Desert cacti need cooler night temperature to process CAM. However tropical cacti like Melocactus and Discocactus belong to areas where night temperature don't fall too much (like Bahia in Brazil where few melos belong to had the max temp of 29C and low of 24C yesterday). Does this mean that they are equipped to process CAM in such high temperatures?
I am asking because I live in a tropical place and have similar weather like Bahia. I often do some kind of arrangements to cool down my desert cacti in night for them to effectively conduct CAM but I have left my Melos and Discos outside without any special arrangements.
Melos & Discos Night Temperature
Melos & Discos Night Temperature
Tropical weather, no winters! 
Re: Melos & Discos Night Temperature
If some cacti are not OK with current temps they will just stop their growth or metabolism for some time. Not sure about Melocactus bt for Discocactus at my house I don't provide special conditions, keep them just like other frost-tender cacti.
If your cacti mess in your job just forget about the job.
°C = (°F - 32)/1.8
°C = (°F - 32)/1.8