High temperature and light intensity during the summer have a negative effect on plant growth and yield (Lopez-Marin et al., 2012). Excessive light and heat load on leaves is a result of high solar radiation (Lopez-Marin et al., 2011) which severely limits the productivity of a crop. Reductions in crop yield are frequently observed under Mediterranean conditions with high levels of solar radiation and low air humidity (Katsoulas and Kittas, 2008). Shading is a popular method to improve plant microclimate in the summer by decreasing leaf temperature and leaf transpiration rate, thus alleviating heat stress (Aberkani et al., 2008). The cultivation area under shade is constantly increasing in Mediterranean countries such as Israel, Morocco, and others (Tanny et al., 2014).
In Texas, commercial tomato production in open field is limited to relatively short periods of favorable temperatures during the spring and early fall. With the advent of high temperatures, usually in June, vegetative vigor and reproductive ability of the plants start to decline with a corresponding blossom abscission and failure of fruit set. Flowering, pollination, and fruit set of tomato are often adversely affected by extreme high temperatures. The optimum temperatures for tomato fruit set are from 18.5 to 26.5 °C (LeBoeuf, 2004). Chili pepper is primarily cultivated in warm and semiarid regions where high irradiance and temperatures during summer growing seasons are common (Dorji et al., 2005). Therefore, the optimal temperatures for chili pepper may be higher than those for tomato. In other words, chili pepper may be more tolerant than tomato to full sun and high temperatures in Texas.
In southern and western Texas, high temperatures in summer months often exceed 35 °C and in some days the high temperatures can even reach to 40 °C and higher. According to our local weather station (El Paso, TX), relative humidity during the day can be in the teens and maximum solar radiation can reach 32 MJ·m−2·d−1, equivalent to 64 mol·m−2·d−1 in the 400 to 700 nm range of the photosynthetically active radiation (PAR). The high temperature and solar radiation lead to high plant temperatures, thus causing heat stress.
Shade screens reduce solar radiation during the day, thus reducing heat stress (Lopez-Marin et al., 2012). Previous studies indicated that shade screens, mostly light shade, led to an increase in productivity and quality of tomato, pear (Pyrus sp.), apple (Malus ×domestica), strawberry (Fragaria ×ananassa), grape (Vitis vinifera), and chili pepper (Rajapakse and Shahak, 2007; Shahak et al., 2008). With temperature reduction under the shade proportional to shade intensity (Kittas et al., 2009), a shade may be necessary to alleviate the heat stress of plants for regions with high solar radiation and temperatures. El-Gizawy et al. (1992) reported that shading (up to 51%) significantly increased the number of fruit and total yield of tomato in Egypt and the highest yield was achieved at 35% shading while heavy shading of 63% had similar yield to full sun. Nevertheless, too much shade can cause yield reduction in some crops, but no research-based information is available in Texas. The purpose of this research is to quantify the effect of relatively heavy shading intensities (50% and 70%) on the growth, yield, chlorophyll fluorescence (Fv/Fm), and Pn of tomato and chili pepper in semiarid west Texas. We also quantified microclimate traits underneath the shade structures.
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