Rain cracking in sweet cherries is caused by surface wetness, not by water uptake

Abstract

Rain cracking limits sweet cherry production worldwide. Cracking is often assessed in laboratory assays, where detached fruit is submerged in water. The objective of this study was to compare cracking of detached fruit that were submerged in water, with fruit that remained attached to the tree, but were exposed to simulated rain or fruit that were detached, placed in a wire cage and positioned in the canopy and were exposed to simulated rain.Water uptake and cracking were determined. For attached fruit, water uptake was calculated from diameter changes measured using linear variable displacement transducers. For detached fruit, uptake was quantified gravimetrically. Cracking was determined by inspecting fruit (detached and attached) at regular intervals.Water uptake into attached and detached fruit increased linearly indicating approximately constant rates of water uptake. Rates of water uptake were highest in detached, submerged fruit, were intermediate in attached fruit exposed to simulated rain and least in detached fruit in a wire cage exposed to simulated rain. The percentage of cracked fruit increased sigmoidally with time. Cracking was markedly faster in detached, submerged fruit, than in still attached fruit, than in detached fruit in a cage. Expressing the percentage of cracked fruit as a function of the amount of water taken up, again revealed a sigmoidal relationship. Cracking of detached, submerged fruit generally required less water uptake than attached fruit or detached fruit in a wire cage. Attached fruit cracked mostly in the pedicel cavity region whereas detached submerged fruit cracked mostly in the stylar scar region.Our study demonstrates that cracking is primarily a function of wetness duration and area wetted. Wetness duration was longest and area wetted largest in detached, submerged fruit, followed by attached or detached fruit exposed to simulated rain.