The Effect of some Plant Growth Regulators and Surfactant on Safflower in Chromium Contaminated Soil

Chromium (Cr) is one of the toxic metals widely used in leather tanning, alloy preparation, electroplating, drilling mud, refractory steel and catalytic manufacture. Besides the toxicity of chromium to human, it also disturbs the soil ecology and plant growth due to its toxic nature even at low concentration. Phytoremediation is effective and can be viewed as a relatively low cost, solar energy driven process for the management of contaminated soils. Hiwever the heavy metal toxicity adversely affects the plant growth and development. We can use some chemical compounds to increase plant resistance to heavy metal and increase the efficiency of phytoremediation. These days, foliar application of plant growth regulators such as Gibberelic acid Indole acetic acid and Benzyl amino purine are considered for various purposes such as enhancing plant growth and resistance to salinity, drought and heavy metals. Exogenous application of phytohormones can modulate the toxicity of Cr on plants most probably by maintaining hormonal balance of plant under metal stress. surfactants effectively enhance metal ion transfer to aqueous and hence increase their availability. Assessing surfactant assisted phytoremediation is important in order to ascertain the extent of its effectiveness under different conditions and to find its optimum level for metal phytoremediation. The application of plant growth regulators and surfactants can be an effective way to cope with stresses such as heavy metal contamination. The objectives of this study were to determine the effects of the growth hormones Gibberelic acid, Benzyl amino purine and Indole acetic acid alone and combined with surfactant on plant growth, concentration and uptake of Cr, Fe, Mn, Cu and Zn and some phytoremediation factor for Cr.

The soil was air-dried and grounded to pass through a 2-mm sieve then was analyzed to determine various soil physic-chemical properties using standard methods. A greenhouse experiment was conducted in a completely randomized design with a factorial arrangement of 3 × 4 including three levels of surfactant (control, 2.5 and 5 mmol kg-1 soil) and four levels of plant growth regulators (control, Gibberellic acid, Indole acetic acid and Benzyl amino purine). All soils were contaminated by 5 mg/kg chromium and incubated for 1 month. During incubation, the soil samples were maintained at field capacity by distilled water. Safflower (Cartamus tinctorius L.) seeds were disinfested with 10% sodium hypochlorite, washed three time with distilled water and planted in the pots. Growth regulators were sprayed at three stage of 30, 20 and 40 days after planting at concentration of 1 mM. The plants were kept in the standard condition of greenhouse and the soil moisture content was maintained at field capacity by distilled water. Sixty days after planting, the plants were harvested and washed with distilled water, and then dried in oven at 65 Celsius until they reached a constant weight. Afterwards, the over-dried plant samples were grounded. Then dry ashing and extracting with 2 normal hydrochloric acid, the concentration of Cr, Fe, Zn, Cu and Mn in shoot was determined by atomic absorption (Shimadzu AA-670). The analysis of variance (ANOVA) was performed using a completely randomized design. Significantly different treatment means were separated using Duncan test (P < 0.05). Biological accumulation coefficient (BAC) and uptake index (UI) were calculated with a specific formula.

The results showed that addition of surfactant and growth regulators caused a significant increase in shoot dry weight, biological accumulation coefficient, uptake index and chromium concentration and uptake. Addition of surfactant reduced the concentration and uptake of iron in the absence of growth regulators, but in the presence of plant growth regulators, application of tween 80 increased iron concentrations. Application of 5 mmol kg-1of surfactant decreased mean concentration of manganese, copper and zinc. While addition of 2.5 mmol kg-1 of surfactant increased metals concentrations. Although addition of 5 mmol kg-1 surfactant increased dry weight, it did not have a satisfied effect on increasing the concentration of the elements in the plant. Plant growth regulators increased uptake of elements which is a protective mechanism against stresses.

It appears that using plant growth regulators increased the resistance of the plant to chromium toxicity probably through increasing absorption of the elements such as Fe, Mn, Cu and Zn. According to the results, application of tween 80 along with plant growth regulators could increase safflower capability to cope with chromium toxicity.