effect of humic acid on the accumulation of cadmium, nitrate and changes of nitrate reductase activity in lettuce

abstract lettuce is one of the best accumulators of heavy metals and nitrate between leafy vegetables. through this research, it was tried to decreased cadmium absorption with humic acid (ha) and studies the effect of ha on the accumulation and metabolism of nitrate. plants were grown in the peat/perlite and irrigated with hoagland solution with addition of cadmium (2 and 4 mg l -1) and ha (100 and 1000 mg l-1). results showed that cd in media increased cd of leaves and decreased protein and fresh weight but it has not any effect on the n, p and nitrate concentration and nitrate reductase activity. with increasing concentration of ha, increasing in fresh weight, n, protein concentration and decreasing in cd concentration in leaves was observed. using of ha and cd together in nutrient solution showed that ha decreased absorption of cd. so toxic effect of cd decreased on the physiological changes of lettuce and do not accumulate nitrate in leaves, as well. more investigations should be done for using ha on the polluted soil for planting of lettuce.