Polyamines provide new insights into the biochemical basis of Cr-tolerance in Kinnow mandarin grafted on diploid and double-diploid rootstocks

The present investigation was executed to determine the effect of rootstock ploidy level on polyamine metabolism and phenolic acid contents in leaves. Kinnow mandarin (Citrus nobilis Lour × Citrus deliciosa Ten) plants were grafted on the diploid (2x) and double-diploid (4x) of Poncirus trifoliata (L.) Raf, Citrus reshni, and Citrus limonia Osbeck. Plants were grown in greenhouse under controlled conditions and fertigated with nutrient solution containing chromium (0.75 mM). After four-months of growth under chromium (Cr) stress, polyamines (free, soluble-conjugated and insoluble-bound) and phenolic acid contents (PAC): p-hydroxybenzoic acid (PHBA), ferulic acid (FA), p-coumaric acid (PCA), gallic acid (GA) and vanillic acid (VA) in leaves were investigated. Enzymes activity of polyamine biosynthesis, i.e., arginine decarboxylase (ADC), ornithine decarboxylase (ODC), S-adenosylmethionine decarboxylase (SAMDC) and spermidine synthase (SpdS), and polyamine degradation, i.e., polyamine oxidase (PAO) and diamine oxidase (DAO), were also determined. Chromium stress caused reductions in polyamine contents (PAC) and enzymes of polyamines biosynthesis. The responses of Kinnow mandarin (KM) plants grafted on diploid or double-diploid rootstocks showed a similar trend, although Cr-stress induced more drastic effects in diploid grafted plants. In the case of polyamine degradative enzymes, Cr-stress enhanced the PAO and DAO activities, but diploid grafted plants showed greater increases than those grafted on double-diploid rootstocks. Cr-stress inhibited the activities of enzymes of polyamine biosynthesis, but inhibition was expressed more in diploid-grafted plants than in double-diploid grafted plants. So, polyamine biosynthesis and degradation were less affected by Cr-toxicity in double-diploid grafted plants than in diploid grafted plants. Similarly, KM plants with double-diploid rootstocks had greater accumulation of phenolic acid contents in their leaves. Combining the findings of our previous reports and the data of the current study, it is suggested that higher polyamine and phenolic acid content accumulation in double-diploid grafted plants strengthens the antioxidative defense system of the Kinnow mandarin plants by regulating the ROS homeostasis and stabilization of membranes under Cr-toxicity. Therefore, higher levels of polyamines and PACs augmented the Cr-tolerance potential of KM plants grafted on double-diploid rootstocks.