The role of temperature in directly modifying apoplastic viscosity in vivo

We noted a significant reduction in the rate of anthocyanin and chlorogenic acid degradation attributable to the enzyme polyphenol oxidase (PPO) in frozen, thawed blueberries (Vaccinium corymbosum) when assessed at 4°C, compared with 20-40°C. We also noted a soft gel that formed in blueberry homogenates only at low temperatures. We propose that the reduction in rate of anthocyanin and chlorogenic acid degradation in frozen, thawed tissue at low temperatures is a consequence not only of the reduced rate of activity of PPO at these temperatures but also of the slow rate of diffusion of PPO through the apoplast following low-methoxy pectin gelation. We note that calcium pectate gels become firmer as the temperature falls below ambient. We suggest that a possible contributor to the reduced rate of fruit softening found at cold storage temperatures may also be an increase in pectin gel viscosity in the apoplast in vivo with a consequential reduction in mobility of wall-degrading enzymes. Viscous drag is likely to be highly significant in the constrained volume of the apoplast.