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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 Supply

2002; Oxford University Press; Volume: 128; Issue: 2 Linguagem: Inglês

10.1104/pp.010602

1532-2548

Marie Mattßon, Jan K. Schjøerring,

Soil and Water Nutrient Dynamics

Abstract 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.