HIGH-TEMPERATURE AND LOW-TEMPERATURE LIMITS TO GROWTH OF TOMATO CELLS

The temperature dependence of the metabolic rates of cultured tomato c ells (Lycopersicon esculentum Mill.) has been studied by differential scanning calorimetry as a continuous function over the range from near 0 to above 45 degrees C. Metabolic rates increase exponentially with temperature over the permissive range for growth (approx. 10-30 degree s C). Outside this range irreversible loss of metabolic activity occur s. The rate of activity loss is time and temperature dependent, increa sing as the exposure temperature diverges from the permissive range an d increasing with time at any nonpermissive temperature. Metabolic hea t rates obtained while scanning down from intermediate (25 degrees C) to low temperature (0 degrees C) yielded Arrhenius plots with pronounc ed downward curvature below about 12 degrees C. The increase in appare nt activation energy below 12 degrees C is a function of the scan rate , showing its time dependency. This time dependency caused by inactiva tion confounds many estimates of apparent activation energy. Scanning up to high temperature shows that activity loss at high temperature is also time and temperature dependent. No first-order phase transitions associated with the changes in metabolism were detected at either low or high temperatures. Studies with lamellar lipid preparations added to cells show that temperature-induced transitions of lipids at levels equivalent to 4% of the lipid content of the cells were detectable. C ells with altered lipid composition showed altered temperature depende nce of inactivation. High pressures (in the range from 10 to 14 MPa) s hift the high temperature threshold and the rate of metabolic activity loss, supporting a postulate that higher-order transitions may be ass ociated with inactivation of metabolism. Higher-order transitions of l ipids or first-order transitions encompassing only a small fraction of total lipid remain among several viable postulates to explain tempera ture-dependent loss in activity. Alternative postulates are discussed.