Plant Physiol, February 2002, Vol. 128, pp. 742-750

Dynamic and Steady-State Responses of Inorganic Nitrogen Pools and NH₃ Exchange in Leaves of Lolium perenne and Bromus erectus to Changes in Root Nitrogen Supply¹

Plant Nutrition Laboratory, Department of Agricultural Sciences, Royal Veterinary and Agricultural University, Thorvaldensvej 40, DK-1871 Frederiksberg C, Copenhagen, Denmark

Short- and long-term responses of inorganic N pools and plant-atmosphere NH₃ 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 NO₃ to NH₄⁺ caused within 3 h a 3- to 6-fold increase in leaf apoplastic NH₄⁺ concentration and a simultaneous decrease in apoplastic pH of about 0.4 pH units in both species. The concentration of total extractable leaf tissue NH₄⁺ also increased two to three times within 3 h after the switch. Removal of exogenous NH₄⁺ caused the apoplastic NH₄⁺ concentration to decline back to the original level within 24 h, whereas the leaf tissue NH₄⁺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 NO₃ or NH₄⁺, 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 NO₃, NH₄⁺, and total N. Ammonia compensation points were higher for B. erectus than for L. perenne and were in both species higher for NH₄⁺- than for NO₃-grown plants. Steady-state levels of apoplastic NH₄⁺, tissue NH₄⁺, and NH₃ emission were significantly correlated. It is concluded that leaf apoplastic NH₄⁺ 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.