Morphogenetic plant responses to mechanical stimulation (MS) have been the subject of several research projects over the past decades. The altered growth behavior induced by MS was first described by Jaffe (1973) as thigmomorphogenesis. The growth-inhibiting effect caused by a direct physical impact by touching, rubbing, or brushing is mainly used to improve plant quality criteria such as stability and appearance, thereby increasing the market value of greenhouse-raised plants (Garner and Björkman, 1996; Jaffe and Forbes, 1993; Latimer, 1998; Latimer and Beverly, 1993; Latimer and Thomas, 1991). Moreover, through the application of mechanical treatments, the obtained compact and stable plants, which are tougher and sturdier because of their increased shear strength and increased modulus of rupture (Heuchert and Mitchell, 1983), are better able to withstand potential impacts during handling, transplanting, transport, and shipping (Latimer, 1998; Latimer and Beverly, 1993; Samimy, 1993). Underlying complex mechanosensing mechanisms are responsible for specific endogenous plant processes such as gibberellic acid (GA) biosynthesis inhibition or calcium (Ca2+) signaling responses (Chehab et al., 2009; Sparke and Wünsche, 2020), which frequently result in anatomical and morphological plant adaptation.
Considering the worldwide growing and competitive horticultural market, producers of high-quality plants are realizing the necessity to establish an efficient and economic production regime to 1) maximize productivity per unit area, 2) effectively use transportation capacity, 3) minimize losses attributable to damage by transportation, and 4) fulfill the market requirements. To meet these numerous challenges, horticulturists have to implement production processes that require a high level of skillful crop management. During greenhouse cultivation, for example, excessive stem elongation can be controlled by regulating the water and nutrient supplies and controlling temperature conditions (Garner and Björkman, 1996). Nevertheless, during ornamental plant production, the application of plant growth regulators (PGRs) is still the most commonly used tool for achieving an effective reduction in plant height (Latimer, 1992; Morel et al., 2012). However, increased public concern and governmental restrictions have limited the use of chemicals in plant production. Additionally, the legally defined use of PGRs often leads to some constraints in the production regime of the company because, in many cases, re-entry into the greenhouse section is not allowed for at least 24 h after PGR application, and, if necessary, re-entry is only allowed with protective clothing. Consequently, the growing demand for sustainable production systems requires the development and practical implementation of alternative crop management strategies, such as organic cultivation methods, that might have the potential to increase profitability (Raviv, 2010). In this context, the use of thigmomorphogenic effects can be a promising application technology in large-scale horticultural greenhouse production as an alternative to PGRs. Furthermore, it was previously demonstrated that various mechanical treatments reduced plant height to a similar extent as PGR applications (Adler and Wilcox, 1987; Biddington and Dearman, 1987; Latimer, 1991).
Beyl and Mitchell (1977) invented an automated system that shakes and touches plants simultaneously to control plant growth, but their device was unsuitable for large-scale crop production. Further technical approaches to control plant shoot elongation involved brushing plants manually with cardboard (Latimer, 1990) or paper sheets (Biddington and Dearman, 1985). These mechanical treatments could only be applied at irregular intensities to a limited number of plants, and the high humidity conditions inside greenhouses led to upwelling and wear-out of the brushing material. Latimer and Thomas (1991) used a moving polyvinylchloride (PVC) pipe for successfully reducing the average stem length and leaf area of tomato (Solanum lycopersicum) greenhouse plants. However, the friction of the PVC on the plant surface caused undesirable damage to the leaf tissue, particularly when plants were turgid. Alternatively, a smooth bar made of wood or steel, which was vertically adjustable to account for time-dependent plant height differences, reduced the growth of several plant species (Baden and Latimer, 1992; Latimer, 1991; Latimer et al., 1991). Koch et al. (2011) further improved the brushing application by using a fleece material and demonstrated a significant reduction in internode length for several organically cultivated herbs. However, plant quality was frequently diminished through the repeated direct physical impact, resulting in damage to the shoot tips, leaves, and petals (Garner and Björkman, 1997; Garner and Langton, 1997; Johjima et al., 1992; Koch et al., 2011; Latimer, 1991; Latimer and Beverly, 1993). The extent of the reported plant damage appears to be dependent on plant species, cultivar, cultivation conditions, and growing season (Garner and Björkman, 1997; Garner and Langton, 1997; Latimer and Beverly, 1993); these factors render the development of a suitable application system difficult. To our knowledge, an efficient and reliable mechanical application system that inhibits plant growth and concomitantly maintains plant quality is not yet available for horticultural crop cultivation under greenhouse conditions. In response to this production constraint, the objectives of this study were to evaluate three air stream application modules that apply a controlled and directed air stimulus to plants sustainably cultivated under greenhouse conditions. During this study, the height growth of ‘Merrybell’ bellflower (Campanula), a favored ornamental plant, ‘Romello’ tomato, which has frequently been investigated for reactions to mechanical stimulation, and creeping inchplant (Callisia repens), a main crop of a horticulture company used for pet feed production, were studied under experimental and commercial greenhouse conditions.
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