Leave PROBLEM!

Could be many many different things that no one would be able to diagnose with the information you provided, I suggest you spend some time in the Plant Problems forum to learn more on how to read your plants. Usually yellowing leaves is some kind of nitrogen deficiency.

Or da might be getting too hot yo......how close iz your light from da topz.....an du u have a fan on dem....?

the plants are outside, and it has also been up to 100 degrese for the last threee days.

Some of the top things that can cause yellowing

1) lack of nute IE. Nitrogen/magnesium
2) to much nutes IE nitrogen/ magnesium causing a lockout
3) PH being off causing a nute lockout
4) lack off water can cause mild yellowing then death
5) heat also will cause it in extreme cases
6) there are more but these are the most common things

Things to help solve this is flush x3 the volume of your bucket
And start from fresh to solve the ph issues same for over feeding
Nutes.

Under feeding simple add more nutes, start by adding little by little
Old yellow leave dont get green so watch new growth

Heat issue can be a bitch short of a AC unit bringing in fresh air
Is key