But what if the plantlife has photosynthesis triggered by a different spectrum than the green of our current world? You could create a symbiosis relationship with a parasitic moss that feeds primarily on proteins that ooze in from the decomposition of the existent plant life, and then it emits a chemical compound that causes the correct spectrum of light for the plants it feeds on with a chemical chain reaction. If the plant life is say, overall dark purple leaf, it will also soak up more light than a lighter leaf surface plant. Adapting better to limited light. So, chlorophyll goes through a phase of fluorescing as it moves from neutral to excited. So maybe if when the parasitic fungal plant feeds on its desired non-light based proteins it triggers a fluorescent phase as its transition, and this light happens to be the correct spectrum for the primary edible plants to trigger their own chlorophyll reactionary phase, and maybe they also fluoresce on the transitional activation, creating this rippling wave of dim light that cascades over the cavern, moving back and forth like the breathing of an enormous collaborative lung. If any plant is removed from this biosphere, it will fail, requiring the teamwork of these specifically chosen plants that work together in this fashion. All of the plants have naturally darker leaves that soak up more light that comes into contact, requiring less light per day as the effect fades as the catalytic plant runs out of its favourite protein. As long as the gardener comes by and feeds that starter plant, though, enough light will ripple through the growth cavern to keep all of the cave-growing plants alive. Loss of that trigger plant will destroy the whole biosphere and it must be cared for as its glow is a vital catalyst. Perhaps alternatives have been designed that glow in a similar spectrum, to greater or lesser results. Does it throw off the heat balance of the growing cavern to have a glowing brazier? Is it possible to use the venting system from the forge to run a pipe through the underground lake, causing steam to reach an ideal rainforest-like climate for the fruiting room? Is all of the corn purple or blue?
...Wow, I don't know where that came from. Uh. I draw fantasy maps, but the most isometric thing I've done is my signature graphic and well, yeah. Not exactly what you want. My fantasy map portfolio!