Exogenous spermidine alleviates fruit granulation in a Citrus cultivar (Huangguogan) through the antioxidant pathway

The role of exogenous spermidine (Spd) in alleviating fruit granulation in the grafted seedlings of a Citrus cultivar (Huangguogan) was investigated. Granulation resulted in increased electrical conductivity, cell membrane permeability, and total pectin, soluble pectin, cellulose, and lignin contents. However, it decreased the activities of superoxide dismutase, peroxidase, and catalase, as well as the (Spd + Spm): Put ratio. The application of exogenous Spd onto Huangguogan seedlings significantly increased proline and ascorbate contents, but decreased the H2O2 and O-2(-.) levels, which suggested that exogenous Spd provided some protection from oxidative damage. In addition, exogenous Spd decreased cell membrane permeability and MDA content, and increased the (Spd +Spm): Put ratio. The activities of antioxidant enzymes, such as catalase, peroxidase, and superoxide dismutase, were increased in Spd-treated seedlings affected by fruit granulation, resulting in a decrease in oxidative stress levels. The protective effects of Spd were reflected by a decrease in superoxide levels through osmoregulation, increased proline and ascorbate contents, and increased antioxidant activities. Our observations reveal the importance of exogenous Spd in alleviating citrus fruit granulation.