Dihydroporphyrin iron (III) (DHFe) chelates have a role in plant growth regulation. In the present study, the physiological mechanisms contributing to the ameliorative effects of foliar DHFe spraying on seedling growth and photosynthetic parameters as well as seed traits of rapeseed plants under control or salt-stressed conditions in pots were investigated. Plant characteristics, including chlorophyll content; photosynthetic parameters; reactive oxygen species content; antioxidant enzyme activity; relative electrolytic leakage; and Na+, K+, and Cl- content were compared in above-ground parts and roots. Seed traits at maturity (e.g., numbers of pods and seeds) and seed dry weight per plant were also evaluated. The results showed that under NaCl stress, foliar application of DHFe could significantly reduce Cl- content in above-ground plant parts and maintain higher K+ content in seedlings; enhance the activities of ascorbate peroxide, catalase, and peroxidase; and decrease the rate of superoxide anion and hydrogen peroxide generation. Foliar DHFe application could restore leaf chlorophyll content and improve Photosystem II maximum photochemical efficiency, stomatal conductance, transpiration rate, and intercellular CO2 concentration. This effect alleviated the salt injury to treated rapeseed plants that then exhibited enhanced vegetative growth, improved leaf photosynthesis, and increased seed yield. These results may provide a valuable theoretical basis and technical guidance for future practical application of DHFe as a kind of plant growth regulator for the chemical regulation of salt tolerance of rapeseed and other crops under saline cultivation conditions.
