Ask Mr. Smarty Plants

The Acer palmatum, Japanese maple is a native of China and Japan, and therefore out of our range of expertise. At the Lady Bird Johnson Wildflower Center, we are focused on the use, protection and propagation of plants native not only to North America but to the area in which they are being planted. Native plants which are accustomed to the climate, rainfall and soil of an area will need less fertilizer, water and maintenance.

From About.com: Landscaping read this article on How To Manage and ID Japanese Maple. Read also our How-To Article Container Gardening with Native Plants.  Although, as we said, the Japanese maple is non-native to North America, still, the same principles should apply in your case. Our thinking is that if you have been relying on rainfall to irrigate your tree, its leaves have every right to shrivel. That, and the extremes of temperature you have been experiencing, would be enough to stress any plant. Remember, in extreme heat and cold, a potted plant has only the potting soil to insulate it, while in the ground, it has the whole Earth insulating it. Since we are not familiar with the plant, we would suggest you first move it into more shade, if it is not in shade now. If the top inch or so of the potting soil is dry, start letting a slow dribble of water trickle into it every day or so. If the soil is very dry and the drainage in your pot is good, most of the water the first time is going to go straight through that dry soil and drain. Try lightly watering the soil, and go back in an hour or so and do so again. You don't want the soil soggy-a plant can die just as fast from roots rotting in too much water as from drying out due to too little water.