Plastic (polyethylene) mulches have been used in vegetable production in the United States since the 1950s. Black polyethylene plastic mulch is the standard plastic mulch used in vegetable production. Black plastic alters the plant's growing environment by generating warmer soil temperatures (Dodds et al., 2003; Hanna et al., 2003) and holding more soil moisture (Ham et al., 1991; Lamont, 1993) than bare soil. Researchers using black plastic instead of bare soil have recorded higher yields (Brown et al., 1995; Leib et al., 2002; Summers and Stapleton, 2002), earlier harvests (Bonanno and Lamont, 1987; Ibarra et al., 2001; Lamont, 1993), and cleaner fruit (Brown and Channell-Butcher, 2001; Loughrin and Kasperbauer, 2002), all attributed to soil temperature and moisture differences under plastic mulch. Sometimes black plastic mulches can create soil temperatures that are too high and this will cause deleterious effects on plant growth (Díaz-Péréz et al., 2000; Orzolek and Murphy, 1993). Other mulch colors besides black have been used by growers and researchers in vegetable production. White plastic mulch has been shown to generate cooler soil temperatures than black plastic (Díaz-Péréz and Batal, 2002; Lamont, 1993). White plastic is preferred during the summer growing season in warmer regions of the world compared with black plastic because it has the ability to maintain soil moisture while having cooler temperatures than black plastic. The use of silver plastic mulch has resulted in less insect-transmitted disease (Csizinszky et al., 1995; Lamont et al., 1990) in certain vegetable crops. Red plastic mulch has shown increased yield in tomato (Solanum lycopersicum) (Decoteau et al., 1989) and other crops (Decoteau et al., 1990; Kasperbauer, 1992).
Row covers are used to insulate a plant's growing environment to promote early yield. Floating spun-bonded polyester row covers are used with various vegetable crops (Shadbolt et al., 1962; Wells and Loy, 1985). Floating row covers have been shown to alter the plant's microenvironment by increasing temperatures for the crop during the day and into the night (Arancibia and Motsenbocker, 2002). The use of row covers with plastic mulch has shown to sometimes have a greater effect (Powell, 2000) and sometimes less effect (Lamont et al., 2000) on plant yield than the use of plastic mulch alone. The response of a plant to row covers is greatly dependent on the temperature in the region during the time the row covers are installed (Bonanno and Lamont, 1987; Contreras Magana and Sánchez del Castillo, 1990).
The use of row covers with plastic mulch has been shown to generate earlier and increased yields than row covers and plastic mulch used separately (Brown and Channell-Butcher, 1999a; Farías-Larios et al., 1998; Purser, 1993). Brown et al. (1998) observed that row covers with plastic mulch increased yield in sweetpotato (Ipomoea batatas). Gerber and Brown (1982) found superior and earlier yields of muskmelon (Cucumis melo) with the combination of row covers and plastic mulch. Cucumber (Cucumis sativus) exhibited a positive increase in early and total yield with the addition of row covers to plastic mulch (Wolfe et al., 1989).
Phytochrome is the photoreceptor responsible for light-regulated growth responses. The phytochrome molecule is a dissolvable chromoprotein with subunits that are made up of a linear tetrapryrrole chromophore covalently linked to a polypeptide of 120 to 127,000 molecular weight depending on the plant species (Pratt, 1982; Vierstra et al., 1984). Phytochrome receptors within plant cells have the ability to detect wavelengths from 300 to 800 nm. Red (R) light is absorbed from 660 to 680 nm and far-red (FR) light is absorbed from 730 to 740 nm (Decoteau et al., 1993; Kasperbauer, 1999; Rajapakse and Kelly, 1994). Tomato plants grown with red plastic mulch produced higher marketable yields than those grown with black plastic mulch (Decoteau et al., 1989). The researchers who conducted this study believed that their findings were related to the effects that light reflected from red plastic mulch had on the plant's phytochrome regulatory system. Orzolek et al. (2000) found silver and red plastic mulches reflected higher FR:R ratios than other mulch colors used in their study. This study also revealed an increase in marketable fruit yield in tomato using silver or red mulch as compared with standard black plastic. Bell pepper (Capsicum annuum) plants grew taller and were heavier when grown on red plastic, which exposed them to a greater FR:R ratio compared with other plastic mulches used in the experiment (Decoteau et al., 1990).
Plants have shown a multitude of different responses when treated with blue (wavelength 400 to 500 nm) light. Blue light treatments have shown to affect morphological, metabolic, and directional reactions in plants (Senger and Schmidt, 1994). Some of the documented findings on plant response to blue light include: phototropism (Lipson, 1980; Shropshire, 1980) enzyme synthesis (Hart, 1988; Ruyters, 1982), chloroplast development in leaves (Akoyunoglou et al., 1980), and stomatal opening (Hart, 1988). Hatt et al. (1993) and Kasperbauer and Loughrin (2004) have found white plastic mulch to reflect more blue light than the other colored mulches used in their experiments. Antonious et al. (1996) reported that turnip (Brassica rapa) roots grown on white plastic mulch, which reflected the largest amounts of blue light, had the least distinct or mildest flavor. Decoteau et al. (1988) reported that shorter stems and more auxiliary growth could result from the blue light reflected from the use of white plastic mulch with tomato. Field-grown cotton (Gossypium spp.) produced more boll fibers and seeds per plant when grown on ground covered with colored plastic mulches that reflected less blue light resulting in higher FR:R ratios (Kasperbauer and Thibodeaux, 1997).
Research on growing vegetable crops with various colored plastic mulches along with spun-bonded row covers is limited. The purpose of this study was to evaluate the effects of colored plastic mulches with and without row covers on growth and earliness of fruit production on okra.
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