Folhas amarelas caindo, início de floração

Nothing to worry about, those lower/inner leaves just don't get enough direct light to be useful to your plant anymore. Your plant will translocate most of the goodness, like carbs, sugars, amino acids etc from those leaves to use elsewhere where there is growth occurring and is a natural process. Plant looks happy and healthy otherwise. It is not a water/temp/pH issue, just normal cannabis efficiency in practice. Thankfully you have not defoliated this pant, otherwise it would have to use some of its energy to make new carbs, amino acids, sugars etc from new, and not be able to put 100% of its energy into growth alone. Have you considered doing some LST? Tying some of the branches down, to open it up like a rose, would allow more light into the centre of the plant, increasing the size of the inner flowers dramatically. Bricks and some wide string/tape work well, wire can cut into the stems and kill the branches. Hope this helps, Organoman.