Nitrate Reductase Activity in Relation to Heat Stress in Barley Seedlings 1

Nitrate reductase activity was assayed in barley seedlings (L.) exposed to supra-optimal but sub-lethal air temperatures of 41 to 43C for 16 to 24 hours. The activity of this enzyme decreased progressively during stress. Reduced activity was not attributed to production of inhibitors during stress or to water deficiency induced by the heat. Pre-stress application of cycloheximide resulted in less inactivation of nitrate reductase during stress.Subsequent to stress, nitrate reductase activity increased gradually, equaling the control in 24 hours, then increasing further to exceed the control. Cycloheximide applied after stress prevented this post-stress increase. Heat hardening did not alter the level of heat stability of nitrate reductase. Heat stress retarded growth alike in plants of average or low nitrate reductase content. Heat stresses which produced marked inactivation in nitrate reductase had little influence on the activity of several other cytoplasmic and chloroplastic enzymes.