Understanding the factors that affect colour can help growers to improve their pack-outs. By Anna Mouton.
Blushed pears are eye-catching and earn a premium in the market provided they have sufficient red colour. In South Africa, blushed cultivars occupy more than 40% of the area planted to pears. Forelle is our second biggest cultivar and it alone accounts for nearly 28% of the total orchard area.
From 2016–2021, the number of hectares of Forelle, Rosemarie, Cape Rose, and Celina increased by more than 20%. The return on these orchards will largely depend on the redness of the fruit — green is not the colour of money when it comes to blushed pears.
Why are some pears red?
Red colour in pears is caused by water-soluble pigments called anthocyanins. Anthocyanins are also responsible for the red, blue, purple, or black colour of many other fruits, including blueberries, blackberries, cherries, aubergines, and apples. The health benefits of anthocyanins have been widely reported, giving consumers another reason to buy brightly coloured fruit.
As for the fruits themselves, most produce anthocyanins to attract non-human consumers who will spread their seeds. This function is probably less important in European pears as the ancestral wild pear did not have red fruit and domestic pears do not depend on seed dispersal for survival.
Anthocyanins also protect plant tissues against oxidative damage and prevent inhibition of photosynthesis by high light levels. Light stimulates anthocyanin synthesis in apples and pears — more on this later.
Anthocyanin dynamics in pears
Blushed and fully red pears have two anthocyanin pigments, usually in the deeper layers of their skin. The concentration of the pigments determines the intensity of the colour.
Anthocyanin formation and breakdown are ongoing processes. Fruit become redder when formation exceeds breakdown and fade when breakdown overtakes formation.
In pears, anthocyanin concentrations tend to peak about midway between blossom and harvest. Thereafter, their ability to produce anthocyanins decreases, so concentrations drop as accumulated pigments are broken down. This differs from apples where the ability to produce anthocyanins increases with maturity.
Pear cultivars vary in their potential to accumulate anthocyanins. Cultivars that attain high anthocyanin concentrations in the fruit can lose more pigment before their redness fades than cultivars that have less anthocyanin to begin. This is why the red colour fluctuates more in Rosemarie than in Flamingo or Forelle — Rosemarie has among the least ability to accumulate anthocyanins of any blushed or fully red pear grown in South Africa.
In contrast, cultivars such as Bon Rouge, a fully red mutation of Williams’ Bon Chrétien, can remain red despite substantial anthocyanin breakdown. This is called buffering — large anthocyanin reserves cushion the fruit against colour loss due to pigment breakdown.
The role of light and temperature
Although plants need light for photosynthesis, too much light can inhibit photosynthesis and damage tissues. Anthocyanin production is one strategy that pears use to protect themselves against the detrimental effects of excessive light. Like the melanin in tanned skin, anthocyanins absorb incoming light.
In South Africa, outer-canopy Forelle pears exhibit the red blush that is associated with this cultivar. The blush only develops on the sun-exposed areas of the fruit. Shaded, inner-canopy fruit remain green and are marketed as lower-value Vermont Beauty.
While anthocyanin production requires light, light also destroys anthocyanins. The effect of light exposure may therefore be either increased or decreased red blush, depending on whether the pears can produce anthocyanins faster than these are broken down.
Since pears generally become less able to synthesise anthocyanins as harvest approaches, protection from excessive light may reduce fading of red colour in some cultivars. For this reason, summer pruning close to harvest may not be a good idea.
Low temperatures promote anthocyanin accumulation in all apple cultivars, as far as we know. But research on Bon Rouge, Flamingo, Forelle, and Rosemarie pears found that only Rosemarie and Forelle became redder in response to cold fronts. This effect was also more evident during early than during late fruit development.
High temperatures accelerate anthocyanin breakdown. Even a single hot day can cause significant fading in cultivars with low anthocyanin levels, such as Rosemarie. Trials have shown that evaporative cooling can mitigate fading in Rosemarie and Sensation Red Bartlett, but this is impractical in a water-scarce country like South Africa.
Other factors affecting red colour
Rootstocks can influence red colour development in at least two ways. One is by affecting gene expression in the scion, and the other is by influencing vigour, which affects light distribution in the canopy.
In one study, colour development was compared in Forelle on three pear and three quince rootstocks, from dwarfing to vigorous. The overall finding was that red colour development was better on quince rootstocks even though anthocyanin levels were not significantly higher than on pear rootstocks.
The researchers speculated that the yellower background colour of the fruit on quince rootstocks intensified the appearance of red pigments, whereas red pigments appeared muddy in greener fruit.
Several pear rootstocks are currently being evaluated in a Hortgro-funded trial — read more about this in our Research Rundown article.
Although protective netting is now widely used in apples, less is known about the potential benefits in pears. Previous trials that assessed the effect of nets on mealiness found that there was almost always a colour penalty.
For more tips on optimising colour development in both apples and pears, see the very first issue of Fresh Quarterly, available on this website.
Bonus: Solving the riddle of red colour in pears
Much of the initial work on red colour development in pears was done in the early 2000s by then PhD-student Wiehann Steyn — today he is the general manager of Hortgro Science. He explains how he came to spend time with Rosemarie:
“Rosemarie was a relatively new cultivar back then and growers were struggling to farm it. I was offered a project on red colour loss in Rosemarie — to figure out why it happens. At the time, everyone was familiar with colour development in apples and how that works, but no one understood that colour development in pears is completely different.
“My research looked at apples and pears, and I also reviewed the function of colour, which I found very interesting. It was a very visual study, and I enjoyed it because I like working with things you can see.”