Pardo, C. Centro Tecnologico de la vid y el vino, Universidad de Talca, Av. Lircay S/N, Talca, Chile.Berries of Vitis vinifera L. cv Shiraz can undergo weight loss during later stages of ripening. Existing published views on how weight loss occurs are based oil changes in capacity of the vascular system to import water during development (McCarthy and Coombe, Australian Journal of Grape and Wine Research, 5, 17-21, 1999). One important element of these views is the proposed cessation of water flow through the xylem after veraison. We have now measured the water flow into berries of Shiraz and Chardonnay as they develop using the pressure probe and the high pressure flow meter (HPFM). The pressure probe connected to the pedicel of individual berries provided measurements of single berry hydraulic conductance. By systematic excision of tissue segments of the berry and pedicel we determined where in the pathway hydraulic conductance changed during development. The HPFM was used on whole bunches showing that berries (including pedicels) represent parallel high hydraulic resistances and that the hydraulic resistance of the bunch axis was rather small. The hydraulic conductance per berry could be determined from excision experiments. There was close agreement between the pressure probe and HPFM measurements. Both showed a ten-fold reduction in hydraulic conductance of whole berries from veraison to full ripeness. Shiraz had hydraulic conductances that were 2- to 5-fold higher than those for Chardonnay. Shiraz maintained a higher hydraulic conductance past 90 days after flowering than Chardonnay. The decrease in hydraulic conductance occurred in both the distal and proximal parts of the berry for both varieties. The pressure probe also provided measurements of the xylem pressure that non-transpiring berries could develop. These pressures were -0.2 to -0.1 MPa until veraison and increased to zero when the juice osmotic potential reached about -3 MPa in Chardonnay and -4 MPa in Shiraz. The results suggest values of the reflection coefficient of the osmotic barrier around the xylem vessels of about 0.1-0.2 at veraison decreasing to 0 at harvest. It is suggested that in addition to changes in xylem anatomy, aquaporins in berry membranes may play a role in regulating hydraulic conductance. Water movement from the berry back to the parent vine via the xylem (backflow) may be an important component of berry weight loss in Shiraz, particularly if the phloem ceases functioning at high osmotic potentials near maximum weight. Backflow could account for a weight loss of 43 mg per day in Shiraz berries for a relatively small gradient of 0.1 MPa.