Xylem and phloem tracing from the vine to the berry and vice versa elucidates pre-harvest berry shrinkage in Vitis Vinifera L.

Physiological mechanism of berry shrinking physiopathy in cv Shiraz (Vitis vinifera L.) is deeply debated. Critical points in Shiraz pre-harvest shrinkage are focused on the role of the xylem during post-véraison, on existence and timing of putative xylem back flow from the berry, and on functionality of the phloem. In order to solve 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 the phloematic functionality, we used the fluorescent tracer 6(5)-carboxyfluorescein diacetate (CFDA). Our results clearly showed that vines gradually lose 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 berry dehydration dynamics become pivotal: in pre-véraison there is a ‘plant/berry’ and ‘berry/plant’ water communication, whereas in post-véraison this connection apparently disappears. A mechanism is proposed and discussed.