NITRATE REDUCTASE-ACTIVITY AND UPTAKE OF NITRATE AND OXYGEN AS AFFECTED BY NITRATE SUPPLY TO DETACHED MAIZE ORGANS

Supply of 1, 2, 5, 10 or 20 mM nitrate to detached roots; scutella or shoots from 5- to 6-d-old Zea mays L. seedlings increased in vitro nit rate reductase (NR) activity in all the organs and NADPH specific NR ( NADPH:NR) activity in roots and scutella but not in the shoots. Usuall y 2 to 5 mM nitrate supported maximum enzyme activity, the higher conc entration did not increase it further. The protein content in the root s, scutella and shoots increased up to 5, 2 and 20 mM medium nitrate, respectively. Nitrate uptake also increased with increasing nitrate co ncentration in roots and shoots, but it increased only slightly in the scutella. In both roots and scutella, methionine sulfoximine had no e ffect, while cycloheximide and tungstate abolished nitrate induced NAD H:NR activity completely and NADPH:NR partially. Methionine sulfoximin e increased nitrate uptake by roots and scutella slightly, but other i nhibitors had no effect. The depletion of dissolved oxygen from the me dium was lower in the presence of nitrate than in its absence or in th e presence of ammonium, especially in the scutella.