The inhibition of in vitro nitrate reductase activity in extracts of Protea spp. is shown to be due to polyphenolic constituents of the roots and shoots of the plants (which can be adsorbed by insoluble polyvinylpyrrolidone) rather than to the activity of endogenous proteases. The in vitro nitrate reductase activity in shoots of Protea repens and Protea cynaroides fed 2mmoldm−3 NO3− for 24h prior to nitrate reductase extraction show a nitrate reductase activity of 2–4μmol NO2− h−1 while the roots of P. repens show an in vitro nitrate reductase activity of 0,2μmol NO2− h−1 (g fresh mass)−1. The low nitrate reductase activity of these plants possibly reflects their adaptation to growth under the low nutrient condition of the soils of the South Western Cape, South Africa
