Our most important observation is that there is a significant loss of cell turgor early in the onset of ripening in the grape berry. Berry cell turgor was evaluated using two developmental time scales. The full report shows the pattern of turgor during berry development as indicated by Brix. The turgor data are for cells in the outer mesocarp below the hypodermis and dorsal vasculature. The Brix data are for the juice of the same berry after turgor measurements were completed. The data show that turgor declines several bars to a stable turgor of about 1 bar or less during ripening. For intact clusters, the highest cell turgor occurred early in development and was less than 4 bars. Values of turgor shown that are greater that 4 bars are from cells of clusters that were excised and hydrated with water. Thus, turgor could be increased 2 to 4 bars by supplying water prior to veraison. For any stage of development beyond 6 Brix, turgor was about 1 bar. The hydration experiments failed to increase turgor in these berries to as much as 2 bars except in one case at 7.5 Brix.
We observed a “rapid” loss of turgor prior to or coincident with “veraison” on both scales. On a Brix scale, veraison is usually about 7 Brix; on the deformability scale, it is usually somewhere above 10%. However, both scales are problematic in testing for earlier events in the onset of ripening. Brix is of limited use as a developmental scale before “veraison” – Brix for all of our green berries was between 4 and 5 regardless of “age”. In addition, Brix determination generally requires destructive sampling of the berry, and it is not possible to know the “age” of the berry before sampling it. It may be possible to measure deformability nondestructively, i.e. on the vine, but it is not clear whether the measurement itself disrupts cell turgor and water relations.
We have had limited success with a vital staining technique for cells of grape berries using fluoresceni diacetate and ethidium bromide. Images of berry sections exposed to the dyes were visualized with a fluorescence microscope equipped with a digital camera and image processing system. The fluorescein stain has been used by Prof. Shackel to visualize impact injury in grape (both red and green varieties). Uptake of this stain has been used by Prof. Vito Polito (Dept. Pomology, UCD) as a measure of membrane integrity and cell vitality in pollen. The ethidium bromide has been used in bacterial and animal studies as a dead cell indicator. When we supplied fresh berry segments with the stains, getting sufficient uptake has been difficult.
We determined that there is a loss of several bars of turgor in the outer mesocarp near of at the beginning of veraison. We determined that there is an accumulation of apoplastic solutes in ripening berries and that the concentration of malate in the apoplast becomes similar to that in the berry as a whole as ripening progresses. We need to learn more about the geography of turgor and water transport in the berry in order to discover the site and timing of the origin of veraison. We need to increase our resolution of berry age and of the changes in apoplastic solutes that occur near veraison in order to draw conclusions about the source of apoplastic solutes and the cause of turgor loss at veraison.