The link between cell vitality and potassium in grape berries

Australia’s unpredictable and dynamic climate has consequences for the maturation of grape berries, a major economic crop for both domestic and export markets. Potassium (K⁺) is an essential ion accumulated in maturing grape berries. Accumulation of K⁺ into wine grape berries slows dramatically in later phases of ripening, coinciding with berry shrivelling and cell vitality loss within the mesocarp. However, the causes and contributors to the cessation of K⁺ accumulation and mesocarp cell death are unclear. Mesocarp cell death has been linked to hypoxia, suggesting that berry cell vitality may be regulated by its energy status. Post-véraison berries may experience a progressive energy limitation which could impact K⁺ transport, since rapid solute accumulation increases energy demand but hypoxia limits the production efficiency of adenosine triphosphate (ATP). Studies in other plants reported that K+ transport and voltages across membranes were involved in stress response and programmed cell death (PCD). The current project tested the hypothesis that mesocarp cell vitality loss is associated with limited K⁺ transport and altered K⁺ homeostasis.
To examine developmental changes in berry energy status, voltages across the tissue and membrane were compared between living and freeze-killed Shiraz berries. Voltages became less negative in more matured berries, providing electrophysiological evidence of energisation loss during berry ripening. Voltage in various mesocarp compartments may be associated with oxygen distribution and solute content. Transcription of genes correlated with membrane energisation, K⁺ transport, and PCD was analysed in the pericarps of individual Shiraz berries. The berry ripening phase-dependent K⁺ transport was displayed by the four gene clusters varying in developmental expression patterns. Vacuolar K⁺ accumulation may contribute to tonoplast energisation in berries at and beyond physiological ripeness (24-25 °Brix).
To explore the relationships between energy regulation and cell vitality in grape berries, pericarp ATP, ethanol and typical ripeness parameters were analysed in pericarps of cultivars showing various berry cell death levels during ripening. Shiraz, with the most severe berry cell death and berry shrivelling, was distinguished by significant dehydration and reduction of ATP level and K/Na ratio in pericarps during ripening. Chardonnay pericarps maintained a high ATP concentration and K/Na ratio after completing solute accumulation. In Flame Seedless, a cultivar without loss in berry cell vitality or pericarp water mass, solute accumulation did not cease and the pericarp maintained a low but stable K/Na ratio throughout ripening.
Levels of K element in grape berries may be altered by fertilisation or through the application of a selective rootstock. A soil fertiliser trial with two K rates was introduced to investigate K⁺ and other solute accumulation patterns in Shiraz pericarps, including ATP and ethanol. Doubling K⁺ supply may impact on pericarp composition and the K/Na ratio depending on the ripening phase, without significantly promoting maturation or dehydration. A rootstock trial on Shiraz scions demonstrated that K⁺ was lower in berries with 420 A relative to Ramsey, Ruggeri 140 or own-rooted vines in two growing seasons.
The results of this set of experiments indicate that the essential ion K⁺ is potentially involved in berry energy regulation and cell vitality maintenance in late-ripening phase. Insight into the physiology of berry maturation can inform future studies on the management of berry composition and adaptation strategies to climate change and nutrient availability.