Why would a sweet pea bush have yellow leaves?
Planted these a few months ago and they were doing great, but one has yellowing leaves. It has been in the 100's and have been watering daily. They said they were drought tolerant.
Yellowing leaves are a sign of watering issues. With temperatures as high as your receiving, I would say it is a lack of water.
Water more and make sure you are watering deep enough. You will need to water twice a day with temperatures above 85 degrees.
Here is a link with care information.