Localized bursting of mesocarp cells triggers catastrophic fruit cracking

The so-called rain-cracking of sweet cherry fruit severely threatens commercial production.Simple observation tells us that cuticular microcracking(invisible)always precedes skin macrocracking(visible).The objective here was to investigate how a macrocrack develops.Incubating detached sweet cherry fruit in deionized water induces microcracking.Incubating fruit in D2O and concurrent magnetic resonance imaging demonstrates that water penetration occurs only(principally)through the microcracks,with nondetectable amounts penetrating the intact cuticle.Optical coherence tomography of detached,whole fruit incubated in deionized water,allowed generation of virtual cross-sections through the zone of a developing macrocrack.Outer mesocarp cell volume increased before macrocracks developed but increased at a markedly higher rate thereafter.Little change in mesocarp cell volume occurred in a control zone distant from the crack.As water incubation continued,the cell volume in the crack zone decreased,indicating leaking/bursting of individual mesocarp cells.As incubation continued still longer,the crack propagated between cells both to form a long,deep macrocrack.Outer mesocarp cell turgor did not differ significantly before and after incubation between fruit with or without macrocracks;nor between cells within the crack zone and those in a control zone distant from the macrocrack.The cumulative frequency distribution of the logtransformed turgor pressure of a population of outer mesocarp cells reveals all cell turgor data followed a normal distribution.The results demonstrate that microcracks develop into macrocracks following the volume increase of a few outer mesocarp cells and is soon accompanied by cell bursting.

Spatial heterogeneity of flesh-cell osmotic potential in sweet cherry affects partitioning of absorbed water

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 nondestructively 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 microosmometry(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.