Effect of NaCl on ammonium and nitrate uptake and transport in salt-tolerant and salt-sensitive poplars.

Nitrogen (N) plays an important role in mitigating salt stress in tree species. We investigate the genotypic differences in the uptake of ammonium (NH₄⁺) and nitrate (NO₃⁻) and the importance for salt tolerance in two contrasting poplars, salt-tolerant Populus euphratica Oliv. and salt-sensitive P. simonii × (P. pyramidalis × Salix matsudana) (P. popularis cv. 35-44, P. popularis). Total N content, growth and photosynthesis were significantly reduced in P. popularis after 7 days of exposure to NaCl (100 mM) supplied with 1 mM NH₄⁺ and 1 mM NO₃⁻, while the salt effects were not pronounced in P. euphratica. The ¹⁵NH₄⁺ trace and root flux profiles showed that salt-stressed poplars retained ammonium uptake, which was related to the upregulation of ammonium transporters (AMTs) in roots, as two of the four AMTs tested significantly increased in salt-stressed P. euphratica (i.e. AMT1.2 , 2.1) and P. popularis (i.e. AMT1.1 , 1.6). It should be noted that P. euphratica differs from salt-sensitive poplar in the maintenance of NO₃⁻ under salinity. ¹⁵NO₃⁻ tracing and root flux profiles showed that P. euphratica maintained nitrate uptake and transport, while the capacity to uptake NO₃⁻ was limited in salt-sensitive P. popularis. Salt increased the transcription of nitrate transporters (NRTs), NRT1.1 , 1.2 , 2.4 , 3.1 , in P. euphratica , while P. popularis showed a decrease in the transcripts of NRT1.1 , 2.4 , 3.1 after 7 days of salt stress. Furthermore, salt-stimulated transcription of plasmalemma H⁺-ATPases (HAs), HA2 , HA4 and HA11 contributed to H⁺-pump activation and NO₃⁻ uptake in P. euphratica. However, salt stimulation of HAs was less pronounced in P. popularis , where a decrease in HA2 transcripts was observed in the stressed roots. We conclude that the salinity-decreased transcripts of NRTs and HAs reduced the ability to uptake NO₃⁻ in P. popularis , resulting in limited nitrogen supply. In comparison, P. euphratica maintains NH₄⁺ and NO₃⁻ supply, mitigating the negative effects of salt stress. [ABSTRACT FROM AUTHOR]