Evaluation of Humic Acid on Chromium Toxicity, Accumulation and Translocation in Lettuce (Lactuca sativa L. var. longifolia)

Chromium pollution of the soil due to natural processes or industrial activities such as metal refining, chrome plating, stainless-steel production, leather tanning, and chemical dye production is a globle environmental issue. Excessive soil Cr levels cause detrimental effects on plant physiological processes including photosynthesis, water relations and mineral nutrition as well as the growth of roots, stems and leaves, which may decrease the biomass and yield of plants. Currently, soil application of organic amendments particularly humic acid seems to be an effective procedure to enhance relative plant tolerance to Cr stress. Humic acids are complexes of heterogeneous poly electrolytes with abundant functional groups that act as a weak poly electrolytic acid. Their structures, the degree to which these functional groups are protonated or ionized and environmental conditions influence the interaction between HA and soil pollutants. The complex compounds form by interaction of HA and heavy metals that cannot be uptaken by plants. Humic acid may play a significant role in the mobility and uptake of Cr which leads to a significant increase in plant biomass and growth. The aim of this research was to investigate the ability of humic acid to reduce Cr uptake and translocation by lettuce (Lactuca sativa L.) from Cr-contaminated soil.

The present study through a greenhouse pot experiment was conducted in the greenhouse of Ferdowsi university of Mashhad. The experiment was arranged in a factorial manner in a randomized complete design with three replications and treatments consisted of 3 levels of Cr (0, 25, and 50 mg kg-1as K2Cr2O7) and 3 rates of HA (0, 5 and 10 %). The soil samples were dried at room temperature, ground and sieved with a 2-mm mesh screen for further analysis. The bioavailable concentrations of Cr in the soils were assessed by DTPA. Three lettuce seedlings were grown in each pot containing five air-dried soil and watered to a near field capacity with distilled water as needed. After 100 days plant tissues were harvested, carefully washed with deionized water and the leaf, stem and root parts separated. All of them were oven-dried at 65-75 °C to constant weight and the dry weight of lettuce tissue samples was recorded. To determine the Cr concentrations, the tissues were ground, passed through a 0.3-mm sieve and digested in di-acid mixture (HNO3:HClO4). Concentrations of Cr in the digested solutions and soil extractions were determined using an Inductively Coupled Plasma Optical Emission Spectrometry (ICP OES). Translocation factor (TF) is determined from the ratio of the concentration of Cr in the plant’s shoots compared to that in the plant’s roots. Bioaccumulation factor (BAF) was evaluated as defined as the accumulated concentration of Cr in plant divided by concentration to that in respective soil. A two-way analysis of variance was done by using a statistical package, JMP version 8.0. The differences between the treatments were determined using LSD multiple range tests at significance level of P ≤ 0.05 and P ≤ 0.001.

The results of the present study clearly demonstrate that all Cr treatments significantly reduced leaf, stem, shoot and root dry weights. In unamended soils, both Cr treatments alone reduced leaf, shoot, stem and root dry weights 83%, 101%, 207% and 65% (for Cr 25 mg kg-1) and 194%, 219%, 355% and 92% (for Cr 50 mg kg-1) respectively as compared to control. Using HA (5 and 10%) and Cr treatments (25 and 50 mg kg-1), showed that leaf, shoot, stem and root dry weights were significantly increased as compared to Cr contaminated control. The lowest values of these parameters were recorded in Cr treatments without addition of HA, whereas at each Cr level, the highest values of them were obtained with application of 10% HA. The Cr concentrations in shoot and root samples significantly were affected by adding HA and Cr levels in soil. It was observed that Cr contents in shoots and roots, transfer factor and bioaccumulation factor of shoots and roots significantly increased by increasing soil Cr levels. Moreover, HA application negatively affected Cr content in shoot compared to Cr treatment alone. The interaction of chromium and humic acid caused a significant decrease in the concentration of chromium in the aerial parts, the shoot accumulation factor and a significant increase in the concentration of chromium in the roots and consequently reduced translocation factor. The highest value of Cr in shoot (47.7 mg kg-1) was obtained in those plants grown in soil with addition of 50 mg kg-1 Cr alone, whereas at each Cr level the lowest value of Cr in shoot was found in those plants grown in soil with the application of 10% HA. HA application in soil increased Cr concentration in root compared with Cr contaminated control. The maximum Cr concentration in the root (367 mg kg-1) and root bioaccumulation factor (28.5) was obtained after exposure to 50 mg kg-1 Cr +10% HA treatment. Also, the regression models showed that the transfer factor and shoot bioaccumulation factor decreased significantly and linearly with increasing shoot dry weight. Moreover, the regression model of shoot dry weight and shoot bioaccumulation factor was able to predict traslocation factor and shoot bioaccumulation factor with Adjusted R2 = -0.78** and R2 = -0.93**, respectively.

Results demonstrated that Cr toxicity markedly reduced plant growth parameters for instance leaf, stem, shoot and root dry weight and enhanced the concentration of Cr in shoot and root as compared to control. Humic acid application in Cr contaminated soil induced increased plant biomass, root bioaccumulation factor, Cr contents in roots and reduced Cr concentration in leaves, translocation factor and shoot bioaccumulation factor. Therefore, the application of HA specially at higher dose (10%) seems to be a cost-effective and environmentally friendly method for the restriction of Cr accumulation and its transfer from contaminated soil to edible parts of lettuce, thus helping to enhance food security.