Cytosolic ions may be further factors involved in the control of NR activity. Artificial acidification of darkened leaf or root pieces with permeating weak acids and low external pH activated NR, and alkalinisation with permeating bases and high external pH reversed this effect (Kaiser and Brendle-Behnisch 1995). Further, treatment of darkened leaf discs with EDTA and the ionophore A23187, also activated NR (Kaiser and Huber 1994). Unfortunately, this does not necessarily prove direct Ca2+ effects e.g. on protein kinases (CDPKs). Efflux of divalent cations may be connected to a proton influx, and NR activation may thus also be related to cytosolic acidification.
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