Changes in Water Relations and Elastic Properties of Apple Fruit Cells during Growth and Development

Water relations parameters of individual cells in the cortex of apple fruit ( Malus domestica Borkh ., ‘Cox's Orange Pippin’) were determined during 2 growing seasons between the end of June and the end of September. Apple and cell diameters doubled during this time. Cell turgor pressure (P) of sliced tissue incubated in a nutrient solution ranged between 6 and 12 bar (0.6 and 1.2 MPa). The elastic cell modulus (∈) increased with both cell turgor and cell volume. For high turgor pressures (P > 5 bar), the average values of ∈ ranged between 30 and 180 bar (3.0 and 18.0 MPa). The half-times (T 1/2 ) for the water exchange of individual cells with their adjacent tissue increased during the seasons from 8–10 s to 20–25 s which was partly due to a decrease of the hydraulic conductivity of the cell membrane (Lp) during ripening. Lp decreased from about 3 to about 1 · 10 −6 cm · s −1 · bar −1 (from 3 to 1 · 10 −7 m · s −1 · MPa −1 ). The probe data allowed estimates of the diffusion coefficient for the propagation of a change in water potential, volume and turgor pressure across the tissue (D c ) · D c ranged between 3 and 9 · 10 −7 cm 2 · s −1 (3 and 9 · 10 −11 m 2 · s −1 ) and increased continuously during the season. The results indicate that the reduction in the elastic extensibility at the cell level (1/∈) does not correspond with the reduction of the firmness of the fruit as maturity is approached. On the contrary, the fruit become softer. Therefore, the physiological meaning of tissue Young's moduli as determined by other methods has to be questioned. The results are discussed with respect to the development of fruit disorders.