Transplanting is a common cultural practice used to improve stand establishment, shorten the field growing cycle, enhance earliness, and eventually increase yield and quality of vegetable crops. After transplanting, biotic and abiotic stressors acting both independently or in combination often limit seedling growth and stand establishment (Leskovar and Stoffella, 1995). Limited root absorption area upon transplanting can be the source of transient drought stress and subsequent poor stand establishment, particularly under high field evapotranspiration conditions, even with optimum irrigation regimes. Restricted water uptake can lead to sudden and severe plant water deficit, resulting in transplant shock (Nitzsche et al., 1991). High air temperatures, accompanied by dry winds and rapid soil drying conditions, encountered in many southern regions of the United States can also be detrimental during or right after crop establishment.
Strategies that reduce plant transpiration have the potential to enhance stand establishment. Foliar application of antitranspirants is a promising tool for regulating transpiration to maintain a favorable plant water status. Previous studies indicated that antitranspirant application can increase leaf water potential, survival, and subsequent yield when bell pepper seedlings were transplanted into the field (Berkowitz and Rabin, 1988; Nitzsche et al., 1991). Antitranspirants have different modes of action (Davenport et al., 1969; Gale and Hagan, 1966), and effective formulations are those that prevent excessive water loss without reducing CO2 uptake.
Reflective materials such as kaolin clay and chitosan can reduce absorption of radiant energy (heat), lowering leaf temperatures and reducing transpiration (Bittelli et al., 2001; Jifon and Syvertsen, 2003). Emulsions of wax, latex, or plastics that dry on the foliage and form thin films can also minimize escape of water from the plant by decreasing stomatal conductance (g s) and thus preventing transpirational losses, improving plant water status, and reducing wilting and leaf abscission (Gu et al., 1996; Hummel, 1990; Laurie et al., 1994; Nitzsche et al., 1991; Plaut et al., 2004). As a result of limited gas exchange between leaf and the surrounding environment, common responses are increased leaf temperature and impaired net CO2 assimilation rate (ACO2), but the magnitude of these responses is highly species or cultivar dependent (Laurie et al., 1994; Plaut et al., 2004; Russo and Díaz-Pérez, 2005). Regardless of the limitation of ACO2, enhanced vegetative growth of pepper, walnut (Juglans regia L.), and tomato (Lycopersicon esculentum Mill.) were reported as well (Irmak et al., 1999; Nitzsche et al., 1991; Voyiatzis and McGranahan, 1994).
Physiologically active chemicals that trigger partial or complete stomatal closure [e.g., abscisic acid (ABA) and its analogs], thus decreasing water loss from leaves, have also been used as antitranspirants (Davies and Zhang, 1991). Berkowitz and Rabin (1988) found that ABA-treated pepper transplants had higher field survival rates than untreated seedlings, which was attributed to reduced g S and increased leaf water potential. Under certain conditions, ABA application can also slow shoot growth (Watts et al., 1981) and can thus be used to control transplant growth in nurseries (Leskovar and Cantliffe, 1992). The shoot growth reduction effect seems to be more responsive when foliar ABA is applied to regularly watered plants compared with plants exposed to moderate water deficit (Leskovar and Cantliffe, 1992). A retardation of moisture use and growth of tomato but not in marigold (Tagetes patula L.) was also found with ABA analog 8′ acetylene ABA methyl ester (Sharma et al., 2005).
Sharp (2002) has suggested that an increase in endogenous ABA concentrations often may help to maintain rather than to inhibit shoot and root growth under soil drying conditions. This role was related to ABA antagonistic interaction with ethylene. Increased ethylene concentration in plant tissues, a frequent response to various environmental stresses, often triggers leaf senescence and abscission (Morgan and Drew, 1997). Inhibitors of ethylene biosynthesis such as aminoethoxyvinylglycine (AVG) may thus mitigate the ethylene-related effects of stresses (Abeles et al., 1992). Islam et al. (2003) found that application of AVG at the root zone reduced transpiration under drought stress and postponed water deficit of white pine (Pinus strobus L.) plants. Application of AVG into germination media also stimulated root and shoot growth of barley (Hordeum vulgare L.) seedlings (Locke et al., 2000).
Manipulation of transplant growth and physiology by irrigation management has been used to condition transplants to withstand posttransplant stress better (Leskovar, 1998; Liptay et al., 1998). The use of antitranspirants to condition transplants could be a safer practice that eliminates the risk of stressing young seedlings to the point of physiological injury. Most of the studies cited earlier have separately evaluated the effects of antitranspirants on physiological (water relations and gas exchange) or growth (morphology and yield) parameters. Parallel analysis of these plant responses can provide a better understanding of the potential benefits of antitranspirants for transplant conditioning. The objective of this study was to determine the effects of physical film-forming and chemical antitranspirants on growth and physiological responses contributing to stress tolerance of pepper seedlings when exposed to transient water deficit stress.
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