Fig. 5.2 Schematic illustrations of plant responses to temperature (adapted from Fitter and Hay 2002). (a) The response of plant growth rate; (b) the influence of temperature on the rate of photosynthesis and respiration second parameter is higher, a drop in activity levels is noted. For most enzymes, the optimal temperature falls within the range of 30-45°C. Enzymes are irreversibly denatured and inactivated at temperatures higher than 60°C. The enzymes of thermophilous organisms (such as thermal spring bacteria) remain active and attain maximum reaction rates at higher temperatures. The highest temperature at which an enzyme is not thermally inactivated under given conditions determines the enzyme's thermal stability.

An alternative approach involves the application of the Arrhenius equation (from chemical kinetics) to plant processes:

k = A exp(-Ea/ RT), where k is the rate constant, E is the activa-

a tion energy for the process, A is the constant, R is the gas constant, and T is the temperature expressed on the absolute temperature scale.

Arrhenius constants (Ea/R for the process) can be useful in biochemical comparisons between j { OPTIMUM


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