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Selenium – an essential trace element for apples

Research & Extension

This article originally appeared in the 2021 Winter edition of AFG.

Dr Gordon Brown reviews an Iranian paper on the impact of selenium fertiliser on harvested apple quality.

Key points:

  • Selenium is a trace element that is often deficient in apple soils.
  • While selenium deficiency in livestock is well recognised there is little knowledge on its effect on apple trees.
  • This paper shows that selenium deficiency in apple trees results in fruit producing substantially more ethylene at harvest and having substantially lower levels of fruit colour, firmness and sugar.
  • In a second publication it has been shown that deficiency of selenium also substantially reduces apple fruit yield.
  • Correction of selenium deficiency in apple trees is relatively easy and will improve orchard productivity and profit.

Figure 1. Map of selenium deficient soils http://www.selcote.com.au/selenium-deficient-areas/

Selenium is a trace element which is deficient in many Australian apple growing soils from Western Australia down to Tasmania and up to Queensland (Figure 1).

Soils typically low in selenium are acidic soils with a parent rock material of granite or basalt. However, lighter soils can also be deficient due to leaching of the nutrient. The importance of selenium in agriculture is well documented for the grazing industries although for apple production its impact is not known. As selenium is essential for many enzyme systems there is potential for fertilisation to make a big impact on apple production. Due to problems encountered in the grazing industries, there are several companies marketing selenium fertilisers which may be useful for apple tree nutrition.

For growers who are currently using seaweed-based materials, it is possible that enough selenium is being applied through these products.

Method

In this trial sodium selenate was sprayed (0, 0.5, 1 or 1.5 mg/L selenium) at 2 weeks after flowering and again 5 weeks later. Application took place on cool mornings to the point of runoff.

At commercial maturity fruit were harvested and assessed for colour, anthocyanins, firmness, titratable acidity, fruit sugars (TSS), vitamin C and ethylene. Fruit were then stored at 1°C in air and the above characters measured monthly for 6 months.

Results

Fruit responses increased less with each increase in application rate. For example, anthocyanin content increased by 33 per cent for the first 0.5 mg/L and by only 4 per cent for the final 0.5 mg/L increase in application rate. This suggests that the optimal rate of application for these trees would have been marginally greater than the highest rate of application used here.

At harvest, the highest application rate was found to increase skin anthocyanin (red colour) by 43 per cent, titratable acidity (15 per cent), fruit sugars (40 per cent), fruit firmness (78 per cent),
vitamin C (32 per cent), and reduced ethylene production by 52 per cent. Hence the selenium application substantially improved fruit quality at harvest with commercial implications.

Figure 2. The impact of selenium application on fruit ethylene production during six months of air storage.

For the untreated fruit the level of the fruit ripening hormone, ethylene, was high at harvest and the rate of ethylene production for the duration of storage remained higher than that of the selenium treated fruit (Figure 2). After 3 months of storage in air, the ethylene production rate of the selenium treated fruit reached that of the untreated fruit at harvest and was always less than half that of the untreated fruit. With our knowledge of the impact of AVG and 1-MCP on ethylene production and subsequent fruit storage potential these results indicate that selenium treated fruit will have superior storage characters to the untreated fruit allowing for a longer marketing window for air stored fruit.

Figure 3. The impact of selenium application on fruit firmness during 6 months of air storage.

Both the critical fruit quality characters of fruit firmness and sugars were found to decrease during the 6 months of air storage (Figures 3 and 4).

Significantly the levels of firmness and sugars for the selenium treatment after 6 months of air storage were still higher than the untreated fruit at harvest demonstrating that they were still of superior quality to the freshly harvested untreated fruit.

Figure 4. The impact of selenium application on fruit sugar content during 6 months of air storage.

The selenium spray applications increased the foliar selenium content from 15 to 360 µg/kg while that of fruit increased from 0.1 to 275 µg/kg. This shows that this treatment is an effective way of
biofortifying apples to assist in overcoming selenium deficiency in human diets.

The increase in fruit firmness, sugars, acidity and vitamin C observed demonstrates that selenium deficiency in these trees was restricting tree productivity, potentially having an impact on fruit
yield. Unfortunately, no data was provided on fruit yield per tree. However, in a separate Chinese paper published in March this year (see further reading), ‘Fuji’ apple trees sprayed with sodium selenite resulted in a 12 per cent increase in yield along with a 21per cent increase in vitamin C content and a 19 per cent increase in fruit sugars. These results confirm some of the findings of the paper reviewed here and also demonstrate that selenium deficiency also restricts the yield of fruit.

A note on application rate

This Iranian paper is reported to have sprayed trees with up to 1.5mg/L selenium to the point of runoff. The Chinese paper sprayed trees with 150 mg/L sodium selenite (73mg/L selenium). A third German paper (see further reading) sprayed trees with approximately 225 mg/L selenium. This caused damage to the fruit so subsequent applications were restricted to 112 mg/L selenium. These differing application rates are consistent with there being little additional benefit in rates of application greater than those used in the Iranian research. They do, however, indicate that rates over 112mg/L have potential to damage trees. Hence the Iranian rates of application have a large safety margin against tree damage. If planning on using the higher rates of application some small scale experimentation would be advised. Sodium selenite or sodium selenite are just under 50per cent selenium such that 1mg/L (1g/1000L) selenium would need approximately 2mg/L (2g/1000L) of these products.

 

Dr Gordon Brown, Scientific Horticulture

E: gordon@scientifichorticulture.com.au

T: 03 6239 6411

 

Further Reading:

1) Influence of canopy spraying with sodium selenate on selenium biofortification and fruit quality maintenance of ‘Starking Delicious’ apple during storage.

2) Effects of sodium selenite spray on apple production, quality, and sucrose metabolism-related enzyme activity.

3) Influence of a Selenium Biofortification on Antioxidant Properties and Phenolic Compounds of Apples (Malus domestica).

 

Research updates are commissioned by APAL as part of its Industry Services.

 

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