Florida ranks second in the United States in strawberry sales value ($362 million), and it produces the vast majority of the crop during the winter months [U.S. Department of Agriculture (USDA), 2011]. During the last 10 years, the surface area planted with strawberries in Florida increased from 6300 acres in 2000 to 8800 acres in 2010 (USDA, 2001, 2011), and it is expected to surpass 10,000 acres in 2011. Most of the acreage occurred in the Plant City-Dover agricultural area in Hillsborough County, located in the west-central region of the state, where deep sandy soils with rapid infiltration are the norm.
In the Plant City-Dover agricultural area, strawberry fields are in close proximity to urban settings and water resources are shared between housing developments and production fields using deep fresh water wells. High-impact sprinkler irrigation (4 to 5 gal/min per head) is used during strawberry production for two primary reasons: transplant establishment and freeze protection. For transplant establishment, bare-root plants from northern nurseries are set into fumigated, polyethylene mulched beds and irrigated with sprinklers during 8 to 10 h per day for the first 10 d after transplanting (Hochmuth et al., 1993). This practice is performed to reduce the temperature around the young crowns during establishment and to provide enough moisture to new roots in that period. Although this is a traditional strawberry practice in many places around the world, it is highly inefficient because it uses large water volumes (between 14 and 20 acre-inch/acre), most of which ends up running off the polyethylene mulch, then into the row middles, and finally into the drainage canals or leaching to the water table (Bish et al., 1997; Hochmuth et al., 2006).
Furthermore, the current structure of water permits for agriculture in the area allocates certain volumes for production regardless of their purpose (i.e., freeze protection, establishment, and crop maintenance) and those volumes are monitored with water meters. Thus, water savings due to reduced volumes for establishment would allow growers either to save water and pumping costs or more flexibility for other uses during the growing season. One example of the fragility of the current system is that during the unusually cold winters of 2009 and 2010, many deep wells went temporarily or permanently dried due to overuse of sprinkler irrigation for freeze protection in Hillsborough County [Southwest Florida Water Management District (SFWMD), 2011]. It is estimated that in Jan. 2010, 750 residential wells were impacted and more than 140 sinkholes were reported in this county, which has prompted discussions about more stringent local regulations on water management for strawberry production (SFWMD, 2011). Therefore, establishment practices aiming to reduce water volumes for strawberry transplanting are desirable both from the environmental and economic standpoints.
Kaolin clay is a naturally occurring mineral that has been suggested as a means to reduce heat stress in horticultural crops. In fruit crops, previous studies determined that application of kaolin clay solutions to apple (Malus ×domestica) trees significantly reduced heat stress and fruit sunburn (Glenn et al., 2002; Schupp et al., 2002). Jifon and Syvertsen (2003) indicated that kaolin clay application on grapefruit (Citrus paradisi) trees decreased leaf temperature by 3 °C, as well as leaf to air vapor pressure, and improved net carbon dioxide assimilation and yield. Rosati et al. (2007) suggested that kaolin clay application on leaves improved the distribution of photosynthetically active radiation in almond (Prunus dulcis) and walnut (Juglans regia). Spiers et al. (2004) demonstrated that when applied to mature blueberry (Vaccinium corymbosum) plants, kaolin clay increased bud and fruit development, as well as plant growth and fruit yield. Steinman et al. (2007) showed that photosynthesis of coffee (Coffea arabica) was enhanced by 71%, while their yields improved by 100% in comparison with the nontreated plants.
In vegetable crops, many reports have focused on tomato (Solanum lycopersicum) and pepper (Capsicum annuum) because these crops are sensitive to sun scalding injury. Nakano and Uehara (1996) showed that kaolin clay reduces the stomatal aperture in tomato. Other studies have indicated that application of kaolin clay consistently lowers leaf and fruit temperatures, leaf net assimilation, stomatal conductance, and transpiration, while improving lycopene fruit content, soluble solid content, and marketable yields (Cantore et al., 2009; Pace et al., 2007; Saavedra et al., 2006). In some cases, the application of kaolin clay did not have a direct effect of plant growth, but improved early flowering and fruit yield of pepper (Makus, 2005). Other investigations on the effects of foliar applications of kaolin clay showed that the product failed to enhance tomato fruit yield (Kahn and Damicone, 2008). Albregts and Howard (1976) indicated that the use of antitranspirants did not enhance strawberry plant survival, leaf retention, fruit size, or yield. Creamer et al. (2005) determined that pepper plants treated with kaolin clay showed less water stress and higher photochemical reflectance than nontreated plants during active growth periods, but without significant yield differences.
Preliminary observations in Hillsborough County suggested that kaolin clay-based crop protectants could potentially reduce heat stress during plant establishment. However, formal studies needed to be conducted to confirm this hypothesis. One alternative practice would be reducing the number of days needed of sprinkler irrigation, followed by application of kaolin clay on the foliage to reduce heat stress. The objectives of this study were to determine whether foliar kaolin clay applications would reduce water volumes during the establishment of bare-root strawberry transplants.
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