Surround wettable powder (Engelhard Corp.; Iselin, NJ) is a processed kaolin [Al4Si4O10(OH)8] clay-based crop protectant approved for use in the organic production of a variety of crops, including apple and blackberry. It was developed for broad-spectrum insect pest control and has other applications including, but not limited to, disease control and protection against heat stress and solar injury (Glenn and Puterka, 2005). The product forms a white, reflective physical barrier called a “particle film” on the plant surface. Although designed not to interfere with leaf gas exchange, the findings to this end are mixed (Glenn and Puterka, 2005). Differences in product application, climatic conditions, crop or cultivar, and physiological state of the plants may account for some of the disparity between these findings.
A number of past pomological studies have investigated the impact of kaolin particle films on insect pests, disorders and diseases, canopy microclimate and environmental physiology, fruit productivity and quality, and water use efficiency (Glenn and Puterka, 2005). The research pertaining to the use of kaolin particle films in blackberry production is limited to its application as a soil amendment in weed control (Takeda et al., 2005) or to its potential use in frost protection (Agricultural Research Service, 2005).
Some studies indicate that kaolin particle film technology is best suited for use in warm, arid climates, in which heat stress, not light, is the principal factor limiting production (Erez and Glenn, 2004; Glenn et al., 2003; LeGrange et al., 2004; Schupp et al., 2004). Other findings suggest that particle films favor gas exchange under conditions of environmental (high temperature) and physiological (drought) stress (Glenn et al., 2001). From ‘Ginger Gold’ [Malus ×sylvestris (L.) Mill var. domestica (Borkh.) Mansf.] leaf gas exchange measurements made in the 2004 season as part of a preliminary study, it appeared that greater rates of net photosynthesis (Pn) were achieved at a higher frequency of kaolin particle film application and that this was particularly the case at leaf temperatures exceeding 35 °C (Privé et al., 2007). Ultraviolet damage and photoinhibition can be additive at plant temperatures exceeding 35 °C (Glenn and Puterka, 2005). Although kaolin particle films are used as a means of pest control in several commercial apple orchards in the Maritime provinces, no previous studies have investigated the effect of these particle films on leaf gas exchange under the region's growing conditions. The New Brunswick growing season is characterized by conditions of low to moderate solar irradiance, moderate temperature, and high humidity. The nature of the physiological response to the kaolin particle film in the region's climate will have a bearing on the use of this product in the Maritime provinces.
No studies to date have investigated the effect of leaf surface (adaxial, abaxial, or both) of kaolin particle film application on leaf gas exchange. Liang and Liu (2002) examined the effect of melon leaf surface treated with a kaolin particle film on oviposition and the repellency of adult silverleaf whiteflies. Abou-Khaled et al. (1970) measured gas exchange of citrus and rubber leaves in the laboratory after having applied a kaolin particle film to the leaves’ adaxial surfaces only. Jifon and Syvertsen (2003) observed that grapefruit leaves with their adaxial surface coated with a kaolin particle film had greater light reflectance than those with abaxial surfaces coated (the latter of which had reflectance levels comparable with those of untreated control leaves), and also found that the application of the particle film to both leaf surfaces did not result in an increase in reflectance over that measured for the adaxial surface alone. Most studies pertaining to the effect of kaolin particle films on leaf gas exchange involve field trials in which the kaolin product is applied indiscriminately to all foliage, as it would in a commercial orchard setting, as opposed to selective application of the particle film to individual leaves. In one instance of the former, a greater density of particle deposition on the adaxial surface (6.5 g·m−2) than on the abaxial surface (4.8 g·m−2) was observed for mature ‘Braeburn’ apple leaves (Wünsche et al., 2004). It is recommended that both surfaces of the leaf be evenly coated with the particle film to achieve optimal performance (Glenn and Puterka, 2005). It can, however, be difficult in practice to achieve thorough, uniform kaolin particle deposition on all leaves in the canopy using common agricultural spraying equipment, and great variability in kaolin residue densities can frequently be observed not only among leaves, but between the two surfaces of any one leaf. The leaf surface on which the stomata are located varies by plant species. They are located primarily on the abaxial surface of apple and blackberry leaves. Any variation in leaf physiological response related to differences in particle film coverage between leaf surfaces would be of practical consideration in informing the method of product application.
The current study is comprised of a ‘Ginger Gold’ apple field trial and a ‘Triple Crown’ (Rubus L. subgenus Rubus Watson) blackberry greenhouse trial. Both of these fruit cultivars display a number of attributes that make them desirable for production in the Maritime region (Galletta et al., 1998; Privé, 2004). ‘Ginger Gold’ is a yellow-skinned, early-maturing cultivar. Damage incited by pests, pathogens, and environmental disorders is all the more prominent on ‘Ginger Gold’ fruit because of its skin color. Not as winter hardy as raspberries, no blackberry cultivars are currently recommended for cultivation in the Maritime provinces; however, interest in blackberry production in the region is growing, and trials are currently underway to evaluate the viability of some of the hardier cultivars. ‘Triple Crown’ is a thornless, late-summer-ripening blackberry cultivar with a potential for cultivation in the region that is currently being investigated. Results of a 7-year field trial carried out in Oregon indicate that ‘Triple Crown’ fruit are susceptible to ultraviolet light injury (Galletta et al., 1998). Various studies report that kaolin particle films are effective in protecting foliage and fruit from solar injury (Glenn et al., 2002; Melgarejo et al., 2004; Wand et al., 2006). The aims of the current study are 1) to characterize the leaf temperature and gas exchange responses [Pn, stomatal conductance (g s), intercellular CO2 (Ci), and transpiration (E)] of ‘Ginger Gold’ apple [Malus ×sylvestris (L.) Mill var. domestica (Borkh.) Mansf.] leaves to a kaolin particle film (95% kaolin clay) applied at various leaf residue densities under New Brunswick growing conditions, 2) to characterize the leaf temperature and gas exchange responses of ‘Triple Crown’ blackberry (Rubus L. subgenus Rubus Watson) leaves to treatment of abaxial or adaxial surfaces with the kaolin particle film at various leaf residue densities, and 3) to determine whether the gas exchange response of apple and blackberry leaves to the kaolin particle film varies according to leaf temperature.
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