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

Liu Jian,Li Jing,Deng Chen,Liu Zhe,Yin Kexin,Zhang Ying,Zhao Ziyan,Zhao Rui,Zhao Nan,Zhou Xiaoyang,Chen Shaoliang

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 and salt-sensitive P. popularis. Total N content, growth, and photosynthesis were significantly reduced in P. popularis after 7 days 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 up-regulation 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.