Figure 1.

Schematic representation of the experimental set-up: poplar plants were grown hydroponically in complete LA liquid medium (N⁺ medium) for 10 days, then transferred to nitrogen-free medium (N⁻ medium) for 3 days; subsets of plants were transferred back to complete medium (N resupply, S1 [${\text{NH}}_4^{+}$-N]; S2 [NH₄NO₃]; S3 [${\text{NO}}_3^{-}$-N]) for another 21 days. The total treatment period is 34 days.