Sex-related differences in physiological and ultrastructural responses of Populus cathayana to Ni toxicity

We used Populus cathayana, a native species with an extensive distribution in northern, central, and southwestern China, as a model species to detect the sex-specific differences in photosynthetic capacity, ultrastructure, nitrogen (N) metabolism, and nickel (Ni) accumulation and distribution in response to Ni stress. Exposure to 100 mu M Ni2+ in a hydroponic system for 1 month significantly decreased the pigment content and the photosynthetic rate, caused visible impairment in cellular organelle structure, and induced obvious disturbance and imbalance in the N content of female plants, while male plants suffered a lower negative influence on all the above measured parameters. However, males accumulated a higher Ni concentration in both leaves and roots than females, while the transportation ratio of Ni from roots to shoots in males was slightly lower than that in females. Our results, therefore, suggest that males have a better tolerance capacity and a greater ability to remediate Ni-polluted soil than females. This greater tolerance capacity in males might be highly correlated with the better maintenance of N balance and more effective physiological detoxification responses (such as the response to proline) under Ni stress. The differences between the sexes in tolerance capacity to heavy metals should be verified after performing a field investigation using adult trees as materials in the future study.