Cell turgor pressure and water relations of developing grape berries

• Author(s): Matthews, Mark; Shackel, Ken; Thomas, Tyler;
• Abstract: Several fleshy fuit exhibit a multiphasic growth habit in which two periods of rapid expansion (Stage I and III) are separated by a period of slow or no growth (Stage II). In grape, fruit ripening (e.g. softening and sugar accumulation) commences approximately coincident with the rapid transition from Stage II to Stage III. It is generally believed that the rate of expansive growth in plant cells and tissues increases with increasing cell turgor, but there is also evidence in a number of fruits that softening is associated with a decrease in cell turgor. Hence the simultaneous softening and increase in growth that occurs between stages II and III in grapes indicates that one or both of these beliefs may be incorrect. The relationship of cell turgor to the growth habit and berry softening was investigated by direct measurements of cell turgor with the pressure microprobe, and measurements of berry softening by force/deformation techniques. The turgor of mesocarp cells exhibited a complex pattern during development in which turgor decreased to about 0.1 MPa during the rapid growth of Stage I, then increased to about 0.4 MPa as growth slowed, and finally decreased again prior to the onset of the second period of rapid growth. Hence, there was a generally negative relation of cell turgor to berry growth. There was also an inverse, but nonlinear relationship of turgor to deformability. Most of the decrease in turgor, from 0.4 to less than 0.1 Mpa, occurred prior to the significant increase in deformability (softening) that occurred at the end of stage II. These developmental changes in turgor were consistent throughout the fruit mesocarp, at depths from 100 to 2,000 micrometers. Measurements of individual cell elastic modulus will be required to determine whether the non-linear relation of turgor to deformability is a result of wall softening at the cell level, or decreased cell-to-cell mechanical coupling at the tissue level.
• Publication Date: Aug 2003
• Journal: Hortscience