Dianthus caryophyllus L., commonly known as carnation, is one of the most popular cut flowers; it ranks third after chrysanthemum and rose in the worldwide market (Liu et al., 2018). Vegetative propagation by cutting is one of the main methods used for D. caryophyllus seedling production, which is economically important for high-scale breeding of elite commercial genotypes from stem cuttings of the mother plants (Garrido et al., 1998).
Adventitious rooting is an essential step in the vegetative propagation, it is a complex physiological, biochemical, and metabolic process affected by multiple factors (da Costa et al., 2013). Endogenous factors such as genetic characteristics, hormone synthesis, and transport unavoidably influence the rooting capacity (Pacurar et al., 2014). Exogenous environmental conditions including temperature, water and oxygen supply, and auxin application are also critical for promoting the quality of rooting and improving the survival rate of cuttings (Geiss et al., 2009).
Several previous studies have been performed to investigate the factors that influence the root formation in cutting propagation, including the effects of rooting auxin, storage temperature, light, and oxygen requirements (Garrido et al., 1998; Kreen et al., 2002; Van de Pol and Vogelezang, 1983). Auxin has a crucial role in the induction of the meristems of root primordia, redistribution of nutrients, and biosynthesis of endogenous hormones in the cuttings, thereby stimulating adventitious root regeneration (Garrido et al., 2002). Indole-3-butyric acid (IBA) and 1-naphthalene acetic acid (NAA) feature different mechanisms in the induction of adventitious roots. For example, IBA was more effective than NAA for the rooting of stem cuttings (Husen and Pal, 2007). IBA has a strong, long-term promotional effect on the initiation, emergence, and development of root primordia. NAA promotes the hydrolysis of starch into sugar, thereby increasing the sugar availability for the formation of roots; it also promotes rooting and induces short, thick roots (Altman and Wareing, 1975). In practice, IBA is commercially accepted as an exogenous rooting hormone for many plant species because of its high root-inducing capacity and light stability (Pacurar et al., 2014). The optimal auxin dose for rooting varies between species and genotypes (Exadaktylou et al., 2009; Luwig-Müller et al., 2005; Rosier et al., 2004). IBA can be used alone or in combination with other auxins such as NAA, depending on the specific condition, to achieve better rooting performance (Cano et al., 2014).
Plant root formation is affected by the physical and chemical properties (bulk density, porosity, water-/air-holding capacity, pH, EC) of the rooting substrates (Altman and Freudenberg, 1983). The ideal substrate is able to provide a proper water and oxygen balance, thus promoting oxygen availability, transpiration, nutrient uptake, growth, and aeration during root initiation of stem cuttings (Mabizela et al., 2017). The water content is a key factor in the survival of cuttings, especially during the initial rooting phase when the transpiration and physiological activities rely on the stem insertion in the substrate (Grange and Loach, 1983). For example, the survival rates of Juniperus horizontalis, Rhododendron, and Ilex crenata stem cuttings were the highest at the highest studied medium moisture level (625%) (Rein et al., 1991). A sufficient oxygen supply is indispensable for root initiation, growth, and development; furthermore, maintaining suitable pH and electrical conductivity (EC) is important during the rooting period (Holt et al., 1998). Generally, a mixture of two or more substrate types is used to obtain the ideal rooting substrate with the expected characteristics, such as organic components (peat), which provide the water-holding capacity, and inorganic components (perlite), which increase aeration, in this study.
The objectives of the current study were to improve carnation breeding quality and provide a theoretical basis for factory breeding. We investigated the effects of the auxin concentration and the ratio of the substrate on carnation rootings by evaluating the rooting percentage and survival rate of the experiments.
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