Seasonal Characteristics of the Daily Fluctuation in Nitrate Reductase Activity

The daily fluctuation of nitrate reductase activity in leaves of tobacco may be controlled by the relative amounts of endogenous cytokinin and gibberellin. While plants growing during the summer exhibit maximum nitrate reductase activity at midday, winter‐grown plants show minimum activity at that time. One reason for this phenomenon seems to be a relatively higher level of endogenous kinin in winter‐grown plants. Salt‐stress treatment of these plants changes the fluctuation pattern of nitrate reductase activity from a winter to a summer type. Kinetin addition to summer‐grown plants changes their fluctuation pattern from a summer to a winter type.