Physical and mechanical changes in strawberry fruit after high carbon dioxide treatments

'Pajaro' strawberry (Frageria x annnassa Duch.) fruit were exposed to 5-40% CO2 for 0-3 days, followed by normal cold storage at 0 degrees C for up to 3 weeks. Strawberry fruit were firmer in air storage at 0 degrees C than at harvest. Firmness was further enhanced by CO2 treatments. Adhesion between cells was measured by the application of tensile tests to plugs of tissue, followed by the examination of fracture surfaces using low temperature scanning electron microscopy. These tests indicated that cell-to-cell adhesion increased by 60% as a result of CO2 treatments. However, there were no differences in the density, electrolyte leakage, propensity for cells to rupture in hypertonic solutions, water potential, osmotic potential or turgor of CO2-treated and control fruit. Electrical impedance spectroscopy was used to assess changes in the electrical resistance of the apoplast and symplast. Carbon dioxide treatments reduced the resistance of the apoplast (resistance at 50 Hz) below that of control fruit, but did not affect the resistance of the symplast (resistance at 1 MHz). This result suggests that concentrations of H+ and HCO3- increased in the apoplast, although no change was detected in the symplast. We speculate that the mechanism for CO2-induced firmness enhancement in strawberry is due to changes in the pH of the apoplast. Such changes in pH may promote the precipitation of soluble pectins and thus enhance cell-to-cell bonding in strawberry fruit. (C) 2000 Elsevier Science B.V. All rights reserved.