In this project we show advances in both our understanding of ‘what happens when’ during berry development and in new technologies and methodologies to study berry growth and development. Ultimately, we intend to discover what veraison is and how the onset of sugar accumulation is regulated in the vine. In the process, we are discovering what veraison is not. Two prevalent ideas about veraison are that: the xylem gets torn apart and stops being conductive and that cell membranes breakdown and cell lose viability (start dieing). Krasnow et al. (J. Exp. Bot. 2008) shows that grape cells are not dieing nor losing membrane function at the beginning of veraison. And in fact cells remain intact and viable until harvest. The two Chatelet papers (Chatelet et al. 2008a,b, J. Exp. Bot.) show that the xylem in the berry remains intact and functional throughout development. We also discovered in the course of that work that the xylem in the berry is comprised of vessels, not tracheids as has been consistently reported (i.e. assumed).
What is veraison? We don’t know yet, but we found that sugar and color accumulation are closely linked, even when water deficits separated the resumption of growth from the onset of those ripening phenomena and caused increased color development. (Castellerin et al. 2008, Planta). We also found that flesh cell turgor pressure decreases in advance of the increased growth, sugar, and color that typify veraison. This together with earlier reports that the stress hormone ABA accumulates at veraison have caused us to speculate that turgor loss and ABA increase may be important in getting ripening started. We sought and were awarded a USDA grant to study the xylem connections of the berry and the how it is that turgor is lost prior to veraison. We report Wada et al. (2008, Planta) that the turgor loss is associated with solutes accumulating outside of the flesh cells. We hypothesize that veraison and the onset of high rates of sugar import is dependent upon this loss of turgor.
To understand the cellular physiological events regarding softening and the onset of berry ripening, we completed development of algorithms that measure and control cell volume and pressure on the automatically-controlled cell pressure probe that this project has funded the development of for three years. We show here application of the instrument to a ‘model system’ that has large cells that are easy to work with. In the no-cost extension period that we are requesting, we will use this new technology to make similar measurements in developing berries. This technological development will put us at the forefront of cell water relations studies and has allowed us to begin work on another USDA grant proposal to investigate the interactions of sugars, cell turgor, and ABA in causing veraison and anthocyanin synthesis. That proposal also exploits our recent progress in developing molecular methods to investigate the genetic basis of veraison. We hope to learn which genes are important in the onset of ripening. We intend to resume work with industry support on sugar accumulation in the near future.