Mechanisms Underlying the Regulation of Root Formation in Malus hupehensis Stem Cuttings by Using Exogenous Hormones

Abstract

Malus hupehensis is one of the most important Malus ornamental and rootstock species in the south China Yellow River Basin. In the present study, we treated the stem cuttings of M. hupehensis with one of three exogenous hormones, indole acetic acid (IAA), naphthalene acetic acid (NAA), or a compound plant growth regulator (GGR) to investigate the mechanisms underlying root formation in stem cuttings and to optimize stem cutting propagation techniques. The results showed that immersing the stem cuttings in 100 mg/L of IAA for 2 h before planting was most effective, which reduced the time to root formation by 21 days and increased rooting percentage by 129.4 %, compared to that in the control group. In addition, the levels of endogenous substances (endogenous hormones, soluble proteins, and carbohydrates) dynamically changed, with the time to peak value or time to valley value of each parameter synchronized well with the initiation of adventitious roots. The synchronized change suggested that root formation was coordinated with physiological metabolism. However, exogenous hormone treatment significantly accelerated the catabolism of the root inhibiting hormone, abscisic acid. On the other hand, exogenous hormone treatment significantly enhanced the accumulation of root promoting hormones [IAA, gibberellic acid (GA3), and zeatin riboside (ZR)] and soluble proteins. Moreover, exogenous hormone treatments accelerated the consumption of starch and soluble sugars. Overall, the results indicated that exogenous hormone treatment (IAA) accelerated the synthesis of endogenous hormones (IAA, GA3, and ZR), therefore, sped up the metabolism of carbohydrates and soluble proteins, and consequently quickened the root formation process.