Indeed it's not really clear. I added one line about this in the Respiration section, maybe it's not enough and could be better explained but I don't want to overload the documentation (i'll do a complete guide later). Thanks for pointing this out.
It'd probably be considerably better if there was an indicator for the amounts of sun/water needed to up the production, or if there were floating point numbers here, rather than only integral amounts of sap being produced.
I chose to use natural numbers to represent the photosynthesis (it seemed to me more logical) but indeed it's disturbing and finally with floating point numbers it will be much clearer. I'll try it (currently I am a little overloaded :/ ). Thanks!