Endogenous antioxidants and phytohormonal regulation induced by spermidine improve cucumber plant growth

Involvement of exogenously applied low dose of spermidine (Spd) on the regulation of endogenous chlorophylls, reactive oxygen species (ROS), antioxidants, gibberellins (GAs), jasmonic acid (JA) and salicylic acid (SA) was investigated in cucumber plants. The plants were exposed to low concentrations of 0.06 and 0.3 μM Spd. The length of shoots, leaf area, fresh and dry weight of leaves was greater in Spd treatments than their controls. A remarkable increase of chlorophyll and protein content was noticed in plants treated with 0.3 μM Spd. However, superoxide content and lipid peroxidation were moderately declined and the activities of peroxidase, polyphenol oxidase and acid phosphatase were elevated in plants treated with 0.3 μM Spd. The plants treated with 0.06 μM Spd also showed the greater level of antioxidants over the untreated controls. A higher accumulation of non-13-hydroxylated and 13-hydroxylated GAs such as GA₂₄, GA₉, GA₄, GA₂₀, GA₈ and GA₇ was observed in Spd treated plants than their controls. Consequently, the concentration of JA and SA declined significantly in 0.3 μM Spd treated plants. These results suggest that exogenous application of 0.3 μM Spd increases plant growth through the enhancement of photosynthetic pigments, protein, enzyme activities and GAs, and reduction of ROS, JA and SA content and it could be useful to enhancement of crop plants cultivation.