Look, I know this might fit into the category of "This would be cool", but honestly, isn't this how a lot of elements were came up with?
When PLNT comes in contact with PHOT, if the photons aren't at too high a temperature, it gives off oxygen. Also, PLNT can absorb CO2, which helps it grow similar to how water does. Thus, this brings plants from being merely a nice green decoration to a biological light detector.
If you don't want this all the time, or if this breaks anything, then make it an option to turn it off.
What's the consensus? Decent idea, or horrible?
Its an decent idea.
A few things:
1. People are going to have different opinions other than just the two you offered
2. Most ideas people give are also given with the ability to be made a option, to be turned on or off.
3. People have suggested this, or at least something similar, before: https://powdertoy.co.uk/Discussions/Thread/View.html?Thread=16901 https://powdertoy.co.uk/Discussions/Thread/View.html?Thread=14973 https://powdertoy.co.uk/Discussions/Thread/View.html?Thread=11147
4. This could ruin, or mess up, a few saves. Even if it is just a few, it would be something someone else spent their time on. I dont believe it would be right, or worth it.
PLNT is already able to absorb CO2, and releases oxygen shortly afterwards. But at the moment, this has no effect on growth, and photons are not involved.
Changing the current behaviour too much could break saves which use PLNT as a CO2 -> oxygen converter. However, most of these probably don't involve PHOT hitting PLNT, so PHOT+PLNT could perhaps be made to do something (such as growing).
"an option to turn it off" - making an option for every single addition is impractical, and would make TPT even more confusing to use. Options are generally only added for new simulation features (such as ambient heat), not for element modifications.