Roots of Lupinus angustifolius L. and enzyme activities in soil contaminated by toxic elements

The impact of toxic elements (TEs) contaminating the root zone of Lupinus angustifolius L. on enzymatic activities, nitrification rate, and changes in the root system was evaluated. Lupine was cultivated in a pot experiment using two types of soil – control and contamination (with a high degree of arsenic (As), cadmium (Cd), lead (Pb), and zinc (Zn) contamination). After harvesting lupine biomass, enzyme activities (β-glucosidase, acid phosphatase, arylsulphatase, lipase, chitinase, cellobiohydrolase, alanine aminopeptidase, and leucine aminopeptidase) in soils were analysed. Enzyme activities decreased with TE soil contamination. According to our results, arylsulphatase was found to be the most sensitive soil enzyme to TEs. The nitrification rate is closely related to soil contamination and plant activity, as it stimulates microbial growth and multiplication through root exudates. The close correlations confirmed this relationship (r = 0.73−0.99). An increasing trend in TE contents in the roots was observed with soil contamination. Plant hormones are crucial in regulating root growth and development under stress conditions. The levels of determined phytohormones in our experiment (auxins, abscisic acid (ABA), salicylic acid (SA), and bioactive cytokinins (bCKs)) were lower in the contamination compared to the control. Correlations confirmed a significant negative relationship between the TE content in the roots and the contents of phytohormones (auxins: r = –0.96 to –0.97; ABA: r = –0.83 to –0.86; SA: r = –0.95 to –0.99, bCKs: r = –0.87 to –0.93). The ratios of these hormones (not their absolute values) appear to be the determining factor for regulating root development and protecting plants from oxidative stress.