Changes in susceptibility of grape berries to splitting are related to impaired osmotic water uptake associated with losses in cell vitality

Background and Aims: High rainfall events after the onset of ripening commonly cause the skin of grape berries to split. The aim was to verify the suggestion, based on vineyard observations, that the susceptibility of grapes to splitting decreases during ripening, and to establish the role of cell vitality in this process. Methods and Results: The susceptibility of detached, ripening berries, cv. Shiraz, to splitting was assessed by immersing them in sucrose solutions or deionised water. At the onset of ripening, susceptibility to splitting was highest and decreased only slightly over 30 days. Thereafter, susceptibility decreased greatly until becoming negligible about 30 days later. Cell vitality within the pericarp, as determined by nitroblue tetrazolium staining, decreased from the onset of ripening. Conclusions: The reduction in the susceptibility of grape berries to splitting is attributed to a decrease in turgor-generating capacity within the berry as an increasing proportion of pericarp cells lose vitality. The loss of cell vitality corroborates previous evidence of this phenomenon, and indicates that this is a general feature of grape ripening. Significance of the Study: This study demonstrates the proportion of vital cells within the pericarp is the driver of turgor-induced splitting, in contrast to previous models in which the whole berry, enclosed in a semi-permeable skin, has been regarded as the osmotically functional system.