Physical Properties Associated With Grape Berry Cracking

Postveraison berry cracking causes economic losses for commercial grape production and is a long standing concern that occurs in both table and wine grapes. Examining the surface of cracked Flame seedless berries, the skin cells were found to fail by cell rupture instead of cell-cell debonding. The dermal cells of several other fruits and leaves also share the same failure mode with cell rupture. Berry skin has been considered as a `Thin Shell' that bears pressure from the internal flesh. Therefore, berry cracking in the form of skin failure was expected to be associated with physical components including berry size, the skin properties and the internal pressure. Cultural treatments including ethephon sprays, irrigation regimes, and girdling that potentially can impact berry physical properties and cracking were performed in 2010, 2011 and 2012 in a Flame seedless vineyard in Arvin, CA. Ethephon spray increased cracking in all three years. Frequent irrigation with about 2.5-3.9 times more water applied compared to grower standard irrigation did not increase cracking in any of the three years; instead, deficit irrigation had 1.6∼1.9-fold higher cracking compared to standard irrigation in 2012. This result indicates that proper irrigation management is important in reducing cracking, and that, contrary to expectation, under-irrigation may be more damaging than over-irrigation. Increasing irrigation substantially reduced the daily swing in vine stem water potential (SWP), and may have reduced daily fluctuations in berry size. Girdling did not have consistent impacts on cracking. The hypothesis based on the Thin Shell model that higher cracking would be associated with larger berry size or higher internal pressure was not supported by our results.;The Berry Balloon System (BBS) was developed to perform two-dimensional tension tests on grape skin. Using the BBS we can expose the inside of an entire isolated grape skin to a known internal pressure in its original three-dimensional conformation, and then calculate the skin stress and strain. The stress and strain correlation for the grape berry skin was non-linear, and in the intact state, the skins of berries were found to be under significant "preload" strain caused by internal pressure of the flesh. The BBS was used to test grape mechanical properties during berry development, across different cultural treatments, and for different genotypes. Flame seedless skin stress and strain at failure both decreased over berry development, indicating that cracking susceptibility increases as berries ripen. Soaking the skin in ethephon solution while performing BBS test significantly decreased skin stress and strain at failure, suggesting that ethephon spray in the field can potentially weaken the skin within a short period of time. Comparing different genotypes, cracking resistance as measured in a soaking test was best correlated with Overall skin strain at failure, followed by Intact to fail strain change and then Preload stress. Nevertheless, the skin strain values measured using BBS were much higher compared to reported daily strain for grape berries under field conditions, and we hypothesize that grape berry cracking in the field is most likely an event resulting from localized stresses and strain on the berry skin, rather than a failure of the Thin Shell solely due to uniform internal pressure from the flesh.