The Role of Carbon Dioxide, Light and Nitrate in the Synthesis and Degradation of Nitrate Reductase in Leaves of Perilla Frutescens

Summary Nitrate reductase in leaves of Perilla is an adaptive enzyme, the induction of which is shown to require carbon dioxide, light and nitrate. Following illumination of a leaf containing nitrate, the activity of the enzyme rises to a maximum after 6 hours and then begins to decline again. However, the enzyme activity is not entirely lost in such a leaf, provided that a supply of nitrate is maintained. When leaves containing nitrate reductase are placed in darkness, the enzyme breaks down at an approximately linear rate, which is influenced by temperature and by the age of the leaf. Synthesis of the enzyme appears to depend on active photosynthesis, since, when a leaf containing nitrate is illuminated in a CO2‐free atmosphere, no detectable enzyme is formed. When a leaf possessing nitrate reductase activity is illuminated in a CO2‐free atmosphere, the enzyme activity falls at a similar rate to that which occurs when the leaf is darkened. Thus it is reasonable to conclude that synthesis of the enzyme is dependent upon both light and CO2 and that a continuous breakdown of the enzyme is taking place even during a period of net synthesis. Using this information it becomes possible to estimate and compare the capacity of leaves of different ages to s)nthesize nitrate reductase.