In many of the world's largest greenhouse and nursery production regions, irrigation water supply (quality and quantity) and management have become significant operational barriers. Increasingly restricted water supplies, coupled with the perennial threat of emerging and existing disease and pest vectors, present significant obstacles in achieving optimal nursery and greenhouse production (Hong and Moorman, 2005; Johansson et al., 2002). These production barriers are exacerbated by shifting consumer and legislative demands that limit the ability of production managers to deal with resource scarcity and pest issues (Province of Ontario, 1990; Yiridoe et al., 2005). Consumers are becoming more conscious of chemical and resource use while evolving government regulations will significantly restrict or alter traditional water use and pest control practices (Johansson et al., 2002; Uri, 1998; Yiridoe et al., 2005). Include the consequences of global climate change and its potential influence on water availability and the distribution and emergence of new pests and pathogens (Boland et al., 2004; Johansson et al., 2002), it becomes clear that nursery and greenhouse managers require new technologies and management strategies that will empower them to meet these resource and pest challenges as well as the environmental, social, and legislative shifts facing the industry (Hong and Moorman, 2005). Adaptation to emerging market, social, and environmental realities will rely on improvements in resource use efficiency, creation of value-added products, and empowerment of growers to rapidly respond to dynamic consumer preferences. Effective water and pest management strategies that can deliver substantial savings in an environmentally benign fashion are an important component of future greenhouse and nursery management strategies. Aqueous ozone [O3(aq)] technology can eliminate pathogens and many chemical contaminants in a wide range of water and wastewater streams without leaving many of the harmful chemical residues associated with other treatment technologies (e.g., chlorination). These properties make the technology attractive to horticultural production; however, data are lacking on the phytotoxicity of aqueous ozone (Fujiwara and Fujii, 2002).
Ozone (O3) is a triatomic allotrope of oxygen most commonly associated with interception of high-energy ultraviolet radiation in the Earth's stratosphere or as a component of photochemical smog, a significant tropospheric pollution issue. As a constituent of photochemical smog, a nearly ubiquitous pollution vector in major greenhouse and nursery crop production regions, ozone pollution as a plant stress is an ongoing concern. Ozone gas has known and well-characterized phytotoxic effects such as reduced photosynthetic capacity and foliar reddening and necrosis (Davison and Barnes, 1998; Fiscus et al., 2005; Fuhrer and Booker, 2003; Heath, 1996; Sandermann, 1996). There is, however, limited yet compelling evidence that the phytotoxic properties of ozone (gas) are altered when the ozone exposure is in an aqueous form (Fujiwara and Fujii, 2002; Sloan and Engelke, 2005), although the mechanisms of this reduced toxicity are a subject for further research and debate. There is additional evidence that low-level ozone (gas) exposure can stimulate oxidative stress adaptation without visible evidence of damage (Chamnongpol et al., 1998; Kovalchuck et al., 2003; Pell et al., 1997; Ranieri et al., 1996; Reiling and Davison, 1995; Zheng et al., 2002). Further to this effect, low-dose ozone has also been implicated in the triggering of systemic acquired resistance responses (Durrant and Dong, 2004; Pell et al., 1997; Rao and Davis, 2001) that convey plant resistance to further pathogen attack. The vast amount of research that has characterized the phytotoxicity of gaseous ozone (as a pollutant) may have inadvertently led to an oversight of the prophylactic use of ozone, in the aqueous form, to address common nursery and greenhouse production issues.
Aqueous ozone has long been used as a water treatment technology in a diverse range of applications, including limited use in the treatment of greenhouse irrigation water (Ehret et al., 2001; Guzel-Seydim et al., 2004; Igura et al., 2004; Rice, 1997; Runia, 1994). A strong oxidation potential (2.07 eV) coupled with a relatively short persistence period (seconds to minutes) has made aqueous ozone an ideal microbial and chemical contaminant control agent in many commercial settings (e.g., municipal water treatment, food processing, sewage treatment, postharvest storage). These same properties also lend themselves to applications in greenhouse and nursery environments. Particular interest lies in aqueous ozone's potential as an irrigation water remediation technology and as a means to control pathogens without leaving a chemical residue on the consumer product, a drawback of many current pest control strategies and a growing concern amongst consumers (Miles and Frewer, 2001; Woese et al., 1997).
When introduced into water, the half-life of ozone is variable, but typically it is extremely short because the ozone rapidly reacts with micro-organisms and oxidation-prone organic compounds in the water. Ozone that is not consumed through these pathways quickly converts to reactive oxygen-containing free radical species, all having very short (nanosecond) half-lives, and eventually to diatomic oxygen (O2). The result of this reversion is the absence of direct chemical residues associated with the treatment. This is not to say that other secondary disinfection byproducts (DBP) are absent, although the general consensus is that the DBPs formed under ozonation are far less problematic than those formed by other common water treatment technologies (e.g., chlorine; Rakness, 2005). The act of creating and dissolving ozone in irrigation water also leads to enhanced dissolved oxygen content. Enhanced oxygenation has been shown to have benefits in terms of improved productivity and pathogen control in greenhouse production (Zheng et al., 2007).
This study presents early steps toward developing a broader understanding of the phytotoxic characteristics of aqueous ozone when applied foliarly. Understanding a crop's tolerance threshold toward aqueous ozone is a key step in developing aqueous ozone applications that will help growers realize greater returns, while meeting the challenges of a changing marketplace and operational environment.
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