Australian researchers discovered that grape berries ‘breathe’ and that they shrivel and die when they lack oxygen during ripening. Factors such as water stress or high temperatures can lead to oxygen shortages for berries. One of the discoveries was that Shiraz berries breathe oxygen differently to Chardonnay and can explain why Shiraz is more susceptible to berry shrivel. This new knowledge on how grapes take up oxygen provides the basis for further research into berry quality and cultivar selection for adapting viticulture to a warming climate.
Berry cell death and subsequent berry shrivel has implications with regards to yield loss as well as berry and wine quality. Berry shrivel leads to concentrated sugars and higher alcohols along with changes in the biochemistry within the berry. Numerous studies have focused on this phenomenon but recent findings shed new light on the mechanism behind berry cell death.
A group of Australian researchers recently tested the hypothesis that cell death was associated with oxygen starvation (hypoxia) in the berry tissue. This theory was based on the observation that cell death increased at higher temperatures. Oxygen is essential for living cells and their normal respiration, which also depends on temperature.
A large gradient of oxygen concentration across the skin indicated that the skin did not allow for much oxygen diffusion which shifted their focus to the tiny gas permeable pores on the pedicel (grape berry stem). These pores are called lenticels and their density on the pedicel varies between varieties. Shiraz has a lower surface area of these pores compared to Chardonnay and possibly indicates that oxygen uptake in Shiraz is restricted.
The research also indicated that air spaces exist within the berry and these air spaces are connected to the pedicel. Most oxygen diffusion also occurred via the pedicel lenticels. When the lenticels to Chardonnay berries were blocked, ethanol concentrations increased inside the berries, indicating that fermentation occurred within the berry due to oxygen starvation. This proved that lenticels on the pedicel are an important pathway for oxygen uptake into the berry. Blocking or restricting these gas permeable structures can lead to decreased oxygen supply in the berry mesocarp.
High respiratory demand in the berry can also create an oxygen deficiency. The seeds contributed substantially to berry respiration around veraison but decreased to a negligible demand late in ripening.
How does this relate to environmental factors?
Ambient temperature and water stress modulate cell death and berry shrivel, which is why researchers then investigated how water stress and elevated temperature affect Shiraz berry cell death and internal oxygen status.
Results indicated that Shiraz berry cell death was correlated with low oxygen concentration in the mesocarp and smaller air spaces within the berry. This was perhaps induced by stress, which could potentially restrict the diffusion of oxygen into the berry, leading to hypoxia and cell death. The progression of cell death during berry ripening correlated with mean berry internal oxygen concentration across all growing conditions. Water stress decreased Shiraz berry internal oxygen concentration and increased ethanol accumulation and cell death.
Significance of the study
This study proved that cell death, and by implication berry shrivel, are strongly linked to oxygen supply and demand. Potentially any stress that influences oxidative processes, including treatments that can cause oxidative stress in the berry, berry respiration or anatomy, will likely impact on cell death. Cultivars are adapted differently and their ability to take up oxygen differs.
This new knowledge on how grapes take up oxygen provides the basis for further research into berry quality and cultivar selection for adapting viticulture to a warming climate.
Image source: Organic Lifestyle Magazine