Skin Mechanical Properties and Fruit Cracking in Flame Seedless Tablegrapes

Fruit cracking leads to bunch rots in both tablegrapes and winegrapes and increased packing costs in tablegrapes.  Biophysical, anatomical, and cultural factors associated with cracking were investigated in the cracking prone cultivar Flame Seedless by varying ethephon, irrigation, and girdling practices over two seasons in a commercial vineyard, and evaluating cracking and skin properties.  Berry skin has been widely regarded as a "thin shell" that bears pressure from the internal flesh, such that berry cracking in the form of skin failure should be associated with either a high internal pressure or a weak skin.  In ripe fruit, tissue failure under an applied load has generally been attributable to cell separation, but scanning electron microscopy of field and water soaked cracked berries clearly showed that skin failure was by cell wall fracture, suggesting either that skin cells have particularly strong intercellular bonding, that failure may be caused by high skin cell turgor, or both. Overall skin strength was tested in the laboratory using a custom device (the “berry balloon system,” BBS) designed to apply multiaxial stress to an entire, intact berry skin.  Under field conditions, ethephon sprays caused the largest increase in cracking, followed by irrigation and girdling.  In all treatments, skin stress and strain at failure decreased markedly over berry development, indicating that cracking susceptibility does increase as berries ripen.  Skin mechanical properties did not differ for berries with or without ethephon spray in the field, however, exposing the berry skin to an ethephon solution (1.5 µM) during BBS testing caused about an 18% reduction in skin strength.  Also, intact berries soaked in an ethephon solution cracked in one-half the time of berries soaked in water.  These results strongly suggest that cracking susceptibility increases very soon after exposure to ethephon.  The failure to detect differences in skin properties using field sampled berries may be due to the fact that only sound (non-cracked) berries were selected for the lab tests, and these may not have been typical of the berries that did crack in the field.  Irrigation treatments in both years caused clear differences in midday stem water potential (SWP) over a range of –1.1 to –0.3 MPa.  Berry firmness was positively correlated with SWP and cell turgor pressure.  In general, cracking, berry size, and firmness were all reduced by reducing water, and there may be a threshold SWP of about –0.8 MPa for these values to show decreases.