To maximize red color, we can influence and manipulate some, but not all of these factors. There are three groups of pigments in apple skin and the concentrations of all three pigments change during the season. Chlorophylls are the green pigments responsible for photosynthesis and are located in chloroplasts within the cell. Carotenoids are yellow, orange or red and are located in chromoplasts within the cells. Anthocyanins are red, blue or purple and are located in the vacuoles in the cell. As apples mature, chlorophyll is degraded and carotenoids increase as chloroplasts transition to chromoplasts. At the same time anthocyanins may increase up to 5 fold. The apple skin has two types of cells: the epidermis is 2 or 3 cells deep and under the epidermis there are 6 to 10 hypodermis cells. The redness of the skin depends on the proportion of these cells containing anthocyanin.
More about anthocyanins
Anthocyanins are water soluble and that is why they become bleached when cooked. In contrast, the red pigments in peppers and tomatoes are carotenoids and remain red when cooked because they are not water soluble. Anthocyanins belong to a class of odorless molecules called flavonoids and taste moderately astringent. Anthocyanins are antioxidants and may protect plant tissues from UV light and high temperatures. Anthocyanins are glycosides, which are organic molecules containing sugar molecules and of the 6 common glycosides the most common is cyanidin-3-galactoside. These glycosides can range in color from orange to blue to purple. The color of these glycosides can change if the pH of the cellular solution is modified; in high pH the color is blue and at low pH the color is red. When peaches are exposed to water with a high pH, streaking or inking of the skin often develops as a dark blue or nearly black color.
The biosynthetic pathway of anthocyanin is fairly well understood. The precursor for anthocyanin is the amino acid phenylalanine and there are seven biochemical steps required to synthesize anthocyanin from phenylalanine. Recent research has shown that anthocyanin synthesis in apple skin and flesh is controlled by a family of genes. Each step in the process is catalyzed by a different enzyme and each of these enzymes is controlled by one or two of the three genes. For three of the first six steps in the process the genes are controlled by light and for three steps the genes are controlled by both light and temperature, but the gene involved in the final step is controlled primarily by temperature. Apple has 17 sets of chromosomes and so far genes for skin color have been identified on chromosomes 2 and 9.
There has been quite a bit of research with 'Honeycrisp' to learn why some apples are striped whereas others are blushed. Sometimes a whole tree produces either striped or blushed fruit, but sometimes both types are found on the same tree and even within a single cluster. Research at Minnesota has shown that striped areas of the skin have higher activity of two of the three genes responsible for red coloration.
Factors affecting red color
There are six primary factors affecting red color development in apple skin and some are related.
Some cultivars or strains of some cultivars lack the ability to synthesize large quantities of anthocyanin. 'Golden Delicious' and 'Granny Smith' are examples of cultivars that develop little red color.
Fruit physiological stage
As apple fruits develop, there are two peaks of anthocyanin development. The first occurs at the end of fruit cell division, when fruits are about 1.5 inches in diameter. Even 'Golden Delicious' fruits develop red color on the sun side of the fruit and light is required for red color development. The second peak occurs in red cultivars as the fruits mature. At this stage both light and temperature are involved. Light is needed to produce sugars that are required for the final step in anthocyanin synthesis. Blue-violet and UV light are most important for red color development, but the amount of light required depends on the cultivar and stage of development.
Light and temperature
If light is the factor limiting red color development, then exposing fruit to additional light during the final two or three weeks before harvest will often enhance color development of apple and peach and this can be done by summer pruning to eliminate upright non-fruiting shoots that shade the canopy interior or by placing reflective mulch under the trees. The other primary environmental factor that can enhance late-season color development is exposure to low temperatures, but the optimum temperature and time of exposure depends on the cultivar and critical values have not been identified for most cultivars. Typically several cool nights followed by warm sunny days will enhance color development. 'McIntosh' requires temperatures below 70 degrees, 'Fuji' requires temperatures in the low 60s, and 'Redchief Delicious' requires temperatures in the low 50s, but one day at 90 degrees will negate the effects of several cool nights. The activity of the gene controlling the conversion of phenylalanine to the second intermediary compound decreases within 24 hours of exposure to temperatures above 90 degrees. This is one reason that 'Honeycrisp' did not color well in some orchards in 2016 when we experienced high temperatures during the harvest season.
Some growers think they can enhance red color development by improving the nutritional status of the tree, but most inorganic elements have little effect on color, especially if temperature and light are not suitable for color development. High late-season nitrogen levels cause phenylalanine to be converted to proteins rather than anthocyanin, so high late season nitrogen levels should be avoided. Adequate levels of potassium are needed for good color development. So if trees are deficient is potassium, applying potassium may enhance color development. If nutritional levels are adequate for good tree growth and fruiting, then the addition of any element will likely not enhance color development. Applying unneeded potassium or magnesium will likely induce bitter pit development rather than red color.
Although there are products on the market, mostly containing microelements, that claim they will enhance red color development, there is little supporting evidence in the scientific literature. In general, the only time we should expect improved red color development from applications of nutrients is when the tree is deficient in one or more element.
Trees with heavy crops often have poorly colored fruit. The reason for this is likely that sugar levels in the fruit are low. 'Honeycrisp' is especially sensitive to heavy crops. Research from Nova Scotia and New York clearly demonstrated that red color is negatively related to crop load and trees with crop densities greater than about 5 fruit/square cm of trunk cross-sectional area had poor fruit color.
Most types of stress, such as water stress or damage from leaf-feeding insects, can reduce photosynthesis resulting in poor red color development. These stresses become more problematic when trees are carrying heavy crops because increased demand for photosynthetic assimilates accompanies the reduced capability of the tree to produce those assimilates.
What about rootstocks?
There is some anecdotal evidence in commercial orchards indicating that rootstock may influence fruit color. A great volume of rootstock research has been published over the past 50 years. In general vigorous rootstocks produce poorly colored fruit because the interior portions of the large canopies are shaded. In studies that were conducted in more than one location and over multiple years at the same location, rootstocks occasionally influenced fruit maturity or red color, but results were always inconsistent. Of the 150+ rootstocks that have been tested thus far, none have consistently affected fruit maturity or red color.
In summary, when highly colored fruit is desired, it is important to plant cultivars and strains that are genetically programmed to produce highly colored fruit. Other than selecting appropriate cultivars, growers have only a moderate ability to encourage red color development. When temperature is adequate for red color development, then pruning and training trees to allow good light penetration throughout the canopy will encourage maximum color development. Preventing stresses on the trees, especially over-cropping, drought, unhealthy foliage and nutritional deficiencies is important, but will not compensate for high temperatures or low light levels.