Development of Nitrate Reductase Activity in Expanding Leaves of Nicotiana tabacum in Relation to the Concentration of Nitrate and Potassium.

Up to 80% of the total nitrate reductase activity (NRA) determined in vivo in different parts of vegetative tobacco plant (Nicotiana tabacum) was located in the leaves. The NRA reached a peak when a leaf had expanded to 27% of its final weight and 33% of its final area. Thereafter, with advancing expansion and age of the leaf, the activity declined. This pattern of development of NRA during the ontogenesis of leaves was not influenced by raising the supply of NO(3) (-) from 3 to 6 milliequivalent per cubic decimeter in the substrate solution. The concentration of NO(3) (-) in leaves, stem and root was inversely related to NRA at both NO(3) (-) levels. Raising the supply of K(+) from 1 to 6 milliequivalent per cubic decimeter at either concentration of NO(3) (-) slowed down the development of NRA in the initial stages of expansion, but promoted it subsequently. The peak of the activity which developed in a leaf of 62% of its final area was higher at the higher supply of K(+). The higher activity was maintained thereafter in the expanding and in matured and older leaves. It was concluded that NRA and the pattern of its development in expanding leaves is related to the availability of metabolites and their incorporation into enzyme proteins. Both these processes are influenced by: (a) the vertical profile of concentration of K(+) in the shoot and (b) the concentration of K(+) in a leaf, which depend upon its supply.