Oscillations in the Activities of Enzymes of Nitrate Reduction and Ammonia Assimilation in Glycine max and Zea mays

Abstract

AbstractThe activities of glutamate dehydrogenase (GDH), glutamine synthetase (GS), and nitrate reductase (NR) and the levels of soluble protein and NO‐3 were assayed in soybean (Glycine max [L.] Merr.) leaves over a 48‐h period with the initial 24 h under a light‐dark cycle (LD 16:8) followed by 24 h of continuous light (LL). Plants had been entrained for 30 days under the LD regime. Maize (Zea mays) leaves (10 days old) under a LD 15:9 cycle were assayed only for NR and nitrite reductase (NiR). Data were subjected to frequency analysis by the least squares method to determine probabilities for cosine function periods (τ's) between 10 and 30 h. NR activities for both soybean and Zea leaves had 24 h τ's with P values < 0.05 indicating circadian periodicity. GDH in soybeans had a 24‐h rhythm under LD conditions which lengthened under LL conditions. The 24‐h rhythm of GDH displayed maximal activity toward the end of the dark period of the LD cycle whereas the highest activity of NR was early in the light period. Total soluble protein displayed a rhythm with a best fitting τ of greater than 24 h under both LD and LL. GDH, GS, NR, NO3, and soluble protein in soybeans and NiR in Zea, all displayed that were ultradian (10–18 h), indicating that a τ of about one half a circadian periodicity may be a common characteristic of the enzymes of primary nitrogen metabolism in higher plants. These data also demonstrate that although both NR and GDH are circadian in their activity, the 24‐h rhythm may be greatly influenced by ultradian oscillations in activity.