Drip irrigation reduces upward translocation of heavy metals in soybeans by enhancing root retention and inhibiting xylem transport

Soil heavy metal contamination poses a serious threat to the safety of crop production. To elucidate the physiological and molecular mechanisms by which drip irrigation reduces the upward translocation of heavy metals in soybeans, this study established a drip irrigation experiment with four irrigation frequencies. A systematic analysis was conducted on the differences in various physiological indicators and gene expression in soybean roots under combined contamination of Cd, Pb, and Cr(VI), in comparison with surface irrigation. Results indicate that: (i) drip irrigation maintained root physiological activity by enhancing antioxidant enzyme activities and osmotic adjustment capacity. (ii) Drip irrigation up-regulated the expression of genes encoding key enzymes involved in the synthesis of cytokinin and ethylene, as well as genes associated with transport functions such as metal ion transmembrane transporter activity, thereby promoting root growth and enhancing heavy metal uptake. (iii) Drip irrigation stimulated the synthesis of small organic molecules such as organic acids, amino acids, glutathione, and nicotinamide in soybean roots, consequently reinforcing the root's capacity for heavy metal retention. (iv) The reduced expression of genes encoding lignin biosynthesis-related enzymes under drip irrigation restricted xylem development, thus diminishing heavy metal translocation to aboveground parts. The findings provide a theoretical foundation for the safe utilization of heavy metal-contaminated farmland.