Loss of rachis cell viability is associated with ripening disorders in grapes

Rachises of grape (Vitis vinifera L.) clusters that appeared healthy or displayed symptoms of the ripening disorders berry shrivel (BS) or bunch-stem necrosis (BSN) were treated with the cellular viability stain fluorescein diacetate and examined by confocal microscopy. Clusters with BS and BSN symptoms experienced a decrease of cell viability throughout the rachis, and their berries contained 70-80% less sugar than healthy berries. The xylem-mobile dye basic fuchsin, infiltrated via the cut base of shoots with one healthy and one BS cluster, moved to all berries on the healthy cluster but generally failed to move into the peduncle of the BS cluster. Peduncle girdling did not interrupt dye movement in the xylem, but stopped solute accumulation in berries and led to berry shrinkage. In contrast, surgically destroying the peduncle xylem at the onset of ripening did not affect berry growth and solute accumulation. These results indicate that cessation of sugar and water accumulation in BS and BSN is associated with phloem death in the rachis. Although xylem flow to the berries may also cease, a functional xylem connection to the shoot may not be required for normal ripening, while water loss from berries by transpiration and xylem efflux may explain the characteristic berry shrinkage that is associated with these ripening disorders. The similarity of internal tissue breakdown in BS and BSN rachises and the correlation observed here between the proportion of shrinking berries on a cluster and the severity of rachis necrosis suggest that there may be a gradual transition between the two ripening disorders. Seeds from healthy and BS clusters showed no differences in colour, morphology, weight, viability, and ability to germinate, which indicates that the disorder may not appear until seeds are mature.