Hypoxia in the grape berry linked to mesocarp cell death: the role of seed respiration and lenticels on the berry pedicel

Mesocarp cell death (CD) during ripening is common in berries of seeded Vitis vinifera L wine cultivars. We examined if hypoxia within berries is linked to CD. Internal oxygen concentration ([O2]) across the mesocarp was measured in berries from Chardonnay and Shiraz, both seeded, and Ruby Seedless, using an oxygen micro-sensor. Steep [O2] gradients were observed across the skin and [O2] decreased toward the middle of the mesocarp. As ripening progressed the minimum [O2] approached zero in the seeded cultivars and correlated to CD. Seed respiration was a large proportion of total berry respiration early in ripening but did not account for O2 deficiency late in ripening. [O2] increased towards the central axis corresponding to the presence of air spaces visualised using x-ray microCT. These connect to lenticels on the pedicel that were critical for berry O2 uptake as a function of temperature, and when blocked caused anoxia in the berry, ethanol accumulation and CD. Lenticel area on Chardonnay pedicels was higher than that for Shiraz probably accounting for the lower sensitivity of Chardonnay berry CD to high temperatures. The implications of hypoxia in grape berries are discussed in terms of its role in ripening and berry water relations.