Endogenous rhythmic activity of nitrate reductase in a selection of Chenopodium rubrum

Abstract

Oscillations of nitrate reductase (NR) activity occurred after transfer of seedlings of Chenopodium rubrum L., ecotype 60°47′ N 137°32′ W, from an alternating temperature and light intensity germination program to constant temperature and light conditions; there were peaks of activity at 18, 30, and 45 h. The presence of half-strength Hoagland's nutrient solution during the germination period increased the range of the oscillations but did not affect the period. Changes in levels of nitrate and nitrite within the seedlings were consistent with the periodicity of NR activity during the light period after germination. Enzyme activity decreased if seedlings were placed in darkness, but the phasing of the oscillations was not altered by the transition from light to darkness. In addition, oscillations of NR activity with a period length of about 18 h occurred during continuous light after dark periods. This secondary rhythmicity is apparently superimposed upon the pattern of fluctuations previously established.On the basis of the rhythmicity that is characterized by these findings, and also in the light of other previously observed effects of altered environmental conditions on the activity of NR, hypotheses are presented to account for the endogenous oscillations of NR activity in Chenopodium rubrum. It is suggested that such rhythmic behavior of NR may occur in higher plants in general.