Effect of mercury toxicity on hypocotyl elongation and cell wall loosening in Phaseolus seedlings

The effect of heavy metal mercury on hypocotyl elongation growth and cell wall loosening in young Phaseolus vulgaris seedlings was studied in light. The study was done with five different concentrations (0.01 mM, 0.05 mM, 0.1 mM, 0.2 mM, and 0.4 mM) of mercury in the presence of distilled water (DW) or gibberellic acid (GA3 200 micromolar). Mercury was given in the form of mercuric chloride. Growth in terms of length and fresh weight and changes in pectic polysaccharides and xyloglucan (low and high) content was studied at an interval of 12 h from germination to 110 h segments. All concentrations of mercury inhibited hypocotyl length in both DW and GA3 treatments. However, the length and fresh weight of hypocotyl in GA3 treatment was more than that of DW treatment. The pectic polysaccharide content was high initially, which showed a gradual decline up to 40 h in both the treatments and in all concentrations of mercury. The treatment, which resulted in minimum growth, had maximum content and vice-versa; however, though a concentration dependent trend was not visible, a clear inverse correlation between pectic polysaccharide content and hypocotyl length was observed, thus supporting the conclusion that pectic polysaccharides may be involved in cell wall loosening. Both low and high molecular weight xyloglucan were high initially at 8 h and showed a decreasing trend till 110 h. Low levels were maintained between 40 h-110 h. The xyloglucan content showed an inverse correlation with hypocotyl elongation growth. High molecular xyloglucan content was more than that of low molecular weight xyloglucan. Inhibitory effect was concentration dependent and it was more pronounced at higher concentrations. It appears that mercury affected cell division in the apical meristem, thus affecting directly the auxin synthesis, which in turn affected the degradation of xyloglucan. This resulted in inhibition of hypocotyl elongation growth. It appears that GA3 to a certain extent reduces the toxic effects of mercury.