Overhead mist revolutionized the propagation industry by providing reliable means to manage transpirational water losses by leafy stem cuttings. This system slows transpiration of cuttings primarily by decreasing leaf temperatures through evaporative cooling from the leaf surface (Hartmann et al., 2011). However, OM has potential drawbacks, including the use of large volumes of water, potentially unsanitary conditions created by persistent water films on leaves (Preece, 2003), the potential for anaerobic conditions in the rooting zone, the depression of root-zone temperatures by evaporative cooling (Hartmann et al., 2011), nonuniform misting coverage, leaching of foliar nutrients (Preece, 2003), difficulty controlling cutting nutrition during propagation (Hartmann et al., 2011; Zhang and Graves, 1995), and the need to extensively acclimate cuttings to a low-humidity, mist-free environment.
Several authors have considered alternatives to OM for propagation of plants by stem cuttings, which we refer to here as SI, SM, and SF (Zhang and Graves, 1995). Graves and Zhang (1996) evaluated the suitability of SI for the propagation of several woody and herbaceous plant species, and found that such a system can be an effective alternative to OM. Moreover, Zhang and Graves (1995) found rooting could be improved in SI when a fertilizer solution was used instead of water. Another alternative system that merits investigation relies on the application of mist from below the cutting, to the base of the stem inserted into an enclosed chamber. Several such aeroponic systems are available on the market for use by home gardeners, but do not seem to be marketed for commercial propagation. Several authors have evaluated SM aeroponic systems for propagation of plants by stem cuttings, with promising results (Mehandru et al., 2014). Another aeroponic product on the market (Cyclone Ultrasonic Fogger; FutureGarden, Lindenhurst, NY) can be used to supply a fog of nebulized water or fertilizer solution to the bases of cuttings inserted into a rooting chamber. To our knowledge, the potential efficacy of this system for propagation has not been formally evaluated in the academic literature.
We speculated that each of these systems might offer several of the following advantages to OM for the propagation of plants by leafy stem cuttings. These include limited water usage, increased sanitation and reduced foliar disease pressures, superior oxygenation to the rooting zone, maintenance of high root-zone temperatures in the absence of evaporative cooling, uniform application of water to each cutting, no leaching of nutrients, efficient delivery of water-soluble fertilizer during propagation, and a reduction in acclimation requirements typical of cuttings accustomed to receiving foliar mist. Moreover, in two of the systems (SM and SF), root development could be assessed in situ without disturbing fragile, developing roots. Our aim here was to conduct a proof-of-concept evaluation to explore the general merits of each system.
The primary objective of this study was to compare rooting and posttransplant performance of coleus propagated in four propagation systems: traditional OM, SM aeroponic, SF aeroponic, and a modified SI system. The second objective was to determine whether the addition of water-soluble fertilizer was beneficial in any of these systems.
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