The rate of ion leakage from chilling-sensitive tissue does not immediately increase upon exposure to chilling temperatures

Exposure to non-freezing temperatures below similar to10 degreesC causes an increase in the subsequent rate of ion leakage from chilling sensitive tissue (e.g. tomato fruit). The conditions of tissue preparation and conductivity measurements necessary to accurately calculate the rate of ion leakage from excised discs of tomato pericarp tissue were determined. Under the proper conditions, the rate of leakage expressed as a percent of the total conductivity per hour was linear for the period of 30-240 min following immersion of the discs in aqueous solutions of 0.2 M mannitol. A kinetic analysis of the efflux data showed that a combination of two exponential equations of the form y = C-0(1-e-(K.t)), one for a 'fast' extra-cellular reservoir of ions (C(0)f, K-f; y = C(0)f 1-e(-Kf.t)), and one for a 'slow' cellular reservoir (C(0)s, K-s; y = C(0)s(1-e(-Ks.t)), fit the original data with an R-2 > 0.95. When measured at 25 degreesC after chilling, chilling at 2.5 degreesC for 3.5 days increased the rates of ion leakage and the values of C(0)f and K., but had no effect on C(0)s or Kf. This implies that chilling increased the permeability (K-s) of the cellular reservoir that allowed ions to leak out during the exposure to chilling and increase the content of C(0)f However, when ion leakage was measured at temperatures from 2 to 20 degreesC, C(0)f remained unchanged and the changes in K-s paralleled those in K-r. These changes in the rate constants reflect the effects of temperature on diffusion and did not show the abrupt increase predicted by the membrane phase-transition model of chilling injury. (C) 2002 Elsevier Science B.V. All rights reserved.