Red fruit color is associated with higher sales because color appeals to consumers. Apples with rich, dark color are perceived as having increased nutritional value and antioxidant properties.
Dr. Macarena Farcuh, assistant professor of horticulture, University of Maryland, addressed fruit growers at a Fruit Quality and Safety Workshop, saying the challenge in growing desirable red apples is that among all the apple cultivars available, there’s poor or marginal development of red color that doesn’t meet the requirements for retail sale.
Several compounds influence in color development in apples. “Green hues are due to the presence of chlorophyll,” said Farcuh. “Yellow hues are from the presence of carotenoids. Red color is due to anthocyanins. The content and composition of anthocyanins will determine the intensity or quality of apple red skin coloration.”
Anthocyanins are developmentally regulated and produced in small quantities when fruit is at fruitlet stage but produced mostly during fruit ripening. Anthocyanins are also highly dependent on the sugar content of fruit.
Several things influence red color, including environmental factors such as temperature and light. “The ideal conditions for red skin coloration are bright, clear days with temperatures around 77º Fahrenheit, and cool nights around 69º,” said Farcuh. “In this situation, the trees will not be stressed. There’s increased photosynthesis during the day and production of sugar, and decreased respiration at night so sugars are not being oxidized.”
She added that sugars being produced and not broken down is key because sugars are an important raw material for anthocyanin production.
Anthocyanin production is suppressed if daytime temperatures above 90º are combined with warm nights with temperatures above 68º. Farcuh said this is a factor in fruit coloration for early cultivars such as Honeycrisp.
Light, both intensity and quality, also influences apple coloration. Fruit requires adequate light to enhance coloration. Ultraviolet rays are the most important aspect for promoting anthocyanin production.
Orchard elevation also plays a role. Higher elevations allow for a more optimal temperature differential between day and night. “That will increase anthocyanin accumulation and development of red skin color,” said Farcuh. “The aspect, or the way the sun hits the earth, of the orchard also matters. In general, the sun’s rays will shine more directly on southern aspects compared to northern aspects.”
The water and nutrient holding capacity of orchard soil also affects anthocyanin production. Excessive or late nitrogen application may reduce red coloring and promote tree vigor and higher potential for shading due to less UV light in the canopy.
“Another important factor is cultivar and rootstock selection,” said Farcuh. “Different cultivars have different maturity dates, and this directly determines the temperature range at fruit maturity.”
Prolonged warm spells in August and September can affect the color of early-ripening cultivars such as Gala and Honeycrisp. This, combined with stricter color grading standards, results in a loss of value in some cultivars due to lack of red coloration.
“The older, less colored strains are being pushed out of the market because they are not complying with red color requirements,” said Farcuh. “But there are newer, highly red colored strains like Gale Gala®, Firestorm Honeycrisp™ and Royal Red Honeycrisp® that genetically will produce higher red coloration.” It’s important to remember the propensity for red coloration involves more than one factor – not just genetic background.
Rootstock also influences apple coloration. “Size controlling rootstocks can reduce the canopy volume and density, and that increases light distribution and improve red coloration,” said Farcuh. “Rootstock should give us vigor that is strong enough to create a productive tree but without a lot of shading. The recommendation is to plant the most dwarfing rootstock that can be managed given the inherent vigor of the soil and the cultivar.”
Balanced mineral nutrition for apple trees is critical in achieving highly marketable, attractive red fruit. Nitrogen and potassium are primary influencers of red color. “We need enough nitrogen to maintain high productivity, but we know excessive nitrogen is detrimental for red skin coloration,” said Farcuh. “It can result in fruit with a dull, muddy color.”
Nitrogen also promotes excessive vegetative growth, which decreases light exposure and may result in lack of color.
Low potassium levels are linked to poor fruit coloration. In general, the optimal range for leaf potassium in apples is around 1.5% to 2.5%. “There’s a need for active management of this mineral to replace what is removed by the crop,” said Farcuh. “There is no evidence that excess potassium can increase red color, but there is a lot of evidence that excessive potassium can increase bitter pit in Honeycrisp.”
Crop load management plays a role in desirable red skin coloration. “With an excessive crop load, there’s a lack of red color because there’s a shortage of resources (sugars and nutrients) that reach the fruit,” said Farcuh. “In the case of Honeycrisp, highly nutritional sugars affect fruit coloration development of the current year’s production but also affect the juvenile buds that develop for the next year’s crop.”
Low crop load and excess vegetative growth leads to a lack of red skin color due to excess canopy. With too much shade, fruit isn’t receiving the necessary sunlight for anthocyanin development.
“In the ideal situation, a balanced crop load target is seven to eight fruits per square centimeter of trunk sectional area,” said Farcuh. “This will allow the sunlight to reach the surface of the fruit and have good fruit size with good returning bloom for the coming years.”
Fruit maturity management also plays a role in red coloration. “The synthesis of anthocyanin is internally regulated and happens early in the fruitlet stage,” said Farcuh. “But the important biosynthesis of anthocyanin is happening during fruit ripening. Ethylene will control ripening and can be regulated with the use of pre-harvest growth regulators.”
Ethylene regulation can also affect red skin coloration. “Ethephon is an ethylene-releasing chemical and will promote fruit ripening and increase red skin color,” she said. “If temperatures are above optimum ranges, it will only advance maturity and will not increase color. Remember, it isn’t just one factor – it’s all integrated.”
Farcuh added that Ethephon can accelerate fruit abscission and may negatively impact fruit storability.
ReTain®, which is the opposite of Ethephon, inhibits ethylene production and delays fruit ripening. It also has negative effects on color development. Harvista™ blocks the perception of ethylene so there’s no response of the fruit to ethylene, which can delay ripening.
Farcuh said the use of ReTain and Harvista allow delayed harvest, which can prevent fruit drop and possibly delay maturity to a cooler weather window and help fruit reach optimum coloration.
Promoting and preserving favorable red coloration in apples involves a combination of factors, and with comprehensive management, growers can produce higher quantities of highly desirable red fruit.
by Sally Colby