A team of scientists from Massey University and HortResearch have been examining some of the physiological causes of Braeburn Browning Disorder (BBD). We now have a clearer idea as to why Braeburn from some regions, for example Central Otago, are more likely to get BBD than Braeburn from other regions such as Hawke’s Bay. We also have a better understanding of why BBD is found commercially in late harvest fruit.

There appears to be two processes involved in the development of BBD:

1)  THE NATURAL SUSCEPTIBILITY OF THE FRUIT.

When apples are placed in storage at high concentrations of carbon dioxide and low concentrations of oxygen, a proportion of the apples develop a disorder "brownheart" which is similar to BBD. Therefore, it is clear that not all apples respond in the same way to the physiological triggers associated with altered gas composition. Thus, it is important in a disorder such as BBD to characterise both the natural susceptibility of the fruit and the physiological trigger associated with the initiation of disorder. The susceptibility of tissue to BBD probably relates to cell function, for example, the condition of membranes and functions of metabolic pathways. However, observations that BBD occurs in some regions but not others, and in late harvested fruit rather than early harvested fruit, suggests that the natural susceptibility of fruit tissue plays an important role in determining whether or not BBD develops in the field.

2)  A PHYSIOLOGICAL "TRIGGER" WHICH MAY INITIATE BBD

In fruit which are naturally susceptible, a physiological "trigger" may be needed to cause BBD. This trigger is likely to be due to atmospheric differences within the fruit itself. For example, high levels of carbon dioxide and/ or low levels of oxygen may act as the trigger for BBD expression.

This change in internal gas concentrations in the fruit is due to the process of respiration (which occurs within the fruit and generates carbon dioxide and consumes oxygen) and the speed that these gases diffuse (or escape) out of the fruit.

The structural "make-up", which appears to influence the susceptibility of the fruit, varies between regions and at different times of the harvest season. For example, Otago fruit are marginally denser, have a lower frequency of open lenticels (pores) in the skin and lighter (thinner) skins than Hawke’s Bay fruit. Denser flesh and fewer pores for gas to escape restrict gas diffusion. This causes carbon dioxide levels to increase and oxygen levels to decrease inside the fruit. Such internal atmospheres can cause tissue damage and bring on BBD.

BBD type more common in Canterbury

Scientists have found damaging internal atmospheres in late harvested apples from Otago. This fruit has very low levels of oxygen and moderately high levels of carbon dioxide as a result of gas restriction. Unlike the late harvest Braeburn, early harvest Otago fruit have normal internal atmospheres. There is no difference in fruit density or lenticel frequency between early and late-harvest Otago fruit, however, the apple skin is thicker at late harvests. This may account for some of the variation between harvest dates.

Fruit from Hawke’s Bay (either late or early harvest) do not have the same potentially damaging gas concentrations, which may be why BBD is not often found in fruit from this region.

The ultimate aim of this research is to be able to predict which apples will develop BBD. To do this we need a better understanding of the causes of BBD. The research team are currently investigating the effects of internal atmospheres on the development of BBD using hot water dips to induce the disorder.

ACKNOWLEDGMENT

HortResearch acknowledges ENZA New Zealand (International) for financial support.