in Honeycrisp on nine rootstocks, ranging from extremely to moderately dwarfing. “We all know how susceptible Honeycrisp can be to bitter pit, especially early in production,” he said.

Trees on B.9 were the least vigorous, with an average shoot length of about 13 centimetres, and had the least bitter pit, about 15%. Trees on the two most vigorous rootstocks, G.87 and G.210, had average shoot lengths of about 51 centimetres and close to 100% bitter pit.

“We saw a clear relationship between vegetative vigour and bitter pit incidence,” stated Kalcsits, “caused by the interactions between water use, carbohydrate loading, and dilution of the calcium pool as the fruit starts to grow, when fruit size is large.”

Implications for managing minerals

“Your rootstock choice is the foundation of everything in your orchard,” said Kalcsits. “It’s how you meet your production target and optimise labour. But it’s also important to know potential risks that come along with specific rootstocks.”

Growers should consider the direct effects of rootstocks on mineral uptake, especially when planting scion cultivars that are sensitive to bitter pit or other nutrient-related disorders.

Kalcsits stressed the crucial role of indirect factors, especially crop load. “Crop load is one of the key horticultural elements growers can manipulate. They must manage crop load and variability within the block, not only for fruit size but also for nutrient-related disorders.”

Where excessive vigour is a problem, changing the leaf-to-fruit balance is the solution. “If you have too much horsepower behind that tree and you’re using vigour management to control nutrient-related disorders, it will only work if it reduces overall leaf area,” he emphasised.

Early in the season, a smaller leaf area means less competition between fruitlets and leaves for the calcium carried in the xylem. Later in the season, a smaller leaf area means a smaller carbohydrate factory, which reduces carbohydrate loading and calcium dilution in growing fruit.

“Ideally, you’re choosing a rootstock that meets your vigour targets for your specific site right from the beginning,” noted Kalcsits. “But to do that in different climates, environments, and soil systems, for different growing systems, it’s important to identify problem combinations ahead of time, through long-term evaluation and testing.”

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