Shadecloth is used in the production of a variety of vegetable, fruit, and ornamental crops, especially in tropical and subtropical regions (Armitage, 1991; Díaz-Pérez, 2013; Halevy et al., 2002; Ilić et al., 2012; Marini and Sowers, 1990; Wolff and Coltman, 1990). Shadecloth provides a microclimate with reduced PAR, air temperature, root zone temperature, and changes in relative humidity (Kittas et al., 2012; Stamps, 2009). Shading reduces PAR levels and in some cases air temperature (Smith et al., 1984), producing taller plants and increased leaf area, which may be desirable in vegetable and cut flower productions (Armitage, 1991; Díaz-Pérez, 2013; Ilić et al., 2012). Díaz-Pérez (2013) reported increased total plant leaf area, individual leaf area, and individual leaf weight of shaded bell pepper (Capsicum annum) when grown from spring to summer in Tifton, GA. However, leaf number per plant and specific leaf weight decreased with increasing shade level (Díaz-Pérez, 2013).
Plant response to shade varies among species and climatic conditions. For example, Kittas et al. (2012) found shading increased leaf area index, number of fruit per plant, and total yield in tomato (Solanum lycopersicum) during spring and summer in New Anchialos, a coastal area of eastern central Greece. Shade levels of 30% to 47% increased yield of lettuce cultivars (Green Mignonette, Salinas, and Amaral 400), WR-55 Days chinese cabbage (Brassica rapa), and Tastie Hybrid head cabbage (Brassica olearacea var. capitata) to different degrees when grown in Hawaii during fall to spring. However, shade (30%, 47%, 63%, or 73%) did not affect crop yield of ‘Parris Island Cos’ lettuce, ‘Waianae Strain’ green mustard cabbage (Brassica juncea), or green bunching onion (Allium fistulosum) (Wolff and Coltman, 1990). Furthermore, Armitage and Son (1992) indicated no difference in yield of blue spirea (Caryopteris incana) when plants were grown under 55% shade compared with full sunlight in field condition approximated in May at the University of Georgia (Athens). However, shade significantly increased stem length.
Different colored shadecloths, also referred to as photoselective nets, have been used to manipulate the light spectrum to induce specific beneficial plant responses (Basile et al., 2008, 2012; Oren-Shamir et al., 2001; Retamales et al., 2008; Shahak et al., 2004). Altering light quality using colored shadecloth is a sustainable tool in modern fruit production (Bastías and Corelli-Grappadelli, 2012). For example, vegetative growth of ‘Hermosa’ peach (Prunus persica) was increased by shade nets of different color (30% blue, gray, pearl, red, yellow, and 12% white) (Shahak et al., 2004). By contrast, shadecloth of grey, red, or white color did not affect vegetative growth of ‘Berkeley’ highbush blueberry (Vaccinium corymbosum) grown in central Chile (Retamales et al., 2008). Red and pearl colored shadecloths (40%) significantly increased the total yield of tomato (Ilić et al., 2012; Kittas et al., 2012). Shading also improved tomato fruit quality by reducing cracking and sunscald, and increase marketable fruit percentage (Ilić et al., 2012; Kittas et al., 2012).
Plant photosynthesis is affected by shading because it alters the light spectrum, PAR, and temperature. Marini and Sowers (1990) reported a nonlinear increase in net photosynthesis of peach ‘Redhaven’ as photosynthetic photon flux (PPF) level increased from 9%, 17%, 23%, 45%, to 100%, where shading was provided by black polypropylene fabric with different densities. Previous research also indicated increasing shade levels decreased net photosynthesis and gs, and increased leaf transpiration of bell pepper (Díaz-Pérez, 2013). In the same study, data suggested shade levels above 47% may have resulted in excessive transpiration and reduced leaf photosynthesis. Wei et al. (2010) reported reduced yellow vine symptoms of american cranberry (Vaccinium macrocapron) leaves collected from shaded areas. As a result of shade effect, overall photosynthetic activity was improved.
Compared with plant vegetative growth, crop yield, and photosynthetic responses, there is lack of information on how plant chemical compositions, especially phenolic and flavonoid compounds, are affected. Changes in flavonoids in response to colored shade/netting have been investigated more in fruit crops than in vegetables (Basile et al., 2008; Ilić et al., 2012; Retamales et al., 2008; Shahak et al., 2004). Flavonoids are a group of low-molecular weight polyphenolic compounds, including flavonols, flavones, flavonones, catechins, and isoflavones, and contribute to fruit color, flavor, and bitterness (Robards and Antolovich, 1997; Weisshaar and Jenkins, 1998; Winke-Shirley, 2002). They protect plants by absorbing ultraviolet light and visible radiation, and prevent over-excitation of the photosynthetic apparatus (Becker et al., 2013). Their antioxidant activity of scavenging free radicals provides human health benefits of reducing risks of cancer and cardiovascular diseases (Haytowitz, 2003; Jovanovic et al., 1994; Ojong et al., 2008; Rice-Evans et al., 1997).
Use of colored shadecloth has been increasingly adopted in vegetable and cut flower production. Thus, it is of great importance to investigate how colored shadecloths affect plant growth and development, plant physiological activities, and beneficial phytochemicals, such as phenolics and flavonoids. Therefore, the objectives of this study were to investigate the effect of colored shadecloth on: 1) growth and yield of lettuce and snapdragon, 2) physiological activities of lettuce and snapdragon, and 3) flavonoid content in lettuce leaf tissue.
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