Understanding the effect of high temperatures on red colour development in a blush pear

Published on May 9, 2023

The findings of research at Tatura SmartFarm will guide practices to improve fruit colour and develop effective strategies to mitigate any potential negative effects.

The pear cultivar ANP-0118 (marketed as Lanya™) develops an attractive blush on the fruit skin. Light exposure is required for blush development, but excessive direct solar radiation increases fruit surface temperature to levels where sunburn damage and necrosis can occur. Sunburn damage often occurs late in the season, when high air temperatures coincide with larger fruit mass and, subsequently, slower transfer of heat away from the fruit. Furthermore, moderate to high fruit temperatures have been associated with degradation of red colour pigments in other crops, leading to loss of red colour. Historical datasets were used to examine the conditions under which colour ‘degradation’ occurs in ANP-0118, showing early season colour degradation in response to high air temperatures.

Figure 1: Cultivar ANP-0118 fruit showing even blush (left) and sunburn damage (right).

It is important to understand the development of blush on fruit skin throughout fruit development stages and the concurrent effect of weather parameters on blush colour in order to develop effective strategies to mitigate any potential negative effects. Researchers at the Tatura SmartFarm have been using a handheld bluetooth colourimeter to understand the development of fruit red colour throughout the season (Figure 2). This device measures the colour of the fruit by projecting a beam of light and measuring the spectrum of reflected light from the surface of the fruit. The colourimeter objectively determines fruit colour in the CIELAB colour space (Figure 3). This three-dimensional space constitutes quantitative colour attributes like a* and b*, which can be helpful in understanding the development of blush colour over time. Here, a* represents redness and greenness present on the fruit skin, scaling from –60 (green) to +60 (red), and b* represents yellowness and blueness present on the fruit skin, scaling from –60 (blue) to +60 (yellow). The higher the a* value, the higher the red colour present on the fruit skin. Similarly, the higher the b* value, the higher the yellow colour on the fruit skin.

Figure 2: Handheld bluetooth colourimeter being used for the measurement of colour attributes of fruit blush.

Figure 3: CIELAB colour space representing a*, b*, hue angle (h°), chroma (C) and lightness (L*). (Adopted, with permission, from Scalisi A, O’Connell MG, Pelliccia D, Plozza T, Frisina C, Chandra S and Goodwin I (2021) ‘Reliability of a handheld bluetooth colourimeter and its application to measuring the effects of time from harvest, row orientation and training system on nectarine skin colour’, Horticulturae, 7:255).

A study was conducted over the 2019–20 and 2021–22 seasons to understand the effect of temperature on the red colour development of the fruit located at different heights on the tree. Fruit were tagged at three different heights, i.e. low, medium and high on the east and west trellises of the Open Tatura Trellis system.

Intensity of red colour on fruit skin, as evidenced by the a* value of fruits, was higher in November 2021 as compared to November 2019 (Figure 4). Temperature recorded in November 2021 was comparatively lower than November 2019 and a high temperature event (40.9 °C) was recorded in the orchard by the in-situ weather station on 21 November 2019. A corresponding reduction in a* and increase in b* was found after the heat event. Heightened synthesis of red colour pigmentation was found to begin in early December and continued to the end of season in both years.

There was consistent increase in a* in early December 2021 followed by a decline in a* and a concurrent increase in b* on 21 December 2021 which indicate a decrease in the red colour and increase in the yellow colour on the fruit skin. High temperature events, corresponding to the reduction in a*, were recorded on 13, 14 and 18 December 2021. This indicates that high temperature can halt synthesis of red colour pigmentation and degrade the accumulated blush colour on fruit skin. These findings are supported by the results of the 2019–20 season where multiple fluctuations in a* and b* were found in December 2019 corresponding to heat events.

Interestingly, no effect of extreme high temperature events on colour development of most fruit was found from late December to early January in the 2019–20 season. This is possibly because of acclimation of fruit against high temperatures due to its greater exposure to high temperature and direct sunlight. There were no high temperature events recorded after 10 January in the 2021–22 season, so most fruit show consistent increase in a* towards harvest. In January 2020, hot events were recorded on 10, 14 and 15 January and a consistent drop in a* was observed from mid to late January in most fruit, except fruit located at higher and medium canopy heights on the eastern trellis. Acclimation is often found in fruit located in the higher canopy of the tree due to their high exposure to sunlight.

The findings of this research can guide practices to improve fruit colour. High air temperature along with direct sunlight can raise fruit surface temperature significantly higher than the air temperature. Previous studies showed that air temperature of 38°C can raise FST to >50°C and the difference between air temperature and FST can reach as high as 19°C. FST threshold for sunburn damage and sunburn necrosis is reported as 47°C and 50°C, respectively. Researchers at Tatura SmartFarm found that netting can be effective to decrease FST; however, shading from the netting can also hinder the red colour development of the fruit. ANP-0118 fruit that were re-exposed to sunlight after an initial shading period were found to perform heightened red pigment synthesis to bring their red colour alongside fruit that were not shaded at all. The timing of netting cover could hold the key to successful development of colour. However, further research to support the findings would be beneficial.

Figure 4: Seasonal development of a* (redness) and b* (yellowness) of ANP-0118 fruit located at low, medium and high canopy heights on the tree and on east and west trellises of the Open Tatura Trellis System in the 2019–20 and 2021–22 seasons. Bars represent maximum air temperatures on days above 35°C.

Acknowledgement

This study was funded by the Victorian Government’s Agriculture Energy Investment Plan (AEIP) and the PIPS3 Program’s Developing smarter and sustainable pear orchards to maximise fruit quality, yield and labour efficiency (AP19005) project, funded by Hort Innovation using the apple and pear research and development levy, contributions from the Australian Government and co-investment from Agriculture Victoria.