Ammonium uptake increases in response to PEG-induced drought stress in Malus hupehensis Rehd.

It is unclear how plants optimize their preference for different forms of inorganic nitrogen to enhance their survival and fitness in challenging environments. Using a hydroponics culture system, we monitored morphological, physiological, and molecular changes in Malus hupehensis seedlings when drought conditions (induced by 5% polyethylene glycol, or PEG) were combined with a low or normal supply of N (0.05 mM or 1 mM NR4NO3, respectively). We found here that PEG-induced drought stress negatively inhibited the growth of Malus hupehensis seedlings and resulted in higher NH4+/NO3- ratios in their roots and leaves. The net ammonium influx at the surface of fine roots rose dramatically under drought treatment, while that of nitrate was less changed. This was confirmed by higher absorption of stable (NH4+)-N-15 isotope than of (NO3-)-N-15 under drought conditions. Consistently, two ammonium transporter genes closely related to ammonium uptake (AMT4;2 and AMT4;3) were notably up-regulated in response to drought stress, while transcript levels of most genes related to nitrate uptake/reduction and nitrogen assimilation decreased. Therefore, in acclimating to drought, Malus hupehensis plants exhibited relatively increased uptake of NH4+ compared to NO3-. In addition, reducing N supply may alleviate the adverse affect of drought stress on the growth of Malus hupehensis.