Harvesting the light to drive production

John Palmer, New Zealand fruit research

“Use the light you have to your best advantage,” he advised growers during the Cornell University In-Depth Fruit School in Geneva, New York, early this spring. “It’s free and it drives your production.”

Centuries ago, growers learned that shading inside a tree and from neighboring trees affected fruit size, color, and flavor, he said.

“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.”

Palmer developed computer models to examine the effect of changing tree height, shape, row orientation, and latitude on light interception. A shaded leaf contributes little.

His early work showed there was a direct linear ­relationship between light interception and fruit yield.

“High yields cannot be achieved without high light interception,” he said.  “It sets the upper limit for production. It can be increased by closer plantings, taller trees, and closer row spacings.

“But orchards are more than light-harvesting systems. The light distribution without our trees has a major influence on the quantity and quality of the fruit we produce.”

Capturing light

Palmer has said that if apple growers could capture all the sunlight energy that falls on orchards, they could produce more than 20,000 bushels per acre each year, instead of about 700. Less than 1 percent of the sun’s ­visible light energy striking an orchard is captured in fruit.

Given the fact that trees are only in leaf part of the year, and that trees start small and need to grow to fill space, that is not surprising.

The desire to intercept a higher percentage of available light and avoid the effects of shading led to the gradual adoption of systems with taller trees, narrower alleys, and thinner, hedgerow-like rows.

The desire to harvest more fruit required that trees put more of their effort into making fruit and less into making wood. Today, about 70 percent of the available carbohydrates are partitioned toward fruit, much more than it used to be, and the major reason is dwarfing rootstocks, he said.

3 keys to good yields

As Palmer sees it, there are three keys to achieve good yields of salable fruit.

• Carbon acquisition—the process by which trees take up carbon dioxide from the atmosphere through photosynthesis and store the sun’s energy in the form of carbohydrates.

• Partitioning—the process in which the tree “decides” whether it will make roots, shoots, leaves, wood, or fruit.

• Fruit quality—making fruit people will buy.

Major changes

Palmer lists seven major developments that occurred during the last 40 years.

• Realization of the importance of light interception and distribution, and their relationship to yield and quality

• The widespread adoption and planting on dwarfing rootstocks

• The understanding of an orchard as a system

• The general move away from pruning toward branch manipulation as a tree training tool

• The use of computer models to aid decision making

• The use of plant growth regulators in the nursery and in the orchard—PGRs to produce high quality feathered trees in the nurseries and Apogee to control shoot growth in orchards

• The application of physiological understanding to new cultivars, some of which are challenging to grow
Palmer believes there are many ­challenges ahead for fruit researchers.

A good, mature orchard—with good pruning and leaves operating at full efficiency—will gather 70 percent of the potential light energy from light, but it takes time to reach full production and biennial bearing reduces fruit production potential.

Other things need work

“Although eye appeal remains important in many fruit, particularly color and freedom from blemish, taste is becoming increasingly important,” he said. “Initial purchase is based on eye appeal, but repeat purchase is based on eating experience. Our production target should therefore be yield, fruit size, appearance, and eating quality—maturity and dry matter concentration. You can ruin a market by giving consumers fruit that looks good but doesn’t taste good.”

His prescription for grower success?

• Make every bud count. Every leaf needs to function, every fruit be salable.

• Improve rootstocks in apples and bring dwarfing rootstocks to pears.

• Grow fruit to product specification, with greater emphasis on eating quality and less on cosmetic appearance.

• Develop more multidisciplinary research teams that include molecular biologists.

In addition, he said, three new factors are making their influence felt: sustainability, carbon footprint, and water footprint.

“Our fruit-growing industries need to continue to produce desirable, healthy, salable fruit, produced in sustainable, reliable, and predictable ways,” he said. “Only by understanding the way in which the tree dynamically responds to its environment and its own internal regulation can we achieve these goals.” •

For more information about Palmer’s research, read “Capturing the light,” Good Fruit Grower, November 2010.

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Dean of physiologists

Dr. John W. Palmer was born in Bristol in the United Kingdom. He began work at the East Malling Research Station in 1968 and earned his doctorate at Nottingham University in 1976.

From the beginning, he studied the effects of shade on apple yield and quality, measuring light interception and developing computer models to predict light interception and distribution.

While at East Malling, he was a coordinator of the first European planting systems trial.
In 1991, he moved to New Zealand. He went from the cool, cloudy climate of England’s 51 degrees north latitude to the warm sunshine of New ­Zealand’s 41 degrees south.

At the Plant and Food Research Institute’s Motueka Research Center, he has pursued interests in planting systems for apples and rootstocks for apples and pears.

“He is one of few scientists to measure total dry matter production and partitioning in apple orchards,” said Cornell University pomologist Dr. Terence Robinson. He called Palmer “the dean of tree fruit physiologists.”

Palmer has also studied fruit dry matter concentration as related to fruit quality and the effects of temperature on the expression of genes associated with fruit color.

He officially retired in 2013, but continues as a Fellow of Plant and Food Research. In March, he received the Outstanding Research Career Award from the Environmental Physiology of Fruit Crops Working Group.

“I have been privileged to be a bench and field scientist throughout my career and have remained within the pipfruit physiology discipline,” he said, adding proudly, “I have never been a manager.”