RHYTHMS IN MAGNESIUM-ION INHIBITION AND HYSTERETIC PROPERTIES OF NITRATE REDUCTASE IN THE CAM PLANT BRYOPHYLLUM-FEDTSCHENKOI

Nitrate reductase (NR. EC 1.6.6.1) was tested in crude extracts of lea ves from Bryophyllum fedtschenkoi plants growing under alternating lig ht/darkness as well as in excised leaves kept in continuous light or d arkness. In most extracts NR activity was inhibited 20-80% by 5 mM Mg2 + A light to darkness shift (30 min darkness) during the first part of the photoperiod gave an increase in the Mg2+ inhibition and a decreas e in NR activity. Magnesium ion inhibition of NR also showed diurnal v ariations. Strongest inhibition was found in extracts made during the latter part of the photoperiod and start of the dark period. Pre-incub ation of crude extracts with ATP increased Mg2+ inhibition, indicating that phosphorylation of NR is involved in regulation of NR in Crassul acean acid metabolism (CAM) plants. In continuous light an increase in Mg2+ inhibition occurred after 20 h and 40 h, indicating a rhythm in the phosphorylation of NR. A delay in the production of nitrite in the assay (hysteresis) was generally seen in extracts susceptible to Mg2 inhibition. The rhythms related to NR activity showed the same period length (20 h) as the rhythm in CO2 exchange. However, in contrast to the rhythm in CO2 exchange, NR rhythms were strongly damped in continu ous light. In constant darkness the rhythms were even more damped. The results show that posttranslational modification of CAM NR is influen ced by light/darkness and by an endogenous rhythm.