I think it's a bit colder as you go west in Texas. It's erratic, to say the least. I find the warm night temperatures beginning in spring and lasting through summer to be one of the most limiting factors in growing many plants. There's no cool off when the sun sets. In many areas with equally hot day temps, the nights cool off. Mostly that doesn't happen in most of Texas. Any plant that is a derivative of an alpine habitat will likely need something to mitigate the relentless warm temps. I think it's not necessarily the sun. Many alpine plants get lots of exposure to sunlight at high altitudes, but they get a chance to do some repair at night.
