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Wine Grapes Gasp for Breath

University of Adelaide researchers have discovered how grapes “breathe”, and found that shortage of oxygen leads to cell death in the grape. The discovery raises many questions about the potentially significant impacts on grape and wine quality and flavour, and may lead to new ways of selecting varieties for warming climates.

“In 2008 we discovered the phenomenon of cell death in grapes, which can be implicated where there are problems with ripening,” says Prof Steve Tyerman. “We’ve since been trying to establish what causes cell death. Although there were hints that oxygen was involved, until now we’ve not known of the role of oxygen and how it enters the berry.”

The research published in the Journal of Experimental Botany (https://goo.gl/ELesxb) describes how grape berries suffer internal oxygen shortage during ripening. With the use of a miniature oxygen-measuring probe – the first time this has been done in grapes – the study compared oxygen profiles across the flesh inside grapes of chardonnay, shiraz and ruby seedless table grapes.

“By manipulating oxygen supply we discovered that small pores on the surface of the berry stem were vital for oxygen supply, and if they were blocked this caused increased cell death within the berry of chardonnay, essentially suffocating the berry,” says PhD student Zeyu Xiao.

“We also used micro X-ray computed tomography to show that air canals connect the inside of the berry with the small pores on the berry stem. Shiraz has a much smaller area of these oxygen pores on the berry stem, which probably accounts for its greater sensitivity to temperature and higher degree of cell death within the berry.”

The research team observed that the level of oxygen shortage closely correlated with cell death within the grapes. Respiration measurements indicated that this would be made worse by high temperatures during ripening. This is expected to happen more frequently with global warming.

Tyerman said that the demand for oxygen in plants doubled for every 10°C increase in temperature. “If a heatwave hits and the temperature rises from 30°C to 40°C, the respiration rate doubles and the grapes need to get oxygen in at twice the rate,” he said.

Tyerman said cell death occurred naturally about 100 days after flowering and that sugar accumulation stopped when cell death begins. He said that heat waves could start the process of cell death any time before this 100-day window, and that would change the flavour profile of the berry.

As the only solution is to keep the grapes cool, the University is also researching different ways to cool vineyards, including using microsprays and night irrigation. However, Tyerman said that watering was slightly problematic because the water blocked the tiny pores and inhibited oxygen uptake.