Abstract:

To illustrate the mitigating effect of the melatonin (MT) on the heavy metal cadmium (Cd), Helianthus annuus was used as the test material to study the effect of exogenous MT on seed germination, seedling growth and leaf tissue structure under Cd stress by means of seed imbibition, foliar spraying and conventional paraffin sectioning. The results showed that soaking seeds with 200 µmol/L MT significantly increased the germination rate and germination potential. Under Cd stress, a 400 µmol/L MT treatment significantly improved the growth indicators such as plant height, root length, leaf length, leaf width, and the physiological indicators such as SOD, POD, CAT and the content of soluble protein, but the content of MDA was significantly decreased. By analyzing the microscopic structure of leaves, the leaf thickness, upper and lower epidermis cell thickness, palisade tissue thickness, cell tense ratio and ratio of palisade tissue to spongy tissue were significantly increased by exogenous application of 200 µmol/L MT. However, the spongy tissue thickness and the spongy ratio were significantly decreased. The above results indicated that the appropriate concentration of exogenous MT could effectively alleviate the seed germination and seedling growth of Helianthus annuus under Cd stress, and enhanced the Cd tolerance.

Key words: melatonin, cadmium stress, Helianthus annuus, seed germination, seedling growth, physiological characteristics, microstructure