Light
Too little light, especially at the bottom of the canopy, is more likely to be a stress factor than excess light because light is reduced 30% or more by the cover and structural elements. Another reason light is limiting in greenhouses is because temperature, CO2, nutrients and water are generally not limiting. A rule of thumb in northern latitudes is that a 1% decrease in light reduces yield 1%. Supplemental lighting is, however, rarely economical except in extreme northern latitudes which also have favorable utility rates, such as Quebec, Canada. It is not clear how much, or if, plants are light-limited in extreme high-light environments such as the southwestern US. Shadecloths are sometimes used to protect fruit at the top of the canopy from sunscald, russetting, and cracking caused by high temperatures and to reduce overall greenhouse temperatures. However, shadecloths also reduce plant transpiration, because stomates close, so leaf temperatures are not necessarily reduced as much as air temperatures by shadecloths. This makes decisions on when and how much to shade difficult, especially because little data is available on plant responses to high light and temperature under greenhouse conditions [31].