Baldcypress, Taxodium distichum (L.) Rich., is a highly adaptable tree of significant ecological importance in the southeastern United States (Arnold, 2008; Pezeshki and DeLaune, 1994). Baldcypress is typically propagated commercially from seed, grafting, or produced through cuttings (Dirr, 2009; Thomsen, 1978). Baldcypress seeds exhibit a dormancy that is easily overcome with stratification (Dirr, 2009); however, seedling material lacks uniformity (Pezeshki and DeLaune, 1994). Grafting baldcypress is a reliable method of propagation (Dirr, 2009) but is the most expensive of the three methods (Thomsen, 1978). Vegetative propagation by cuttings yields uniform plants and through selection can be used to expedite narrow sense heritable genetic improvement in this species (Pezeshki and DeLaune, 1994). Relatively high percentages of successful rooting have been reported for Taxodium supporting the practice of commercially propagating baldcypress by cuttings. Rooting percentages and rooted cutting quality do, however, vary among genotypes (Copes and Randall, 1993; King, 2010; Pezeshki and DeLaune, 1994; St. Hilaire, 2003; Zhou, 2005).
There are many methods for manipulating cuttings to encourage optimal adventitious root formation. Most of these methods fall under three main categories: management of the ortet (or stock plant), treatment of cuttings during propagation, or management of the environment surrounding the cuttings during propagation (Hartmann et al., 2011). The majority of the research conducted on asexual propagation of baldcypress by cuttings has focused on the treatment of cuttings during propagation. Testing different types and concentrations of synthetic auxin (King, 2010; Pezeshki and DeLaune, 1994; St. Hilaire, 2003; Zhou, 2005) or wounding the basal portion of the cutting (Zhou, 2005) are the most common of these treatments. Management of the ortet has also been tested by making the timing of cutting harvest the independent variable in an experiment (King, 2010; Zhou, 2005). It has not been common, however, to manipulate the environment surrounding the cuttings during propagation. In most instances, the environment has been constant across all treatment combinations (Copes and Randall, 1993; King, 2010; Pezeshki and DeLaune, 1994; St. Hilaire, 2003; Zhou, 2005). Lu et al. (2004) conducted the only study to our knowledge investigating the effects of manipulating the surroundings of baldcypress cuttings by planting them in three different rooting substrates.
Rooting substrates are typically comprised of an organic component (i.e., peatmoss) and an inorganic component (i.e., perlite), which increases aeration. A rooting substrate serves a number of purposes, including anchoring the cutting in place, holding water for the cutting, supplying sufficient aeration for adventitious rooting, and reducing the amount of irradiance that reaches the base of the cutting (Hartmann et al., 2011). The uptake of water in cuttings is proportional to the content of water, by volume, in the rooting substrate (Grange and Loach, 1983; Rein et al., 1991). Water in excess, however, prevents proper aeration (Erstad and Gislerod, 1994). Copes and Randall (1993) found that baldcypress rooted at greater percentages when positioned in the wettest portion of the mist bench tested (50% water content, by weight) as opposed to two more aerated portions (40% and 29% water content by weight, respectively). Cuttings rooted at 58%, 33%, and 6% when stuck in the mist bench with 50%, 40%, and 29% water content, respectively. Little else is known about the specific substrate requirements for rooting baldcypress cuttings. The lack of available research on vegetative propagation of baldcypress by cuttings necessitates a look at the literature dealing with vegetative propagation of other coniferous species by cuttings. Kolasinski (2006) rooted softwood dawn redwood, Metasequoia glyptostroboides (Hu and W.C. Cheng) cuttings, a closely related species to baldcypress, in a 1:1 (v:v) mix of peat and perlite, respectively. Rooting for the 3-year study averaged 93%. Mazăre et al. (2007) conducted rooting experiments on Picea glauca (Moench.) Voss. ‘Conica’. Treatments included six different substrates (100% sand, 100% peatmoss, 100% perlite, 1:1 sand with peat, 1:1 sand with perlite, 1:1 peat with perlite). Rooting percentages were greatest in the sand with peat substrate, whereas root quality (determined visually by the number of primary and secondary roots) was greatest in the perlite with peat. Although research on other closely related species is helpful, it is difficult to determine the environmental factors that will lead to optimum rooting percentage and root quality for Taxodium cuttings based on that research (Ragonezi et al., 2010).
All cuttings are by definition wounded. Increasing the area of mechanical wounding however has been shown to enhance wound responses in some species, including increasing the division of the cambial cells, which may lead to adventitious root formation (Mackenzie et al., 1986). Zhou (2005) included a wounding treatment in a number of experiments conducted on a baldcypress clone [T302, T. distichum var. distichum × T. distichum var. mexicanum (Carrière) Gordon]. Cuttings were incision-wounded (1-cm basal cut along the vertical axis) or not. One experiment included a wounding treatment that did not significantly affect (P ≤ 0.05) rooting percentage or root density ranking (RDR) (a qualitative method of ranking root system quality) (Zhou, 2005). Another experiment however included a wounding treatment that did significantly affect (P ≤ 0.05) both rooting percentage and RDR. Wounded treatments in Zhou (2005) produced cuttings with a 1.6 and 1.8 times greater rooting percentage and RDR, respectively, than did non-wounded treatments. Kolasinski (2006) included similar incision wounding treatments in a propagation study of dawn redwood. No significant difference (P ≤ 0.05) was found among the wounded and non-wounded treatments for rooting percentage, mean root number per cutting, or mean total root length. Wounding increased mean root length per cutting. Wounded cuttings averaged 9.0 cm, whereas non-wounded cuttings averaged 8.4 cm. De Silva et al. (2005) included three wounding treatments (none, single wound, and double wound) on pre-callused cuttings of leyland cypress, × Cupressocyparis leylandii (A.B. Jacks. and Dallim.) Dallim. The single wound treatment was applied by detaching callus tissue on one side of the cutting, whereas the double wound treatment was applied by detaching the callus on opposite sides. Rooting percentage and the percentage of cuttings with acceptable root symmetry (a visual rating) were significantly (P ≤ 0.05) affected by an interaction among wounding and indole-3-butyric acid (IBA).
Treating cuttings with differing auxin concentrations or a.i. has been one of the most common treatments in research on baldcypress propagation by cuttings (King, 2010; Pezeshki and DeLaune, 1994; St. Hilaire, 2003; Zhou, 2005). Pezeshki and DeLaune (1994) found that when baldcypress cuttings were taken from 1-year-old trees and treated or untreated with a 1000 mg·kg−1 powder concentration of IBA (ROOTONE®; Ferti-lome Co., Bonham, TX), they rooted at similar percentages (88% and 75% for the 0 IBA and 1000 mg·L−1 IBA treatments, respectively). These cuttings did however show significantly greater (P ≤ 0.05) shoot dry weights when treated with IBA. Mean shoot dry weights were 10.90 g and 2.86 g for treated and untreated cuttings, respectively. St. Hilaire (2003) observed an increase in rooting percentage of T. distichum var. mexicanum softwood cuttings after treatment with increasing concentrations of IBA. In 1 year of the study, rooting percentages were 48% and 82% for 3000 and 8000 mg·L−1 IBA, respectively. Zhou (2005) found that rooting percentages increased with increasing levels of K-IBA. Cuttings treated with 5,000 and 10,000 mg·L−1 K-IBA rooted at 57.6% and 68.1%, respectively. Cuttings treated with 0 and 2500 mg·L−1 K-IBA rooted at the significantly lower (P ≤ 0.05) rates of 16.6% and 22.2%, respectively. Kolasinski (2006) studied the effect of the growth regulator Seradix B No. 1 (2000 mg·kg−1 IBA) on softwood dawn redwood cuttings. Rooting percentage, mean root number per cutting, and mean total root length were all significantly (P ≤ 0.05) greater when treated with the growth regulator. Rooting percentage increased from 91% in the control to 95% with the application of IBA. De Silva et al. (2005) also included a growth regulator treatment of 0, 5,000 and 10,000 mg·L−1 IBA when rooting pre-callused leyland cypress cuttings. A significant (P ≤ 0.05) interaction was found among IBA concentrations and wounding treatments. The greatest rooting percentage of 63.9% was found in the 10,000 mg·L−1 IBA, double-wounded treatment. The percentage of cuttings with acceptable root symmetry was also highest for this treatment.
The objective of the current research was to determine combinations of rooting substrate, wounding treatment, and K-IBA concentration that yielded optimal percentages of rooted cuttings and root systems of optimal quality with baldcypress cuttings.
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