A decline in tissue nutrient concentration was observed in petunia and poinsettia during the first phase of root development (Santos, 2009; Svenson and Davies, 1995). Nutrient applications during this phase have the potential to alleviate tissue nutrient decline and maintain nutrient levels within recommended ranges. Combined macronutrient and micronutrient mineral nutrient applications to petunia apical stem cuttings during the first 7 d of propagation maintained tissue nutrient concentrations at higher concentrations [4.8% nitrogen (N)] compared with cuttings that received micronutrients only (3.7% N) (Santos, 2009). Tissue nutrient decline has been attributed to dilution and foliar leaching (Blazich, 1988; Svenson and Davies, 1995; Tukey, 1967). Foliar nutrient applications before root emergence potentially serve to 1) replenish preplant nutrients leached from the substrate, which are subsequently taken up by the cut stem through the transpiration stream or newly emergent roots; or 2) supply nutrients for direct foliar uptake.
Once severed from the stock plant, hormones such as ethylene, jasmonates, and auxins increase at the stem base and subsequently play diverse roles in initiating adventitious root development (Ahkami et al., 2008; Blakesley et al., 1994; Clark et al., 1999; De Klerk et al., 1999; Schilmiller and Howe, 2005; Shibuya et al., 2004; Sorin et al., 2006). Physiologically, the plant begins to respond to the severance first by callus and then by root formation, which are two independent processes (Dole and Gibson, 2006). As soon as the cutting is removed from the stock plant, the outer cells form a protective layer of necrotic cells and suberin (hydrophobic substance) (Dole and Gibson, 2006). The living cells beneath the protective layer begin to divide and form callus (Dole and Gibson, 2006). When the swollen callus area turns white or tan, the epidermis ruptures and causes the callus to crack because of root differentiation (Dole and Gibson, 2006). In petunia, callus is typically formed in 5 to 7 d, and roots develop in 9 to 14 d (Dole and Gibson, 2006).
Water mist applications during the first phase of propagation (before root emergence) are intended to maintain plant turgidity and minimize transpiration. During a typical petunia or calibrachoa production cycle (28 d), the water volume applied can exceed the container capacity of the substrate, causing leaching of preplant nutrient charge (Santos et al., 2008). Transpiration was observed to increase by nearly 50% on visible root emergence in poinsettia (Wilkerson and Gates, 2005). Water moves in plants along gradients of water potential typically generated by transpirational water loss from leaves (Sheriff, 1984). Therefore, potential for nutrient uptake from the base of the stem (through the transpiration stream) should also increase at initial root emergence.
Environmental, structural, and morphological characteristics within a given plant species contribute to the efficacy of foliar nutrient applications. Physiological factors that affect the efficacy of foliar fertilization are the nutrient forms applied, the root temperature, root osmotic potential, leaf age, and current nutrient status in the tissue (Mengel, 2002; Weinbaum, 1996). Plant leaves are specialized organs primarily functioning in the production of organic compounds through photosynthesis. Foliar-applied compounds penetrate the leaf surface through the cuticle through cuticular cracks and imperfections or through stomata, leaf hairs, and other specialized epidermal cells (Burkhardt and Eichert, 2001; Tukey et al., 1961).
In contrast to roots, the outer walls of the epidermal cells in all aerial plant organs are covered with a hydrophobic cuticle (Marschner, 1995), which has the potential to limit nutrient absorption. The water-phobic cuticle is a limiting barrier for two-way transport of water and solutes in and out of the leaves (Marschner, 1995); however, neutral, noncharged molecules have been found to penetrate the cuticle by diffusion and dissolution of cuticular waxes (Schonherr, 2001). The mechanism of cuticle penetration of water and ions is not fully understood but may occur as a result of the existence of aqueous pores (Schonherr and Schreiber, 2004). In general, the micronutrient requirement can be better met by foliar application than macronutrient requirements because in absolute terms, higher quantities of macronutrients are needed (Mengel, 2002).
Given that our research has found an increase in nitrogen, phosphorus, and potassium when fertilizer was applied by root formation, our question was whether this uptake was occurring through some combination of basal and foliar uptake. Therefore, we wanted to determine if applications were recharging nutrients in the substrate or if true foliar uptake occurred. The objective of this research was to quantify rooting response to fertility treatments and tissue nutrient concentration changes in response to basal or apical nutrient supply during three rooting phases in propagation of Petunia ×hybrida ‘Supertunia Royal Velvet’ and ‘Supertunia Priscilla’ cuttings.
Ahkami, A.H. , Lischewski, S. , Haensch, K.T. , Porfirova, S. , Hofmann, J. , Rolletschek, H. , Melzer, M. , Franken, P. , Hause, B. , Druege, U. & Hajirezaei, M.R. 2008 Molecular physiology of adventitious root formation in Petunia hybrida cuttings: involvement of wound response and primary metabolism New Phytol. 181 613 625
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Ahkami, A.H. Lischewski, S. Haensch, K.T. Porfirova, S. Hofmann, J. Rolletschek, H. Melzer, M. Franken, P. Hause, B. Druege, U. Hajirezaei, M.R. 2008 Molecular physiology of adventitious root formation inNew Phytol. Petunia hybridacuttings: involvement of wound response and primary metabolism 181 613 625
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Sorin, C. Negroni, L. Balliau, T. Corti, H. Jacquemont, M.P. Davanture, M. Sandberg, G. Zivy, M. Bellini, C. 2006 Proteomic analysis of different mutant genotypes ofPlant Physiol. Arabidopsisled to the identification of 11 proteins correlating with adventitious root development 140 349 364
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