PARTITIONING CONTROL BY WATER POTENTIAL GRADIENT - EVIDENCE FOR COMPARTMENTATION BREAKDOWN IN GRAPE BERRIES

Xylem exudate was obtained from berries of Riesling grapes at different stages of development after the onset of ripening using a pressure bomb technique. The osmotic potential of the exudate bore a 1:1 relationship to that of juice from the same berries which were afterwards crushed and centrifuged. This result provides the first direct evidence of compartmentation breakdown in grape berries after the onset of ripening. Changes in berry deformability which occur at the same time and measurements of the dynamics of exudation flow lead to the same conclusion regarding compartmentation breakdown. The breakdown in compartmentation occurs at the same time as the rate of phloem translocation to the fruit suddenly increases. A mechanism was recently proposed to account for this increase. It required the existence of a water potential difference between source and sink such as would result from compartmentation breakdown in the sink tissues. The results, therefore, may be taken to indicate that this mechanism is indeed involved in the control of assimilate partitioning in Vitis. Evidence in other publications suggests that the mechanism may be reasonably widespread in plants.