Pre-harvest berry shrinkage in cv ‘Shiraz’ (Vitis vinifera L.): Understanding sap flow by means of tracing

The berry shrinking phenomenon in cv Shiraz (Vitis vinifera L.) is to date much debated. Currently, the critical points in Shiraz pre-harvest shrinkage are: a) the role of the xylem during post-véraison; b) the existence and timing of xylematic back flow and c) the functionality of the phloem. In order to try to resolve these issues, we traced the xylematic flows from the vine to the berry and vice versa by using the fuchsin acid as a xylematic tracer. At berry maturity, in order to verify also the phloematic functionality, we used the fluorescent tracer 6(5)-carboxyfluorescein diacetate (CFDA). The results showed clearly that the vine gradually loses the ability to deliver water to the berries via pedicel during ripening. The xylematic back-flow is active in the pre-véraison but not in the post-véraison berries. Furthermore, the CFDA experiments showed the absence of flow from the plant to the berry and vice versa at berry maturity. In cv Shiraz véraison seems to be the crucial point in the berry dehydration understanding: in pre-véraison there is a ‘plant/berry’ and ‘berry/plant’ water communication, whereas in post-véraison this seems to cease progressively.