Nitrate Reductase Activity in Soybeans (Glycine max [L.] Merr.)

Abstract

The optimum in vivo nitrate reductase (NR) assay medium for soybean (Glycine max [L.] Merr.) leaves was 50 mM KNO3, 1% (vol/vol) 1-propanol, and 100 mM potassium phosphate buffer (pH 7.5). Loss of in vivo NR activity from leaves of soybeans exposed to dark was fastest at 40.degree. C and slowest at 20.degree. C. By the end of a 16-h dark period, even those plants exposed to the lowest (20.degree. C) temperature had lost 95% of the initial activity. Upon re-exposure to light, following a 16 h 30.degree. C dark period, in vivo NR activity increased rapidly to maximum levels after 4 h light. The rate of increase was proportional to light intensity (6, 16 and 45 Klx) and independent of temperature (20.degree., 30.degree. and 40.degree. C). Studies with field-grown soybeans indicated that nighttime temperature (16.degree.-27.degree. C) had no effect on the subsequent in vivo NR activity in sunlight at ambient temperature. There was a marked decrease in in vivo NR activity in late afternoon with the field-grown plants. This decrease continued throughout the night with elevated temperature (27.degree. C) while NR activity increased when a cooler (16.degree. C) night temperature was imposed. The changes in in vivo NR activity in response to light and dark treatments were quite rapid and thought to be related to energy limitations as well as enzyme level.