Cation distribution and balance in apple fruit in relation to calcium treatments for bitter pit

Abstract Calcium (Ca), magnesium (Mg) and potassium (K) concentrations and distribution have been measured in apple fruit treated with Ca sprays during the growing season, and in stored fruit after post-harvest vacuum-infiltration of Ca or Mg, for bitter pit control. The highest concentrations of each cation occurred in the skin and core, the lowest in the outer cortex. Ca sprays increased Ca levels in each tissue segment; K levels were lower in Ca-sprayed fruit, but effects on Mg were variable. Concentrations of all 3 cations in the outer cortex increased during storage. Vacuum-infiltration of Ca substantially increased Ca levels in the outer 12 mm of cortical flesh after 2 weeks in storage, but did not affect Mg or K concentrations. Vacuum-infiltration of Mg resulted in an increase in Mg concentration in the outermost segment of flesh, but had little effect on levels elsewhere in the tissue, or on the concentrations of other ions. Ca added to fruit, either by spray or vacuum-infiltration, could easily leak from tissue in experiments with tissue disks. This leakage reduced tissue Ca concentrations to a level common to all treatments, indicating that Ca externally applied was probably located in the free space, and not involved with maintaining cell structure and function.