Floriculture crops may encounter harsh environmental conditions during shipping and while on display in retail stores. These poor postproduction environments can result in losses of 5% to 20% (Armitage, 1993; Healy, 2009). A major cause of postproduction decline is drought stress resulting from infrequent or inconsistent watering (Barrett and Campbell, 2006). Plants respond to water deficit by closing their stomata and decreasing transpirational water loss. This drought stress response is regulated by the plant hormone abscisic acid (ABA) (Malladi and Burns, 2007).
The exogenous application of ABA or ABA analogs delays drought stress-induced wilting in a variety of bedding and potted plants (Blanchard et al., 2007; Kim and van Iersel, 2008; Monteiro et al., 2001; Sharma et al., 2005, 2006; Waterland et al., 2010). Although ABA applications allow plants to survive temporary periods of drought stress, negative side effects, including leaf necrosis, chlorosis, or abscission, have been observed in some species (Blanchard et al., 2007; Kim and van Iersel, 2008; Waterland et al., 2010). We observed leaf chlorosis on drought-stressed Tagetes patula (marigolds), Pelargonium ×hortorum (seed geraniums), and pansies treated with drench or spray applications of s-ABA. Leaf chlorosis in pansies was not directly induced by drought treatment, because symptoms were observed in both watered and drought-stressed plants that were treated with s-ABA (Waterland et al., 2010). It is unclear why s-ABA induced leaf chlorosis and senescence in pansies or how this side effect might be prevented.
ABA is the main plant hormone associated with drought stress, but most stress responses are the result of cross-talk between hormone signaling pathways (Huang et al., 2008). Although the application of a plant hormone may result in a specific response, these applications can also modify the synthesis and perception of other hormones. The resulting hormone modifications or the interactions between multiple hormones may cause the observed response. To understand a plant's response to an exogenous hormone application, it is therefore necessary to understand these potential hormone interactions. In most cases, drought stress results in an increase in the synthesis of ABA and ethylene and a decrease in the synthesis of auxin, gibberellins, and cytokinins (Nilsen and Orcutte, 1996). ABA applications enhance leaf senescence and abscission by increasing ethylene production and/or increasing ethylene sensitivity (Zacarias and Reid, 1990). Under severe and moderate drought stress, Petunia ×hybrida (petunia) show an increase in ethylene production as well as an increase in s-ABA, the biologically active form of ABA (Vardi and Mayak, 1989). Changes in the balance of ABA, ethylene, gibberellins, and cytokinins within the plant may be responsible for the development of leaf senescence (Blanchard et al., 2007; Waterland et al., 2010) and leaf abscission (Kim and van Iersel, 2008) after s-ABA applications.
The application of other plant hormones, including cytokinins and gibberellins, or the inhibition of ethylene perception may provide a means of preventing ABA-induced leaf chlorosis and abscission. Ethylene perception inhibitors such as 1-methylcyclopropene (1-MCP) have been used to prevent senescence and abscission in a variety of floriculture crops, including cut flowers and potted flowering plants (Blankenship and Dole, 2003). Cytokinins reduce leaf yellowing and senescence by delaying chlorophyll degradation (Gan and Amasino, 1995, 1997). Transgenic petunia and Nicotiana tabacum (tobacco) plants that overproduce cytokinins show delayed leaf senescence and reduced symptoms of leaf chlorosis after drought stress (Clark et al., 2004; Gan and Amasino, 1995). In some species, the application of gibberellic acids (specifically GA4+7) reduces postproduction losses by preventing leaf senescence (Han, 1997; Ranwala et al., 2003; Ranwala and Miller, 1998). Even the use of cytokinins and gibberellins in combination is effective at reducing leaf chlorosis in a variety of floriculture crops (Funnell and Heins, 1998; Han, 2001; Kim and Miller, 2009; Ranwala et al., 2000). It is not known why senescence is prevented or delayed in some species by cytokinins and in others by gibberellins nor is it possible to predict which species will respond to a particular hormone or combination with delayed senescence. Plant responses to plant growth regulators (PGRs) are highly variable; therefore, all PGRs must be examined in different species, cultivars, and even at various developmental stages before useful recommendations can be developed (Gent and McAvoy, 2000).
The goal of this research was to determine how to prevent s-ABA-induced leaf chlorosis in pansy and viola. Although s-ABA applications allow pansies to survive temporary periods of drought stress, negative side effects like leaf chlorosis decrease overall crop quality (Waterland et al., 2010). Our specific objectives were 1) to determine if cultivar, developmental stage, method of application, and concentration influence the effectiveness of s-ABA and the occurrence of leaf chlorosis symptoms; 2) to identify an application method and/or concentration of s-ABA that would not induce leaf chlorosis; and 3) to determine whether s-ABA-induced leaf chlorosis can be prevented by the application of other PGRs, including cytokinins, gibberellins, or ethylene perception inhibitors.
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