The Influence of Water Conditions on Heavy Metal Tolerance Mechanisms in Hybrid Poplar (Populus nigra × Populus maximowiczii) in the Light of Sustainable Development Goals

Sustainable management of soils degraded by heavy metals is a major environmental challenge. The aim of this study was to evaluate the acclimatization ability of the hybrid Populus nigra L. × Populus maximowiczii under variable soil moisture conditions. In a greenhouse experiment, it was shown that both soil moisture level and the presence of metals significantly affected plant growth and metabolism. The hybrid showed high nickel (Ni) accumulation at low and medium soil moisture content (LMC, MMC) (BCF 4.56 and 4.99), while copper (Cu) accumulation was highest at MMC (BCF 5.53). Nickel translocation to aerial parts increased after exposure (TF up to 0.63), while Cu translocation was limited (TF below 0.94). Increased humidity promoted the biosynthesis of low molecular weight organic acids (LMWOAs) in roots, with the highest total content recorded in the Cu treatment under high soil moisture content (HMC) (230 μg g−1 FW). In the stems, the highest levels of sum LMWOAs were found under HMC conditions (6764 μg g−1 FW in the control sample), while among the phenolic acids, the highest content of chlorogenic acid (~144 μg g−1 FW) was determined under LMC conditions under Ni stress, which indicates a strong defense response of the plant. The obtained results emphasize the importance of selecting appropriate water conditions in remediation strategies and indicate that the tested poplar hybrid may be a promising tool in improving the quality of degraded soils.