Apoplastic pH and inorganic ion levels in tomato fruit: A potential means for regulation of cell wall metabolism during ripening

Abstract

Apoplastic pH and ionic conditions exert strong influence on cell wall metabolism of many plant tissues; however, the nature of the apoplastic environment of ripening fruit has been the subject of relatively few studies. In this report, a pressure‐bomb technique was used to extract apoplastic fluid from tomato fruit ( Lycopersicon esculentum Mill.) pericarp at several developmental stages. pH and the levels of K + , Na + , Ca 2+ , Mg 2+ , Cl − and P were determined and compared with the values for the bulk pericarp and locule tissues. The pH of the apoplastic fluid from pericarp tissue decreased from 6.7 in immature and mature‐green fruits to 4.4 in fully‐ripe fruit. During the same period, the K + concentration increased from 13 to 37 m M . The levels of Na + and divalent cations did not change, whereas the anions P and Cl − increased in ripe fruit. Ca 2+ levels remained relatively constant during ripening at 4–5 m M , concentrations that effectively limit pectin solubilization. The electrical conductivity of the apoplastic liquid increased 3‐fold during ripening, whereas osmotically active solutes increased 2‐fold. Pressure‐treated fruit retained the capacity to ripen. The decline in apoplastic pH and increase in ionic strength during tomato fruit ripening may regulate the activity of cell wall hydrolases. The potential role of apoplastic changes in fruit ripening and softening is discussed.