Let there be light
While growers cannot turn cloudy days into sunny ones, they can optimize light interception and distribution to maximize yield and quality.
Light can improve the yield and productivity of an apple orchard in two ways, according to Dr. Jens-Norbert Wünsche of the University of Hohenheim in --Germany.
"One is to increase total dry matter production or total yield, which is dependent on light availability and the amount of light captured by the tree canopy. The second option is to increase the magnitude of partitioning of dry matter toward the fruit, also known as carbohydrate partitioning. This is a process you can influence by tree management and also by utilizing more of the available light."
Wünsche shared his research and knowledge about apple productivity during the Great Lakes Fruit, Vegetable, and Farm Market Expo held in Grand Rapids, Michigan. Some of the research that he is involved with includes biennial and irregular bearing of apples and the effects of hail netting on apple productivity and fruit quality. Growing fruit under hail netting, a widespread practice in European orchards, can result in fruit color problems.
Incident light or solar radiation might start out at 100 percent during a clear, sunny day, but the amount incident light that is actually used in the carbohydrate partitioning is deceiving.
"Surprisingly, less than 0.5 percent of the incident light is potentially used for food production," Wünsche said. "If you are able to increase this percentage by managing your crop properly, you will get a higher yield and better quality of fruit."
Only about half of the annual solar radiation is in the wavelength that is usable by the tree canopy, which is known as photosynthetically active radiation or PAR. But that number is further reduced by the length of the growing season, which is around nine months in many regions. Only 40 percent of the remaining light falling on the orchard is intercepted or captured by the tree canopy. By the time photosynthesis, canopy respiratory losses, and carbohydrate partitioning are figured into the tree's use of light, the amount of light used by the tree is quite small, he said.
Michigan and other northern states that are at a latitude of around 40° to 45° N receive good light, but other fruit-producing countries in the world, particularly those in the Southern Hemisphere at lower latitudes, have higher incident light.
"The trees seem to be able to make use of this additional light, and this is the reason why some of the orchard systems in countries like New Zealand have higher productivity," Wünsche said.
Growers in light-limited regions can overcome reduced light availability by changing their tree planting systems to make up for less light, he said.
Light interception can be enhanced by increasing the tree density, optimizing the leaf area to land area ratio, reducing the distance between tree rows, increasing tree height, and orienting tree rows to north-south.
Studies have shown the slender spindle orchard system on dwarfing rootstocks, planted at densities of around 2,500 to 3,500 trees per hectare (equivalent to about 1,000 to 1,400 trees per acre), provide about the right tree number to achieve maximum productivity.
"You will not gain any further increase in yield by planting more trees," Wünsche said.
Leaf area index is important to light interception, he noted, recommending the ratio of three square meters of leaf area per square meter of land area.
While light interception is important for fruit yield and photosynthesis, light distribution is even more important, Wünsche explained. Light distribution influences flowering and fruit set, internal and external fruit quality (fruit growth, weight, firmness, and color), carbohydrate partitioning, and nutrient supplies from the xylem.
Light profiles within the canopy are influenced by the ratio of tree height to alley width and tree canopy leaf area. Best light distribution occurs at a ratio of one to one. But tree form also affects light distribution. "Trees with a triangular shape have the best light distribution," he said.
Light distribution is especially important three to five weeks after bloom during the cell division stage, when cells are being developed. "The more cells laid down at this earlier stage of development, the greater the fruit size," Wünsche said, adding that the more light within the canopy, the greater the ability to convert photosynthesis building blocks into carbohydrates.
"But, at the same time, temperature has a detrimental effect," he said. Though it's providing energy for the tree, the greater the temperature, the greater the respiratory loss, which burns off carbohydrates. "This is part of the reason why New Zealand has some of the highest apple yields—because they have moderate temperatures, 25° to 30°C (77° to 86°F), during fruit development."
Research studying the light transmission within the tree canopy shows the influence that light has on fruit quality. More light on the outside and upper portion of the canopy translates into better fruit distribution, heavier fruit weight, higher soluble solids concentration, and a higher percentage of fruit with red blush on the upper and outside of the canopy.