AbstractThe speed of acclimatory changes in apparent energy of activation (Ea)of NAD malate dehydrogenase was analysed in four clones ofLathyms japonicusWilld. collected over a 16° latitudinal gradient in eastern North America. Plants were grown initially for six months at either 7° night/15° day or 22° night/30° days. Following reciprocal switches in these two contrasting thermo‐periods, energy of activation was relatively stable for 12 h, then oscillated until 48 h, and stabilized in about 72 h. The results suggest that the acclimation response inEais not due to near‐instantaneous conformational changes in isozymes, but is directed primarily by temperature‐mediated structural and/or functional modifications duringde novosynthesis of the forms of t