Effects of chromium on hypocotyl elongation, wall components, and peroxidase activity of Phaseolus vulgaris seedlings

Abstract

In this investigation, the effect of the heavy metal chromium (Cr) on hypocotyl elongation of Phaseolus vulgaris seedlings in light was studied. The seedlings were subjected to two concentrations of Cr (0.5 and 1.0 mM) in the form of K2Cr2O7. Hypocotyl elongation (growth) was measured in terms of length. Change in wall components (pectic polysaccharides and xyloglucan) and peroxidase activity (soluble and bound) were also studied. Both concentrations of Cr inhibited hypocotyl length and a clear concentration effect was observed. Maximum pectic polysaccharide content was observed at a high concentration of Cr and a minimum was observed in the distilled water control. A clear inverse correlation was observed, supporting the conclusion that pectic polysaccharides may be involved in cell wall loosening. Even xyloglucan (high and low) content showed a clear inverse correlation with hypocotyl length. It appears that Cr inhibits hypocotyl elongation and xyloglucan and pectic polysaccharide contents are not degraded to smaller oligosaccharides as they occur more in seedlings treated with Cr compared to seedlings treated with distilled water. Further soluble and bound peroxidase activity was assayed with four hydrogen donors— ferulic acid, caffeic acid, pyrocatechol, and pyrogallol during the initial phase of hypocotyl elongation. A clear inverse correlation between length and peroxidase activity was observed. All hydrogen donors showed a clear concentration effect, specificity, and variation, and maximum activity occurred with caffeic acid. The role of peroxidase in the defense mechanism in response to Cr toxicity is discussed.