It is normal for the bottom leaves of your mater plant to yellow and die. They are old and tired.

Keep an eye on things because other things can happen. If you start seeing lesions, spots, etc. then it may be another problem.

100 degree temps are not good for maters. They pretty much quit doing anything except survive.

Make sure when you water that you do so with the intention of soaking the garden to a depth of 10-12 inches. Water long and deep - not shallow.

Good luck,

Gary