Contrasting Responses and Phytoremediation Potential of Two Poplar Species to Combined Strontium and Diesel Oil Stress

Abstract

The soil pollution caused by diesel oil and heavy metals has become an increasingly serious environmental issue, with negative global-scale impacts. The remediation of contaminated soil requires special attention, in which phytoremediation has emerged as an ecofriendly solution. However, the response of plants to the combined stress of diesel oil and heavy metals remains largely unknown. In this study, the aim was to investigate the potential of Populus alba and P. russkii for phytoremediation by examining their response to combined diesel oil and heavy metal stress. In a greenhouse experiment using soil contaminated with 15 mg kg^-1 of diesel oil and varying concentrations of Sr (0, 10, or 100 mg kg^-1), we studied the physiological and biochemical changes, as well as the Sr absorption, of P. alba and P. russkii. The results showed that at high concentrations of Sr and diesel oil, the growth of both species was substantially inhibited, but P. alba exhibited higher resistance due to its higher antioxidant enzyme activities and increased accumulation of soluble sugar and proline. Additionally, P. alba concentrated Sr in the stem, whereas P. russkii accumulated Sr in the leaf, exacerbating its negative effects. Diesel oil treatments were beneficial for Sr extraction due to cross-tolerance. Our findings indicate that P. alba is more suitable for the phytoremediation of Sr contamination due to its superior tolerance to combined stress, and we identified potential biomarkers for monitoring pollution. Therefore, this study provides a theoretical basis and implementation strategy for the remediation of soil contaminated by both heavy metals and diesel oil.

Keywords: Populus; combined stress; cross tolerance; diesel oil; physiological response; phytoremediation; strontium.

Figure 1

Growth responses of P. alba and P. russkii to different strontium and diesel oil treatments including plant height (a), leaf area (b) and biomass (c). Treatments are shown as oil/Sr combinations. CK means zero oil and Sr added. Different small letters above the bars (means of five replicates ± SE) indicate significant differences from one other at p < 0.05. Capital letters in figure C indicate significant differences in total biomass among treatments.

Figure 2

Strontium concentration (a–c), enrichment index (d–f), and extraction amount (g–i) of Populus alba and P. russkii under different strontium and diesel oil treatments. The strontium concentrations in 0/0 and 15/0 are below the detection line and are thus omitted. Treatments are shown as oil/Sr combinations. Different letters above the bars (means of five replicates ± SE) indicate significant differences from each other at p < 0.05.

Figure 3

Antioxidant enzyme activities (APX, (a); POD, (b); SOD, (c)) and MDA contents (d) of Populus alba and P. russkii under different strontium and diesel oil treatments. Treatments are shown as oil/Sr combinations. Different letters above the bars (means of five replicates ± SE) are significantly different from each other at p < 0.05.

Figure 4

Correlation analysis between antioxidant enzymes (APX, (a,d); POD, (b,e); SOD, (c,f)) and leaf enrichment of Populus alba (a–c) and P. russkii (d–f) under different strontium and diesel oil treatments.

Figure 5

Contents of sugar (a) and proline (b) of Populus alba and P. russkii under different strontium and diesel oil treatments. Treatments are shown as oil/Sr combinations. The different letters above the bars (means of five replicates ± SE) are significantly different from each other at p < 0.05.

Figure 6

Contrasting physiological responses of Populus alba and P. russkii indicated in a heat map of the relationships among physiological indices (a,b); and principal component analysis (PCA) of the different treatments (c,d). * (p < 0.05), ** (p < 0.01), and *** (p < 0.001).