Genetic variability in tolerance to cadmium and accumulation of heavy metals in pea (Pisum sativum L.)

Ninety-nine wild growing and primitive varieties of garden pea (Pisum sativum L.) were screened for tolerance to cadmium (Cd) toxicity in quartz sand culture. Cadmium tolerance was determined by the time to plant death when treated with a lethal Cd concentration (13 mg kg-1), and by a tolerance index (TI) calculated as a ratio between biomasses of Cd-treated and untreated plants in the presence of toxic Cd concentrations (7 mg kg-1 and 5 mg kg-1). The Cd-tolerance index varied significantly between pea genotypes from 35% to 90% and from 54% to 100% in the presence of 7 and5 mg Cd kg-1, respectively. Shoot Cd concentration of tolerant and sensitive genotypes grown in the presence of 5 mg Cdkg-1 varied between 35 mg Cd kg-1and 135 mg Cd kg-1 (dry weight) and was negatively correlated with TI. Certain tolerant pea genotypes were characterized by a high Cd concentration in shoots. All varieties were also screened for their ability to take up heavy metals (HMs) from a slightly contaminated soil. The concentration of Cd, chromium, copper, nickel, lead, strontium and zinc in plant shoots varied between pea genotypes by a factor of 2.8, 4.9, 2.7, 3.5, 9.7, 3.9 and 4.0, respectively. The coefficients of variation between pea genotypes for HM concentration were high, varying from 23%to 39% depending on the metal. The distribution patterns for varieties based on Cd tolerance (sand culture) and HM concentrations (soil culture) were characterised by positive skewness coefficients, suggesting that the majority of pea genotypes was relatively sensitive to Cd toxicity and tended to avoid excessive accumulation of HMs in shoots. These results show that a high genetic variability exists in pea with regards to Cd tolerance and HM accumulation. Concentrations of different HMs in plants grown in soil correlated positively with each other, with the exception of chromium. There was no correlation between Cd tolerance of the varieties in sand culture, shoot concentration of HMs in soil culture, biomass production, subspecies and geographical origin of the varieties. The genetic systems controlling Cd tolerance, HM accumulation and morphological traits are therefore independent to some extent, suggesting a possibility for breeding pea cultivars characterised by high tolerance to and low concentration of HMs in shoots.