Dynamic and steady-state responses of inorganic nitrogen pools and NH(3) exchange in leaves of Lolium perenne and Bromus erectus to changes in root nitrogen supply.

Short- and long-term responses of inorganic N pools and plant-atmosphere NH(3) exchange to changes in external N supply were investigated in 11-week-old plants of two grass species, Lolium perenne and Bromus erectus, characteristic of N-rich and N-poor grassland ecosystems, respectively. A switch of root N source from NO(-)(3)to NH(4)(+) caused within 3 h a 3- to 6-fold increase in leaf apoplastic NH(4)(+) concentration and a simultaneous decrease in apoplastic pH of about 0.4 pH units in both species. The concentration of total extractable leaf tissue NH(4)(+) also increased two to three times within 3 h after the switch. Removal of exogenous NH(4)(+) caused the apoplastic NH(4)(+) concentration to decline back to the original level within 24 h, whereas the leaf tissue NH(4)(+)concentration decreased more slowly and did not reach the original level in 48 h. After growing for 5 weeks with a steady-state supply of NO(-)(3)or NH(4)(+), L. perenne were in all cases larger, contained more N, and utilized the absorbed N more efficiently for growth than B. erectus, whereas the two species behaved oppositely with respect to tissue concentrations of NO(-)(3), NH(4)(+), and total N. Ammonia compensation points were higher for B. erectus than for L. perenne and were in both species higher for NH(4)(+)- than for NO(-)(3)-grown plants. Steady-state levels of apoplastic NH(4)(+), tissue NH(4)(+), and NH(3) emission were significantly correlated. It is concluded that leaf apoplastic NH(4)(+) is a highly dynamic pool, closely reflecting changes in the external N supply. This rapid response may constitute a signaling system coordinating leaf N metabolism with the actual N uptake by the roots and the external N availability.