Nitrogen uptake and metabolism of Populus simonii in response to PEG-induced drought stress

• Drought stress negatively affected P. simonii seedlings growth at both low and normal nitrogen levels.
• The negative effects could to some extent be eased by increased nitrogen supply.
• P. simonii maintained high N uptake and assimilation ability at low nitrogen level under control condition.
• The ratio of NH4+ and NO3− was increased in response to drought stress.
• Two ammonium transporter genes closely related to NH4+ uptake (AMT1;2 and AMT1;6) were up-regulated under drought stress.

In the present study, changes in nitrogen uptake, assimilation and the transcript abundance of associated genes were investigated in Populus simonii seedlings treated with 5% polyethylene glycol (PEG)-induced drought stress under hydroponic culture conditions with low and normal nitrogen levels (0.01 mM NH4NO3 and 1 mM NH4NO3, respectively). Drought stress negatively affected plant growth at both low and normal nitrogen levels. NH4+ and NO3− concentrations were significantly decreased in the roots and leaves of PEG-treated P. simonii seedlings; however, the ratio of NH4+ and NO3− was increased in these plants. Drought stress also decreased NR and GOGAT activity in roots and leaves, irrespective of the nitrogen levels. However, the NH4+ concentrations and NR activity in roots and leaves were increased in response to 1 mM NH4NO3 under drought stress. PEG-treated plants also showed better growth at normal nitrogen level (1 mM NH4NO3), with increased biomass, total fine root length and larger root surface area. Most genes related to nitrate uptake, reduction and N metabolism were down-regulated; however, two ammonium transporter genes closely related to NH4+ uptake (AMT1;2 and AMT1;6) were up-regulated in response to drought stress. These results suggested that increased N nutrition may have contributed to drought tolerance in P. simonii by increasing NH4+ uptake and decreasing nitrogen metabolism.