Drought is a major environmental stress worldwide resulting in decreased plant productivity and diminished plant health. Climate model predicts this to become a more severe problem in the future (Farooq et al., 2012). Drought stress causes cellular dehydration, loss of turgor pressure, and ion toxicity (Bartels and Sunkar, 2005). Plants’ protective responses to water deficiency can include stomatal closure to reduce transpiration (Sharp and Davies, 1989), leaf rolling/orientation to reduce water loss and heat exposure (Kao and Forseth, 1992), and osmotic adjustment to reduce water potential (Delauney and Verma, 1993). In addition to these drought tolerance strategies, cuticle augmentation in response to water deficit has been well documented in a number of plant species, specifically cotton (Gossypium hirsutum L.) (Bondada et al., 1996), Arabidopsis [Arabidopsis thaliana (L.) Heynh.] (Kosma et al., 2009), tree tobacco (nicotiana glauca L.) (Cameron et al., 2006), soybeans [Glycine max (L.) Merr.] (Kim et al., 2007a), sesame (Sesamum indicum L.) (Kim et al., 2007b), rose (Rosa ×hybrids) (Jenks et al., 2001), citrus (Bondada et al., 2001), and peanut (Arachis hypogaea L.) (Samdur et al., 2003).
The plant cuticle is a continuous extracellular membrane located on the above-ground organs of most higher plants. Only roots and secondary plant tissue as well as some mosses are devoid of this protective barrier (Koch and Ensikat, 2008). The main function of the cuticle is to protect against uncontrolled water loss to the atmosphere through transpiration (Burghardt and Riederer, 2006; Cameron et al., 2006; Riederer and Schreiber, 2001). Secondary characteristics of the cuticle include antiadhesive properties, repelling water, particles, pathogens, and other molecules, which could hinder the uptake of foliar applications of nutrients and pesticides in an agricultural system (Bargel et al., 2006; Koch et al., 2008). The plant cuticle is comprised of two main portions: the cutin, which provides structure, and waxes, which provide protective functions to the plant. Many of the protective functions of the cuticle, especially repellency, can be attributed to the cuticular and epicuticular waxes that develop on the plant surface. The chemical composition of cuticular waxes is a mixture of aliphatic and aromatic components comprised of various combinations of long chain alkanes, fatty acids, primary and secondary alcohols, aldehydes, and ketones, the proportions of which are dependent on species, developmental stage, and organ (Bargel et al., 2006; Jetter et al., 2006; Riederer and Markstadter, 1996). Epicuticular waxes form thin two-dimensional films and/or three-dimensional structures, depending on chemical composition (Koch et al., 2008). Crystalloids are common three-dimensional structures and are characterized as granules, plates, platelets, rodlets, threads, and tubules (Barthlott et al., 1998; Jeffree, 2006). The three-dimensional structures add roughness to the cuticle making it more hydrophobic to foliar applications.
Foliar fertilization is widespread in turfgrass maintenance programs because of the labor efficiency and cost-effectiveness resulting from the ability to tank-mix and apply the fertilizer concurrently with additional chemicals. Low-rate application of nutrients to turfgrasses in standard intervals promotes uniform growth that increases playability and aesthetics (Bowman, 2003). Increased canopy color, leaf N concentration, and leaf micronutrient concentrations were reported from creeping bentgrass (Agrostis stonlonifera L.) fertilized frequently using liquid solutions (Schlossberg and Schmidt, 2007). However, the combination of liquid and granular fertilizers provides the best turfgrass quality and reduces total fertilizer input (Totten, 2006; Totten et al., 2008). Minimal N losses from volatilization were documented on creeping bentgrass and hybrid bermudagrass [Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt Davy cv. TifEagle] putting greens applied with foliar urea applications, providing evidence of lower environmental impact by foliar fertilization (Stiegler et al., 2011).
Nitrogen absorption with various N sources and factors affecting uptake have been studied on various species in the past. Stiegler et al. (2013) found 15N-labeled urea uptake was superior to other tested N sources, where absorptions levels ranged from 31% to 56% of N sources applied after 8 h. Absorption of 15N-labeled urea was variable through summer months ranging from 36% to 69% on a creeping bentgrass putting green, which it was suggested could partly have been associated with increased cuticle wax loads (Stiegler et al., 2011). In ryegrass (Lolim perenne L.), foliar-applied 15N-labeled urea was found to be absorbed by 30.3% and 53.1% by new and old leaves after 48 h, respectively. A significant negative relationship was found with urea uptake and epicuticular wax amounts in citrus leaves and cotton (Bondada et al., 1997, 2001). Aqueous pores (greater than 1 nm) have been demonstrated on plant cuticles and provide entry of water and small molecules into the plant (Schönherr, 2006). There is also evidence of a stomatal pathway for the uptake of foliar-applied solutions. Eichert and Goldbach (2008) found significant differences in N uptake with stomatal aperture and stomatous vs. astomatous leaf surfaces, indicating the role of stomata in foliar uptake.
The use of adjuvants has also been demonstrated to increase the uptake of foliar-applied chemicals in several plant species under various environmental conditions (Fernandez et al., 2006; Liu, 2004; Neal et al., 1990). For example, surfactant added to potassium nitrate applications increased potassium content in cotton leaves compared with applications with water alone (Howard et al., 1998; Howard and Gwathmey, 1994). Rawluk et al. (2000) documented a 25% increase in N recovery when a nonionic surfactant was added to foliar applications of 15N-labeled urea in wheat (Triticum aestivum L.). Surfactant added to 14C-labeled glyphosate applied to barnyardgrass increased the uptake, movement, and herbicide activity (Kirkwood et al., 2000). The addition of adjuvants has become a common practice to increase uptake and reduce losses of foliar-applied fertilizers and pesticides (Wang and Liu, 2007).
Creeping bentgrass is the most widely used turfgrass species for putting greens. There is currently a lack of research examining the influence of water stress to the cuticle layer within the species. Data are also lacking to determine if morphological or compositional changes in the cuticle from acute moisture stress influence foliar N uptake and if added surfactant can amend those changes. The objectives of this study were to 1) determine creeping bentgrass cuticle total wax, chemical composition, and morphology; 2) determine the influence of drought stress on creeping bentgrass cuticle total wax, chemical composition, and morphology; and 3) quantify foliar absorption of 15N-labeled urea with and without a surfactant.
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