An increase in heat to high levels or a sudden "shock" of a severely hot day can trigger leaf yellowing and drop. Many times the leaves will drop very quickly with only slight yellowing.
This "yellowing of leaves" is a simple survival reaction by the plant that allows it to conserve moisture. Wilting and scorching can also present itself as further effects of intense heat.
For indoor grows, this is commonly from placement or growth too near the lights. This is especially true of High Intensity Discharge (HID) lighti9ng systems which generate significant heat. A few days of strong growth and the upper leaves may be well into the 130 degree plus range of these bulbs.
For outdoor plants, consider the location in relation to reflective items or areas that store and contain heat such as sunken beds, areas around sheds and buildings, or near reflective surfaces and asphalt.
Solutions for both of these factors usually involves moving the plant or providing shade. I'll cover some shade recipes and ideas in the future.
Some simple steps for avoiding heat injuries to your plants......
Move the plants away from the culprit (duh!).
Add additional soil to lower the overall plant temperature.
Keep plants at least 8-12 inches below HID bulbs and at least 2-3 inches from LED, and T5 and T8 bulbs.
Move the pants to full or partial shade if the type requires that environment. No sense in killing a plant that has no business in the sun!