Abstract
A fleshy fruit is commonly assumed to resemble a thin-walled pressure vessel containing a homogenous carbohydrate solution. Using sweet cherry (Prunus avium L.) as a model system, we investigate how local differences in cell water potential affect H2O and D2O (heavy water) partitioning. The partitioning of H2O and D2O was mapped non-destructively using magnetic resonance imaging (MRI). The change in size of mesocarp cells due to water movement was monitored by optical coherence tomography (OCT, non-destructive). Osmotic potential was mapped using micro-osmometry (destructive). Virtual sections through the fruit revealed that the H2O distribution followed a net pattern in the outer mesocarp and a radial pattern in the inner mesocarp. These patterns align with the disposition of the vascular bundles. D2O uptake through the skin paralleled the acropetal gradient in cell osmotic potential gradient (from less negative to more negative). Cells in the vicinity of a vascular bundle were of more negative osmotic potential than cells more distant from a vascular bundle. OCT revealed net H2O uptake was the result of some cells loosing volume and other cells increasing volume. H2O and D2O partitioning following uptake is non-uniform and related to the spatial heterogeneity in the osmotic potential of mesocarp cells.
Highlights
In most climates, the leaves, stems and fruits of commercial fruit trees and vines are frequently wetted by rain and/or by dew
The magnetic resonance imaging (MRI) signal intensity reflected the H2O partitioning in the mesocarp and essentially mirrored the distribution of the vasculature within the fruit (Fig. 2b, d, f, h): A net type pattern parallel to the surface in the outer and a radial pattern in the inner mesocarp (Fig. 2f, h, k)
Results presented establish (1) a preferential accumulation of water in a net-type structures in the outer mesocarp and radial channels in the inner mesocarp of mature sweet cherry fruit. These distribution patterns reflect the more negative osmotic potentials in close proximity to the vascular system, (2) a longitudinal gradient of increasing water uptake rate and decreasing osmotic potential from the pedicel cavity region to the stylar scar region and (3) water uptake by individual cells can differ markedly between neighbouring cells and over short distances, with some cells showing considerable water loss, while others close by showing marked water uptake
Summary
The leaves, stems and fruits of commercial fruit trees and vines are frequently wetted by rain and/or by dew. The fruit skins of many such species are sensitive to wetting, especially when nearing harvest maturity. Examples of such free-water sensitive species are sweet cherries (Prunus avium), grapes (Vitis vinifera) and plums (Prunus × domestica)[1]. The water potential of rain or dew on the outside is usually close to zero (almost no solutes). The water potential inside the fruit is a composite comprising the summed components of cell/tissue osmotic potential, cell/tissue turgor pressure and cell/tissue matric potential. It is usually considered safe to ignore the matric component of water potential in most mature fleshy-fruit tissues as there are no air spaces (i.e., no gas/liquid interfaces)
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