NITRATE UPTAKE AND ASSIMILATION BY INDIVIDUAL ROOTS OF ZEA-MAYS

Ten day old maize plants were used to investigate the nitrate uptake a nd assimilation in morphologically different types of maize root syste m. Maize roots not previously exposed to nitrate had a very low consti tutive transport system (CHATS) but influx increased after induction o f high-affinity transport system (HATS). This enhancement represented 3-fold increase of nitrate uptake during six hour contact with 100 mu mol.l(-1) of NO3-. During this period the HATS was fully synthesized. Introduction of similar concentration of NaNO2 resulted in induction o f nitrate HATS, but had no effect on the induction of nitrate reductas e (NR). Induction of NR has shown similar pattern as induction of nitr ate uptake. NR activity increased with both time of induction and conc entration of KNO3. When the plants were induced with 100 mu mol.l(-1) of NO3- the NR activity reached its maximal value after 7-8 h. Under t he increased concentration of NO3- (1 mmol.l(-1) of KNO3), the NR acti vity reached its constant value after 3 h induction. The highest activ ity of NR was found in seminal roots (primary seminal and seminal adve ntitious roots) while nodal roots showed lower capacity for reduction as well as for uptake of NO3-. These results confirm an expected diffe rences in uptake and assimilation of nitrate by individual maize root types.