Ornamental bedding plants represent the largest sector of the floriculture industry in the Unites States and have a wholesale value of $1.96 billion accounting for 45% of all floriculture crops (U.S. Department of Agriculture, 2014). In the last decade, there has been a shift in the retailing of ornamental crops. Customers tend to purchase bedding plants more in mass market retailers or superstores and general retail outlets (such as supermarkets) than in traditional garden centers and florists because of convenience and lower prices (Yue and Behe, 2008). In addition, the major growers have moved their production into areas characterized by lower labor cost and more favorable climate conditions to reduce the cultivation cost (Ferrante et al., 2015). As a result, the location of crop production could be further away from markets, forcing plants to spend an extended period of time without proper irrigation during shipping and/or retailing (Waterland et al., 2010a; Weaver and van Iersel, 2014). Additionally, during postproduction periods, plants are often exposed to adverse environmental conditions, including high temperatures and inadequate irrigation, which accelerate substrate drying and plant wilting. Crop losses caused by these poor postproduction conditions are estimated to result in 5% to 20% of unsalable crops (Healy, 2009), and water stress is one of the major causes of diminished aesthetic quality and salability of plants. Therefore, it is highly desired to minimize crop damage caused by water deficit to maintain high quality and prolong longevity of bedding plants during postproduction.
Water stress causes plants to synthesize a phytohormone called abscisic acid (ABA) in the root system, and it is translocated to leaves through the transpiration stream (Taiz and Zeiger, 2010). When ABA reaches guard cells, it binds to ABA receptors that activate an ion efflux, which reduces turgor pressure in the guard cells. Due to loss of turgidity, the guard cells become flaccid and stomata are closed. Closing of stomata inhibits transpiration and allows the plant to withstand water stress by decreasing water loss. Using this principle, growers can utilize antitranspirants to reduce transpiration, thereby limiting water loss during shipping and retailing (Iriti et al., 2009; Odlum and Colombo, 2007; Waterland et al., 2010b).
Antitranspirants are chemical compounds that increase water stress tolerance by preventing transpirational water loss in plants. Based on their mode of action, antitranspirants can be classified into two major groups, physical and physiological antitranspirants (Anderson and Kreith, 1978; Shinohara and Leskovar, 2014; Waterland et al., 2010b). Physical antitranspirants contain waxes, resins, latexes, or polymers that coat the leaf surface and minimize water loss from the plant by blocking stomata (Goreta et al., 2007). Such physical antitranspirants have shown positive effects on water stress tolerance in pepper [Capsicum annuum (del Amor et al., 2010)], peach tree [Prunus persica (Steinberg et al., 1990)], and herbaceous plants (Anderson and Kreith, 1978). Physiological antitranspirants minimize transpiration by inducing plants to close stomata. These compounds may contain ABA or other chemicals that increase the ABA concentration in plants (Waterland et al., 2010b). Exogenous application of ABA has enhanced water stress tolerance in various horticultural crops (Agehara and Leskovar, 2012; Astacio and van Iersel, 2011; Goreta et al., 2007; Shinohara and Leskovar, 2014). Goreta et al. (2007) found that foliar application of ABA enhanced water deficit tolerance of pepper, which was attributed to decreased gS and increased leaf water potential. Overall, antitranspirants have been shown to reduce wilting caused by water stress. However, some studies have demonstrated that plant responses to antitranspirants vary depending on species, concentrations of antitranspirants applied, developmental stages, and growing environmental conditions (Blanchard et al., 2007; Dunn et al., 2012; Shinohara and Leskovar, 2014; Waterland et al., 2010a).
Antitranspirants have been used to help plants withstand stress caused by water deficit, and many studies have focused on fruits, vegetables, turf, field crops, and woody plants. Little research has been conducted on the effect of antitranspirants on the postproduction quality of bedding plants. Furthermore, most research has evaluated individual antitranspirant and an efficacy comparison study among different products of physical and physiological antitranspirants is lacking. The physical antitranspirants in this study contained either β-P or VP as a coating agent and the physiological antitranspirants were two sugar alcohol-based compounds (SAC1 and SAC2), which are supposed to increase the concentration of ABA in plants, and s-ABA. SAC1 contains xylitol, and SAC2 contains polyhydric alcohol and extracts from seaweed [e.g., red algae (Gracilaria sp.)], corn (Zea mays), and berries [e.g., brambles (Rubus sp.), blueberry (Vaccinium sp.), strawberry (Fragaria ×ananassa)]. The goal of this research was to evaluate the efficacy of these commercially available antitranspirants on enhancing water stress tolerance in bedding plants.
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