During the past century, horticulturists have investigated various systems to propagate plants from stem cuttings. Initially, cutting propagation involved inserting cuttings into soil within a high-humidity enclosure (Bailey, 1896). However, high humidity and poor air circulation in enclosed cases often resulted in cutting decay and disease symptoms, likely caused by gray mold [Botrytis cinerea (Preece, 2003)]. The first published research using overhead mist systems for propagation dates back to about 1940, followed by numerous studies through the 1950s that established the effectiveness of overhead mist systems (Preece, 2003), a technique that continues to be used for propagation today.
Although overhead misting of cuttings inserted into a soilless substrate is standard, research has demonstrated that leafy stem cuttings of woody plants can be propagated in environments that provide mist to the basal ends of the cuttings, with or without overhead mist. At the 14th annual meeting of the International Plant Propagator’s Society, Eastern Region, Nitsch (1964) speculated that supplying cuttings with moisture only to the basal end of the stem, instead of to the leaves, might improve propagation results, particularly for slow-to-root species. In response, Briggs (1964) constructed a mist chamber into which he inserted stem cuttings of several species from above, both with and without overhead mist or heated water. Although no statistical data were provided, Briggs (1964) found that cuttings rooted in his system transplanted well, after producing roots that were “tougher and more hardened” than those produced during conventional propagation. Coston et al. (1983) obtained 97% rooting of semihardwood peach (Prunus persica) cuttings collected in early August and provided with overhead mist while inserted into a submist chamber, with roots on many cuttings evident within the first week. Among cuttings collected in September, Coston et al. (1983) reported rooting of 65% and 27% when cuttings received overhead mist or no overhead mist, respectively. Recently, we reported that stem cuttings of manchurian lilac and inkberry (Ilex glabra) inserted into submist systems with no overhead mist produced more root growth than cuttings in a soilless substrate with overhead mist (Peterson et al., 2018).
Although Pn has been measured in several species during propagation from cuttings, the results are mixed. Svenson et al. (1995) reported that Pn of poinsettia (Euphorbia pulcherrima) cuttings rooted in a growth chamber was low initially, but increased with the emergence of root primordia. Smalley et al. (1991) showed that Pn of red maple (Acer rubrum) cuttings collected in May was low until root emergence after 42 d. In contrast, Pn remained high among cuttings collected in September, all of which rooted within 2 weeks. Although Pn often is low in unrooted cuttings, the accumulation of photosynthates before roots have formed can be substantial and may impact rooting. For example, Klopotek et al. (2012) reported that dry mass of petunia (Petunia ×hybrida ‘Mitchell’) cuttings increased by ≈60% before the emergence of roots, indicative of prerooting carbon assimilation. Moreover, Pn of cuttings without visible primordia doubled from ≈3 to 6 μmol·m−2·s−1 when CO2 concentrations were increased from 300 to 1200 ppm, and increased from <1 to 7.8 μmol·m−2·s−1 when irradiance was 150 instead of 80 μmol·m−2·s−1 (Klopotek et al., 2012). Tombesi et al. (2015) demonstrated that both the accumulation of carbohydrates and rooting percentages in terminal stem cuttings of hazelnut (Corylus avellana) increased when irradiance was increased from <100 to 200–300 μmol·m−2·s−1. Although these studies provide insight into the physiology of cuttings during propagation, they only investigated cuttings under mist or in humid enclosures, not in aeroponic propagation systems within a greenhouse environment.
In our previous trials, we observed that leaves may wilt soon after cuttings are inserted into submist systems, indicating that water stress may be greater among cuttings in submist systems than among those in overhead mist systems. However, this wilting is not necessarily followed by leaf abscission, and the progression of wilting may halt or reverse with no obvious detrimental effects on rooting. Nonetheless, a propagation system that combines the features of overhead mist and submist could be compared with each alone to identify the separate influences, and potential synergies, of overhead mist and submist on adventitious rooting. Likewise, the rates of Pn among cuttings in each system, compared with rooting outcomes, may suggest potential hypotheses to account for measured rooting differences among systems.
According to Dirr and Heuser (2006), manchurian lilac can be propagated from softwood stem cuttings treated with 8000 mg·L−1 indole-3-butyric acid (IBA). Hartmann et al. (2011) stress the importance of collecting lilac cuttings before bud set, while cuttings are still somewhat tender, because cuttings collected after bud set are difficult to root. Fordham (1959) describes the need to collect cuttings at a precise stage at which stems are just sufficiently lignified to snap when bent, but seasonal primary growth has not ceased. Interestingly, the advent of intermittent mist now permits more tender lilac cuttings to be collected and rooted, but more lignified cuttings still tend to root poorly even under mist (Fordham, 1959). Methods of propagation that could extend the collection and propagation window for lilac and other taxa that are sensitive to seasonal cutting stage might make better use of nursery growing facilities and labor hours by permitting cutting collection times to be staggered across longer propagation seasons.
This study had three objectives. Our first objective was to validate the results of Peterson et al. (2018) during a second propagation season, using difficult-to-root cuttings of manchurian lilac collected from shoots with terminal buds already set. Our second objective was to determine whether a combination system, which applied overhead mist to cuttings placed in a submist system, would improve rooting further compared with overhead mist. Our third objective was to evaluate environmental conditions in each of the three systems and investigate the possible role of cutting photosynthesis in differential rooting responses.
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