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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.
Early Response of the Populus nigra L. × P. maximowiczii Hybrid to Soil Enrichment with Metals
This study aimed to investigate the response of Populus nigra L. × Populus maximowiczii to the addition of selected metals in soil. Rooted cuttings were planted in pots containing soil enriched with equimolar concentrations of Pb, Zn, Al, Ni, and Cu (500 mL of 4 mM solutions of single metal salts: (Pb(NO3)2; Zn(NO3)2 × 6H2O; Al(NO3)3 × 9H2O; Ni(NO3)2 × 6H2O; or Cu(NO3)2 × 3H2O). Growth parameters, metal accumulation, and physiological and biochemical parameters were assessed after four weeks of cultivation, simulating early response conditions. The results showed diverse metal accumulation in poplar organs, along with an increase in biomass and minor changes in gas exchange parameters or chlorophyll fluorescence. Among low-molecular-weight organic acids, citric and succinic acids were dominant in the rhizosphere, and roots with malonic acid were also present in the shoots. Only p-coumaric acid was found in the phenolic profile of the roots. The shoots contained both phenolic acids and flavonoids, and their profile was diversely modified by particular metals. Sucrose and fructose content increased in shoots that underwent metal treatments, with glucose increasing only in Cu and Al treatments. Principal component analysis (PCA) revealed variations induced by metal treatments across all parameters. Responses to Pb and Zn were partially similar, while Cu, Ni, or Al triggered distinct reactions. The results indicate the adaptation of P. nigra L. × P. maximowiczii to soil containing elevated levels of metals, along with potential for soil remediation and metal removal. However, further studies are needed to evaluate the effect of differences in early responses to particular metals on plant conditions from a long-term perspective.