Shade can increase total and market yield of tomato (Lycopersicon esculentum Mill.) grown in hot climates. Tomato plants grown in Egypt for the entire season under shade that attenuated 30% to 40% of sunlight had higher yields and more fruit than those grown without shade (Abdel-Mawgoud et al., 1996; El-Aidy, 1986). Increasing shade density above 40% decreased flowering and yield. For crops grown in Oklahoma, 63% shade decreased yield in 1 year but not in another (Russo, 1993).
In climates with more moderate temperatures, shade typically reduces yield of tomato grown in a greenhouse. The structure and covering of a greenhouse reduces light intensity, so additional shade could be deleterious. In Brazil, a humid subtropical climate, 52% shade reduced yield by 20% (Sandra et al., 2003). In England, a cool and low-light environment, 23% shade reduced the yield by 20% (Cockshull et al., 1992). Movable shade, applied only during sunny periods, is less deleterious than constant shade. In England, a 1-week period of shade had no effect on yield (Adams et al., 2001). In Spain, mobile shade used only under intense sunlight increased marketable yield by 10% (Lorenzo et al., 2003). The benefit of shade on quality in the latter study was attributable to less blossom end rot and cracked skin.
High light intensity can lead to several disorders in development and appearance of tomato fruit that affect quality (Dorais et al., 2001). Sunscald injury and uneven ripening are two disorders brought on by direct effects of light on fruit. Sunscald injury of tomato fruit increased with irradiance and air temperature and their combined effects (Adegoroye and Jolliffe, 1987). High temperature in conjunction with high irradiance also contributed to blotchy or uneven ripening (Lipton, 1970).
Cracks in the skin are one of the most common defects of tomatoes produced in the eastern United States (Peet and Willits, 1995). Sunlight plays a role in this defect. Field-grown fruit exposed to sunlight were more than twice as likely to develop cracks as shaded fruit (Whaley-Emmons and Scott, 1997). Plant water status also plays a role. Plants that were watered excessively had 20% more fruit with cracked skin than those with just sufficient water (Peet and Willits, 1995). Excess water also increased fruit weight. Conversely, when sodium chloride was used to increase electrical conductivity (EC) of the nutrient solution for tomato grown in rock wool, fewer fruit had cracked skin (Chretien et al., 2000). This treatment also resulted in a smaller fruit size. An increase in concentration of nutrients had a similar effect on cracking of cherry tomato fruit grown in hydroponics (Ohta et al., 1993). Thus, increasing solution EC or reducing water supply prevents cracked skin but results in smaller fruit. These practices may not be welcomed by growers in the northeast United States, because they believe their customers want large fruit. One benefit of shade to increase the quality of tomato fruit is that shade may increase quality without reducing fruit size.
The climate in the northeast United States varies from cool and cloudy in winter and spring, with nights colder than 0 °C, to warm and humid in summer with days warmer than 30 °C. During a multiyear cultivar trial of greenhouse tomato, the average fraction of marketable fruit was 56% to 58% in years in which the houses were covered with 30% shadecloth, but only 32% to 44% in years in which no shade was applied (Gent, 2003, 2004). It is not clear whether shade applied in summer, or what density of shade, would improve quality of greenhouse tomato grown under these conditions. In the present report, yield and quality of tomato produced in summer in a nonshaded section of greenhouses was compared with that in sections covered with various densities of shadecloth.
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