Seasonal pattern of apoplastic solute accumulation and loss of cell turgor during ripening of Vitis vinifera fruit under field conditions

Using a novel pressure membrane (PM) apparatus for the extraction of apoplastic fluid from field-grown grape (Vitis vinifera L.) berries, our hypothesis that significant apoplast solutes accumulate at the beginning of the ripening process (i.e. veraison), and that this accumulation might contribute to progressive berry softening due to a progressive loss of mesocarp cell turgor pressure (P) was tested. It was necessary to correct the solute potential (Psi(s)) of fluid collected with the PM for dilution due to the presence of a dead volume in the apparatus, but after correction, the Psi(s) obtained with the PM agreed with that obtained by low speed centrifugation. A clear decline in fruit apoplastic solute potential (psi A) began approximately 10 d prior to fruit coloration, and it was found to be coincident with a decline in mesocarp cell P and fruit elasticity (E). By late in fruit development when berry growth ceased (90 d after anthesis), both apoplast and fruit Psi(s) reached almost -4 MPa. These results support the hypothesis that a decrease in psi SA is responsible for the observed loss in mesocarp cell P, and is the mechanistic cause of berry softening.