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Light, colour and the role of temperature in blush pears

Research & Extension

This article was written by Alison Barber and was first published in AFG Autumn 2022.

Blush pears colour when exposed to light, but what role is played by light and what by temperature? Agriculture Victoria researchers in the Goulburn Valley are investigating.

Key points

  • Light is essential for blush colour development
  • Temperature may contribute to bleaching
  • Understanding the role of temperature has orchard implications

Understanding the role played by temperature in colour development in blush pears is the next step in helping growers more consistently deliver the highly desirable red blush so attractive to consumers.

While the contribution of light to colour development in pears has been well established in earlier work at the Tatura SmartFarm by Maddy Peavey, research scientist Dr Lexie McClymont said teasing out the effects due to light and those due to temperature would help better understand and manage colour development.

Early results from testing carried out this season indicate a relationship may be established, but are yet to be fully analysed.

“We know that pears are very reliant on light exposure for that blush development and very responsive to light and shade,” Lexie said.

“Maddy’s work showed if we shade a piece of fruit in the tree, it starts to lose colour and when we take that shade away, the colour comes back.

“The complication is that when you put those shade covers on to show the light effect, you’re also affecting temperature, so that raises the question – what effect does temperature have on colour? Is it as strong as light? Where does it sit in the scheme of things?”

Lexie said although there had been some international work on the relationship between temperature and colour in pears that pointed to colour degradation in warmer climates, little was known about the relationship in blush pears.

Colour degradation from red to orange had been observed locally in some pear cultivars close to harvest, but whether this was due to genetics or temperature was unclear and so difficult to manage.

“Our focus with this is to try to separate the temperature and light effects and look at the effect of temperature alone on blush colour development,” she said.

Fruit surface temperature

To explore this, a new heating and cooling experiment has been set up under the levy-funded Productivity, Irrigation, Pests and Soils (PIPS3) program at the Goulburn Valley-based Tatura SmartFarm.

 

Fruit surface temperatures of the cultivar ‘ANP-0118’ marketed as Lanya™ are recorded by thermocouples.

 

Heating and cooling equipment set up in a row of ‘ANP-0534’ at the Tatura SmartFarm to assess the effect of temperature on colour development.

 

Individual fruit in blocks of two blush pear cultivars, ‘ANP-0118’ (LanyaTM) and ‘ANP-0534’, developed by the Australian Pear Breeding Program based at the Tatura SmartFarm were treated to a range of artificial temperatures applied over two periods of five and three days during the pre-harvest period.

Fruit surface temperature was either:

  • heated at night
  • cooled during the day
  • ambient

Temperatures were measured by thermocouples placed just under the surface of the fruit peel and linked to data loggers and colour by handheld sensors.

“We are looking to see if there is a colour change over the time period and, if so, enough of a colour change that we can confidently relate it to temperature differences. Or not,” Lexie said.

“We have thermocouples measuring the fruit temperature where the blush develops, and then we look at those temperatures and the change in colour over a series of days, to see if there’s a relationship there.”

Heating was applied continuously by rigging a hair dryer to tubing directed at the fruit and cooling using a portable air-conditioning unit.

“We cooled them down to the mid-20°Cs and heated up into the 40°Cs. We even had some go into the low 50°Cs, but we’re trying to avoid that because we know that we’ll just cook the fruit.

“Fruit surface temperatures can be 15–18°C hotter than air temperature. Even on a 32°C day, fruit surface temperature can be getting up into the high 40°Cs and at risk of sunburn.”

Results will be analysed for relationships between colour change and fruit surface temperatures.

“I think temperature will be playing a role to some extent, it is just a matter of how important that is,” Lexie said.

Establishing a relationship between colour development and temperature could have implications for orchard management, including the possible use of evaporative cooling by blush pear growers to manage colour.

“We know that light is important for colour, but light is also a source of energy that heats up the fruit surface,” Lexie said.

“We know that netting will help reduce the surface fruit temperature and so help avoid sunburn, but that if you go too heavy with netting you really start having detrimental effects on the colour because of the requirement for light exposure to develop colour.

“Evaporative cooling is another option which has been predominantly used in apples, especially for sunburn protection, but it would be quite a new step for pear growers.

“But to turn around and put evaporative cooling into pears only to find out the effect was minimal would be pretty disappointing, so it is important we fully understand the role of temperature first.”

Acknowledgement – The heating and cooling experiment is part of the Developing smarter and sustainable pear orchards to maximise fruit quality, yield and labour efficiency project (AP19005), funded by Hort Innovation using the apple and pear research and development levy and funds from the Australian Government.

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