Growth and Physiological Responses of Populus nigra L. Male and Female Seedlings under Cadmium Stress

Accumulation of cadmium (Cd) as one of the most toxic pollutants in the soil can lead to the reduction of microbial activities and soil fertility and as a result the death of plants; This issue has always caused global concerns. The present study was conducted with the aim of investigating the resistance to Cd of male (62-167) and female (62-149) individuals of black poplar (Populus nigra L.) based on vegetative and physiological characteristics.

For this purpose, a greenhouse experiment was conducted in the form of a randomized complete block design with factors of cadmium (concentrations of 0, 50, 150 and 200 mg/kg soil) and gender (male and female bases of poplar) for a 120-day period. Physiological variables of seedlings including net photosynthesis rate, transpiration rate, stomatal conductance, water use efficiency, water potential, mesophilic conductance, intracellular CO2 concentration, and vegetative characteristics including root collar diameter growth and height growth, total biomass and leaf area were investigated. The stress treatment was done in late May through irrigation with cadmium salt solution on a weekly basis for the first five weeks. After that, the seedlings were irrigated with rainwater, respecting the soil capacity, at intervals of three days until the end of the period (late September).

Regardless of the root species, with the increase of Cd concentration, the leaf area and height growth of seedlings decreased. The highest height growth (30.4 cm) and leaf area (51.8 cm2) were observed in the control, and the highest total biomass of seedling (59.2 g) was observed at the concentration of 50 mg/kg of Cd. Regardless of the concentration of Cd, the size of collar diameter growth (male 2.1, female 2 mm), seedling height growth (male 21.5, female 19.9 cm), leaf area (male 43.08, female 43.64 cm2), total seedling mass (male 46.48, female 44.19 g), water use efficiency (male 0.31, female 0.26 μmol mmol-1), mesophilic conductance (male 0.003, female 0.002 mol m-2 s-1) and stomatal conductance (male 0.45, female 0.39 mol m-2 s-1) was higher in males than females and only, the amount of transpiration (male 3.31, female 4.01 mol m-2 s-1) was higher in females than in males. Although, with the increase of Cd concentration, the size of most of the physiological and growth characteristics including photosynthesis, water potential, water use efficiency, diameter, height, leaf surface, and biomass decreased in general, the seedlings of both clones maintained their physiological and growth activities to some extent in high Cd concentrations.

It can be concluded that both clones can tolerate high concentrations of Cd; nevertheless, for afforestation development with Populus nigra in Cd-contaminated soils, the use of male individual (clone 62-167) can be more successful than female one (clone 62-149).