P. Gherbin

Accumulation of cadmium, zinc, and copper by Helianthus annuus L.: impact on plant growth and uptake of nutritional elements.

We investigated the effects on physiological response, trace elements and nutrients accumulation of sunflower plants grown in soil contaminated with: 5 mg kg−1 of Cd; 5 and 300 mg kg−1 of Cd and Zn, respectively; 5, 300, and 400 mg kg−1 of Cd, Zn, and Cu, respectively. Contaminants applied did not produce large effects on growth, except in Cd-Zn-Cu treatment in which leaf area and total dry matter were reduced, by 15%. The contamination with Cd alone did not affect neither growth nor physiological parameters, despite considerable amounts of Cd accumulated in roots and older leaves, with a high bioconcentration factor from soil to plant. By adding Zn and then Cu to Cd in soil, significant were the toxic effects on chlorophyll content and water relations due to greater accumulation of trace elements in tissues, with imbalances in nutrients uptake. Highly significant was the interaction between shoot elements concentration (Cd, Zn, Cu, Fe, Mg, K, Ca) and treatments. Heavy metals concentrations in roots always exceeded those in stem and leaves, with a lower translocation from roots to shoots, suggesting a strategy of sunflower to compartmentalise the potentially toxic elements in physiologically less active parts in order to preserve younger tissues.

GROWTH AND PHYSIOLOGICAL RESPONSE OF HYDROPONICALLY-GROWN SUNFLOWER AS AFFECTED BY SALINITY AND MAGNESIUM LEVELS

The effects of NaCl and magnesium levels (Mg2+) on the physiological response of sunflower were investigated. Plants were grown for 54 days in hydroponic culture with NaCl (100 mM) or without NaCl and four concentrations of Mg2+: 0, 0.4, 1.0 and 5.0 mM. At the end of the vegetative growing cycle of sunflower, salt stress reduced leaf area development by 51% and dry matter accumulation by 37% as compared to non saline-treated plants; at this stage, considering the percent reduction of partitioned plant dry matter, roots (42%) and leaves (35%) showed to be more salt-sensitive than stem. Growth reduction was related to the drop in net CO2 assimilation rate and stomatal conductance, which started declining later during the vegetative growth period when leaf ion concentration started increasing. The investigated genotype was unable to exclude ions and significant amounts of Cl− (about 1700 μmol g−1 DW) and lesser Na+ (700 μmol g−1 DW) accumulated in the leaves. The decline in net CO2 assimilation was well correlated to the increase in leaf Cl− concentration (r2 = 0.71) and not to leaf Na+ concentration (r2 = 0.33). The results suggest that, though sunflower develops an endogenous protection system by which it redistributes this ions in the whole plant, with more ions accumulating in roots and older leaves, growth reduction may be attributed to specific toxic effects of Cl− on photosynthetic functionality. In both saline and non saline conditions, little or no significant differences in growth parameters of plants exposed to a range from 0.4 to 5 mM of Mg2+ were observed. Whereas, its deficiency caused a drastic reduction of dry matter accumulation up to 90%, due to progressive decline in CO2 assimilation rate and chlorophyll content, with imbalances in Ca2+, Mg2+ and K+.