LIGHT REGULATION OF NITRATE REDUCTASE IN ULVA-FENESTRATA (CHLOROPHYCEAE) .1. INFLUENCE OF LIGHT REGIMES ON NITRATE REDUCTASE-ACTIVITY

Observations were made on the behavior of nitrate reductase activity in the green alga Ulva fenestrata under controlled light:dark regimes. The activity of nitrate reductase (NR) was examined in response to normal seasonal photoperiods as well as in response to shortened or extended periods of darkness. NR activity exhibits a light-dependent diurnal rhythm under both normal summer and winter photoperiods, with a maximum in the early morning (2 to 2.5 h after the start of illumination). This peak of activity is followed by a lower steady-state level of activity which is sustained throughout the light period. There is a sustained minimal level of activity in darkness. The morning peak in activity is always observed as long as tissue is illuminated, irrespective of the previous light or dark treatments. As such, it appears that nitrate reductase activity in U. fenestrata is under circadian control. There is no major difference in the NR activity pattern between summer and winter plants, except that the peak activity values in winter plants are consistently much higher (5 times) than in summer plants. The study also suggests that illumination prior to the normal start of photoperiod triggers a different set of regulatory mechanisms, indicating that the physiological state of plants is important in dictating the NR activity response to illumination.