Apple growers who submit their Braeburn and Cox Orange Pippin for export are sometimes puzzled by the variation in the results of calcium mineral samples.

Under the current export guidelines, samples of thirty Braeburn fruit are analysed seven to ten days prior to harvest. Calcium levels must be over 1.8mg/100g fresh weight for the fruit to be acceptable for export. Levels between 1.8 - 2.5 mg/100g fresh weight means the fruit requires a post harvest calcium drench. Often a duplicate sample sent to another laboratory will give a different result. This is not necessarily the fault of that laboratory, but could be due to the variation between fruit.

As part of his PhD thesis, HortResearch scientist Fraser Broom investigated the variation in Braeburn and found there can be a huge difference in calcium levels between individual apples on any one tree.

Why is calcium an issue ?
The level of calcium in apple fruit at harvest has repeatedly been shown to influence the level of bitter pit in the fruit. Bitter pit is a physiological disorder which occurs in some apple varieties, appearing as small brown pits in the apple tissue which are usually close to the surface. Pitting is often not present at harvest but will develop over time in coolstore.

This present study confirmed that Braeburn fruit containing low calcium concentrations were more likely to have bitter pit and develop a greater number of pits.

You can't generalise about a population
We make general statements about populations all the time, such as " All Swedish people have blond hair and blue eyes" . Of course some of that population will have dark hair and brown eyes. The same applies to a population of fruit. Some of the statements made in relation to Braeburn and bitter pit could fall into this category for example :

"Large fruit will develop more bitter pit than small fruit"
"Fruit on one-year old wood is susceptible to bitter pit"

Instead of looking at averages derived from many fruit, Fraser Broom examined individual fruit on six different trees with relation to fruit size, position of fruit on tree and seed numbers.

Small fruit can develop bitter pit
We often assume that larger apples are more prone to the development of bitter pit. This assumption has been backed up many times by research studies which show a clear relationship between increased fruit size and decreased calcium levels (and hence bitter pit development).

Dr Broom also observed this when looking at the average values over the whole tree. Trees with a large fruit average fruit size tended to have lower average fruit ratio and a higher percentage of pitted fruit, (see Table 1).

Table 1.
This shows the between-tree variation of % pitted fruit, total fruit number, mean fruit size and calcium concentration at harvest.

(Note : the calcium concentration was analysed using a non-standard procedure so results are higher than would normally be expected. It is relative rather than absolute levels which are important).

Tree	% fruit Pitted	Total fruit no.	Mean fruit size (g)	Calcium conc (mg/100g fresh weight)
1	44.6	359	190.82	2.99
2	24.5	309	192.94	3.25
3	24.0	404	169.26	3.41
4	23.0	321	171.25	3.66
5	15.0	313	178.73	3.50
6	10.5	497	164.24	3.94

However, a different relationship exists when comparing individual fruit within a single tree. In this case, the larger fruit tended to have slightly higher calcium concentrations. While this relationship was weak, it differed from previous research which compared calcium concentrations as an average of the population. This is illustrated in Figures 1 and 2.

Figure 1.
The graphs are of the six trees sampled with each dot representing an individual fruit. The scatter of the dots shows the variability between harvest fresh mass (size) and calcium concentration.

Tree 1
Tree 2
Tree 3
Tree 4
Tree 5
Tree 6

When put together, the graphs show the "generalised" relationship which applies to the whole population. See Figure 2 for graph.

The effect of crop load
Previous studies have shown that fruit from light cropping trees are more at risk of developing bitter pit. Current export guidelines do not allow growers to harvest Braeburn from light cropping trees.

None of the trees in this study could be considered "light cropping". However, it is interesting to note that, in general, trees with less fruit had a lower than average calcium level and a higher incidence of bitter pit (Table 1). This further confirms that light cropping trees are more susceptible to quality problems.

Position of fruit on the tree
Dr Broom investigated how the particular position of fruit on the tree would affect mineral levels and quality.

Fruit produced on one year old wood were smaller than spur fruit and were slightly more prone to developing bitter pit. This was despite having higher fruit calcium concentrations.

Fruit quality increased along one year old shoots. Fruit closer to the terminal end had higher calcium concentrations, and were larger than one year fruit closer to the base of the same shoot.

As the number of fruit on a one-year-old shoot increased (up to four apples), the average calcium levels and size in those fruit decreased. The number of Braeburn carried on spur wood did not influence quality in the same way.

Flat seeds influence fruit quality
Fruit size and fruit concentrations of calcium and boron increased with higher seed numbers in Braeburn. Full, as opposed to "flat" seeds, are thought to influence fruit quality through hormones. Improving pollination will increase the numbers of full seeds and hence the quality of Braeburn.

In contrast, flat seeds were associated with poor fruit quality ie. smaller fruit with lower calcium levels. This may be due to a number of mechanisms which are not necessarily hormone-related, or restricted to the direct effects of pollination and fertilisation. Scientists need to investigate further the relationship between seed number, type and fruit quality.

What does this mean for the Braeburn grower ?
The relationship between calcium levels and bitter pit is well established. What is less established is the variation between calcium samples. This work confirms that like all living things, each individual apple is different. While we can make a generalisation based on the whole population, when we get down to the individual level, that generalisation does not always hold true. For sampling and predictive purposes, it is practical to "generalise" about the whole population and indeed the current sampling programme is based on this.

Dr Broom has shown that variation in calcium concentration in individual apples will vary according to factors such as :

• Position on tree
• Wood type
• Fruit size
• Seed numbers

Source:
Summarised from Broom, F.D (1995) . Spatial Variation, Bitter Pit and the Quality of Individual "Braeburn" Apple Fruits. PhD thesis. Submitted by Helen Percy of HortResearch, Ruakura on 25 May 1996.

HortResearch acknowledges the Foundation for Science and Technology.