The researchers studied how three enzymes in plants that help them use nitrate are regulated. They found that nitrate reductase and nitrite reductase are low when plants lack nitrogen, but can be increased by their substrates. Glutamate dehydrogenase is always present and only slightly affected by ammonia. Nitrate is the best inducer of nitrate reductase, while nitrite reductase is induced by nitrite. Both enzymes need RNA and protein synthesis to be induced. Nitrate reductase is not affected by ammonia but is repressed by nitrite. The enzymes are not inhibited by normal end products, but cyanate can inhibit nitrate reductase.