Hypoxia in grape berries: the role of seed respiration and lenticels on the berry pedicel and the possible link to cell death

Abstract

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 the profile of CD across the mesocarp. Seed respiration declined during ripening, from a large proportion of total berry respiration early to negligible at latter stages. [O₂] increased towards the central axis corresponding to the presence of air spaces visualised using x-ray micro-CT. These air spaces connect to the pedicel where lenticels are located that are critical for berry O₂ uptake as a function of temperature, and when blocked caused hypoxia in Chardonnay berries, ethanol accumulation and CD. The implications of hypoxia in grape berries are discussed in terms of its role in CD, ripening and berry water relations.