Electrical impedance of Shiraz berries correlates with decreasing cell vitality during ripening

Abstract

Background and Aims: Cell death in the mesocarp of Shiraz berries has been correlated with berry mass loss that occurs late in ripening and can be linked to berry composition. The current techniques to assess the level of berry cell disruption are time-consuming and can give only relative measures. Here we test if electrical impedance of the berry, which is a function of the state of cells within, can be used to detect the loss of cell vitality. Methods and Results: Shiraz berries (774 berries) from two locations (one of them over two seasons) from veraison to harvest were studied. Electrical impedance spectroscopy was performed between 100 Hz and 1 or 2 MHz of alternating current across the berry, while for each berry proportion living tissue was estimated with fluorescein diacetate. The results indicated that, from veraison to the onset of cell death, berry impedance follows the accumulation of TSS. Thereafter, impedance decreases proportionally to the extent of berry cell death. Conclusions: Changes in cell vitality of the Shiraz grape berry can be objectively determined through impedance spectroscopy. The measurements also indicate that cell membranes, although becoming leaky, may still remain intact. Significance of the Study: Impedance spectroscopy as applied to the grape berry has demonstrated a promising application for measuring berry composition.

Application of sprinkler cooling within the bunch zone during ripening of Cabernet Sauvignon berries to reduce the impact of high temperature

“…Berry cell vitality is widely linked to the degree of heat stress and water stress on vines over the ripening period (Bonada et al 2013a, b, Caravia et al 2015, but its use as an indicator of berry health has been restricted because of time demanding vital staining and imaging techniques. Here we examine berry cell vitality using impedance spectroscopy, which is a more rapid and easily applied technique that has been shown to be able to accurately determine berry cell vitality in Shiraz (Caravia et al 2015) and impacts of heat stress (Caravia et al 2016). Of particular interest was our observation that sprinkler cooling (or canopy cooling) had no significant effect on the extracellular electrical resistance of Cabernet Sauvignon berries.…”

“…Fresh berry mass was lower in S2 matching the hottest season, and larger berries were observed in S3 which was the coolest season (Figures 4,5c). There were no differences in the proportion of berry water (Figure 5c), neither pH, TA nor the extracellular electrical resistance ( Figure S1) as an indirect indication of berry cell death (Caravia et al 2015).…”

“…Samples were stored in a Ziploc plastic bag and then placed in an insulated box cooled with ice in the field (the ice was not in contact with the samples). Once in the laboratory, subsamples were then taken to measure fresh and dry berry mass, berry impedance (Caravia et al 2015), TSS, anthocyanins, phenolic substances, tannins, pH and titratable acidity (TA). During S1, berries were sampled four times (70, 80, 94 and 111 DAA), S2 eight times [70,78,84,89,96,106,115 and 126 DAA], and in S3 only at harvest (129 DAA).…”

“…A group of ten berries was randomly selected from each field replicate during S1, and each berry was subjected to impedance spectroscopy (Caravia et al 2015). Briefly, a slice of skin (2-3 mm diameter) was removed on opposite sides of the berry equator with a razor blade.…”

“…There are a limited number of practical tools to mitigate heat stress in the vineyard. Shade cloth as an overhead layer was effective at lowering maximum temperature compared to several other canopy shading options (Greer et al 2010, Caravia et al 2015. Shade application during ripening increased Shiraz berry cell vitality, reduced loss of berry mass and lowered sugar concentration, resulting in reduced alcohol concentration in wine (Caravia et al 2016).…”