DEVELOPMENTAL-CHANGES IN THE DIURNAL WATER-BUDGET OF THE GRAPE BERRY EXPOSED TO WATER DEFICITS

The diurnal water budget of developing grape (Vitis vinifera L.) berri es was evaluated before and after the onset of fruit ripening (veraiso n). The diameter of individual berries of potted 'Zinfandel' and 'Cabe rnet Sauvignon' grapevines was measured continuously with electronic d isplacement transducers over 24h periods under controlled environmenta l conditions, and leaf water status was determined by the pressure cha mber technique. For well-watered vines, daytime contraction was much l ess during ripening (after veraison) than before ripening. Daytime con traction was reduced by restricting berry or shoot transpiration, with the larger effect being shoot transpiration pre-veraison and berry tr anspiration post-veraison. The contributions of the pedicel xylem and phloem as well as berry transpiration to the net diurnal water budget of the fruit were estimated by eliminating phloem or phloem and xylem pathways. Derry transpiration was significant and comprised the bulk o f water outflow for the berry both before and after veraison. A nearly exclusive role for the xylem in water transport into the berry was ev ident during pre-veraison development, but the phloem was clearly domi nant in the post-veraison water budget. Daytime contraction was very s ensitive to plant water status before veraison but was remarkably inse nsitive to changes in plant water status after veraison. This transiti on is attributed to an increased phloem inflow and a partial discontin uity in berry xylem during ripening.