If apple growers could capture all the sunlight energy that falls on their orchards and convert it into salable fruit, they could sell more than 20,000 bushels per acre each year, instead of about 700. Less than one percent of the sun’s visible light energy striking an orchard is captured in fruit.

That leads John Palmer to say: “I do not believe we have reached the limit of the biological system.”

Still, Palmer thinks great progress has been made in apples in recent years. He came into the world of scientific research as a tree fruit physiologist in 1968, when most trees were still big producers of wood.

The measure of how much fruit a tree produces from all of the dry matter it accumulates is known as the harvest index. In the past, the harvest index was low, because trees put most of their resources into growing structural wood.

Today, the harvest index in modern, well-designed, high-density apple orchards can reach 70 percent at maturity. That means the trees partition 70 percent of the light energy they do convert into dry matter into fruit. Only about 30 percent goes into leaves, shoots, roots, and trunk. Each year the industry’s pruning pile gets smaller as growers learn ways to grow more fruit and less wood.

Published work from the early 1980s suggests a harvest index value of 33 percent for McIntosh on MM.106, Palmer said, but those trees were mostly biennial bearing, so that value may have been on the low side. “My guess is that we have moved from a harvest index of 40 to 50 percent to 60 to 70 percent harvest index in today’s orchards.”

Palmer started his career at the East Malling Research Station in England, where he worked from 1968 to 1991. Since then, he has been with the New Zealand Institute for Plant and Food Research, working with apples and pears in the Nelson region of New Zealand.

Last winter, he gave the Bob Carlson Memorial Lecture during the International Fruit Tree Association’s conference in Grand Rapids, Michigan. Carlson, who died a decade ago, was a Michigan State University rootstock specialist and the first secretary of the IFTA, the organization that began in 1957 and has since done so much to usher dwarfing rootstocks into growers’ orchards.

In Palmer’s assessment, the widespread adoption of intensive planting on dwarfing rootstocks was one of the following seven major changes and developments during his career that have boosted the harvest index of apples:

Quantification of the effects of light interception and distribution on yield and fruit quality
Widespread adoption of intensive planting on dwarfing rootstocks
Understanding of the orchard as a system
A move from pruning to branch manipulation
Use of computer models to aid decision making
Use of plant growth regulators
More rapid turnover of cultivars

Three steps

As Palmer views it, there are three steps on the road to achieving good yields of salable fruit. The first is carbon acquisition—the process by which trees take up carbon dioxide from the atmosphere in photosynthesis, storing the sun’s energy in the form of carbohydrates.
The next step is partitioning—the process in which the tree “decides” whether it will make fruit or make wood.

And the third step is fruit quality—making salable fruit.

“The influence of shade on fruit quality had been known for several hundred years before it was ‘rediscovered’ in the 1960s and 1970s,” he said. Centuries ago, growers learned that shading inside a tree and from neighboring trees affected fruit size, color, and flavor.

“The 1970s and 1980s was a period of quantifying what had been visually noted many years before,” he said. “The quantification looked at the two main areas—light interception and light distribution—and how they influenced yield and fruit quality.”

One of the novel developments in those years was the use of computer models to examine the effect of changing tree height, shape, row orientation, and latitude on light interception.

Different areas of the world receive different amounts of light and at different times.

“Although light interception sets a limit on the dry matter production of the orchard system, the partitioning of that dry matter into fruit (harvest index) is often below the biological limit, as evidenced by the amount of shoot growth on the trees and the size of the pruning heap each year,” he said.

“Harvest index—the amount of dry matter that is converted into fruit—is determined primarily by crop load and the strength of the alternative sinks for carbohydrate, particularly vegetative vigor,” he said.

“Nearly all of our recent changes to tree management have encouraged an increase in the harvest index,” he said. These changes include minimal uses of pruning, tying down limbs, use of growth regulators like Apogee (prohexadione calcium) that shorten shoot growth and, especially, use of dwarfing rootstocks.

Out of the shade

The smaller trees have allowed growers to greatly change the fruitful part of the tree canopy, reducing the interior structure and getting fruit out of the shade. Shade reduces fruit weight, red color, soluble solids concentration, flower bud numbers, and fruit set in future years, he said. Shade has the positive effect of reducing bitter pit and skin russet.

Palmer believes growers will continue to make progress in production of quality fruit, moving toward more precise fruit production systems. Some of the challenges are to make every bud count, reducing the production of unwanted wood that ends up as prunings.

“We need to ensure that for intensive systems of production we are using large well-feathered trees and all the buds we produce in the young tree are used for further vegetative or generative development,” he said. “Too often we allow feathers to develop that are too low in the tree, rather than rubbing them out when small and directing growth into more useful parts of the tree where they can contribute to early cropping. Early intervention is the key to much of tree training.”

Apples are fortunate to have a wide range of size controlling rootstocks, but improvements are still needed, particularly in the area of resistance to disease and soil-related problems, he said.

Undoubtedly there will be rapid development of automation in the future, he said. And there will be advances in molecular biology that will multiply the turnover of apple cultivars still further.

“We really need to get to grips with issues of sustainability, beyond integrated pest management to carbon footprints,” he added. “Global warming skeptics are ignoring the major challenges we face ahead of the end of cheap fuel based on petroleum.”