Growth of lettuce (Lactuca sativa L.) affected by cadmium concentrations and toxicity alleviation by calcium supplement

Rapid urbanization and population growth contribute to shortage of fresh water globally. Farmers in many developing countries resort to the utilization of untreated urban wastewater in irrigation. This unconventional waters contains toxic elements from households and industries such as mercury, chromium, lead and cadmium (Cd) that can be toxic to both plants and animals. Lettuce (Lactuca sativa L.) is a good accumulator of these toxic elements, especially Cd in the leaves; and therefore was selected in this research to study the effect of Cd on morphological, physiological, biochemical and anatomical attributes of lettuce. Seeds of five (5) lettuce varieties (Var. 163, Var. 168, Dankabo-D/Kabo, Italian lettuce-167 and Bombilasta-BBL) were germinated for varietal separation in a preliminary experiment. Two-weeks old seedlings were transplanted into a trough and supplied with the Cooper’s nutrient formulation solution (Cooper, 1979) containing (mg L-l): 236 N, 60 P, 300 K, 185 Ca, 50 Mg, 68 S, 12 Fe (EDTA), 2.0 Mn, 0.1 Zn, 0.1 Cu,0.3 B and 0.2 Mo, using a nutrient film technique system. The pH was maintained at 6 and electrical conductivity (E.C.) of 1.5 – 2.5 dS m-1. Varieties BBL and Italian 167 were eventually selected and treated with CdCl2 concentrations of 0, 0.5, 1, 2, and 4 mg L-1 to determine morphological, anatomical and physiological changes due to Cd toxicity. However, no visible toxicity sign was observed throughout the growing period on both varieties. Higher Cd concentrations of 0, 3, 6, 9 and 12 mg L-1 was used to re-examine toxicity effect on the two varieties. Significant negative effects were recorded in morphological, physiological, biochemical and nutrient element contents in the varieties. Despite the level of Cd used, variety BBL was taller and recorded higher leaf area, fresh and dry root weights than Italian variety 167. There was higher increase in antioxidant activity and vitamin C with increase in Cd concentrations compared with the control plants; but it was higher in variety 167 than BBL variety. The varieties displayed similar responses to Cd toxicity symptoms of chlorosis and stunted growth from 6 mg L-1 to 12 mg L-1 Cd level compared with the untreated plants. Macronutrient elements in the roots and shoots of lettuce were significantly decreased compared with the control plants at the highest Cd concentration of 12 mg L-1. Significant differences were found among the varieties of lettuce in the contents of micronutrients in the roots and shoots, however, variety 167 was found to absorb higher contents of Cd than variety BBL. Variety 167 had higher contents of Fe, Mn, Cu and Zn, implying synergistic effect of Cd and micronutrients in the roots and shoots of lettuce varieties. Introduction of calcium supplement of 0, 6 and 9 mg L-1 to Cd polluted water significantly alleviated the negative effects imposed by Cd at 0, 6 and 9 mg L-1 on all the parameters of lettuce studied. Calcium improved morphological, phytochemical, anatomical and physiological attributes of lettuce. At the same time enhanced photosynthetic pigments efficiency, proline and flavonoids contents; increased mineral nutrient elements and yield and decrease in MDA. Addition of 6 mg L-1 and 9 mg L-1 of Ca into solution containing 6 mg L-1 Cd provided more significant results very close to the results of the control plants. This study indicated that application of Ca2+ had significant and antagonistic effect on Cd by improving plant growth and development. Italian 167 variety is not recommended for planting in Cd polluted environments as it absorbed more Cd than lettuce variety BBL. Calcium can therefore be recommended for utilization in heavy metal polluted environments, particularly Cd as a strategy to alleviate their harmful effects, to enhance plant metabolism and perform better in such polluted areas.

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