Bada bing, thank you so much for the link to the shade cloths! I've been using window screens on some cacti I transplanted; this is SOOO much better.

I purchase a 30% shade cloth from Gemplers. The shade cloth that is shown in Bada Bings post, is Gempler's black shade cloth. They also sell a shade cloth called Aluminet, which is a silver sided shade cloth. Alumine's silver reflects the sunlight away, while a black shade cloth absorbs the sun's heat. I actually have both types of shade cloth (black and silver) for my 32' X 72' greenhouse. Aluminet works much better for reducing the sun's light energy. - Millet (270 BO-)

This article could be of a good use, I think:
Keyword: heat stress, high temperature

2002:
"Photosynthetic response of citrus grown under reflective aluminized..."
http://www.polyphane.com/files/6fe4f07c91b4cda44f062f5f3dc98e39.pdf