Defoliate yellow leaves?

To me, these look like older leaves that are not receiving enough direct light. You should remove them once they have gone yellow, for if you leave them, this could invite fungal/mould to form. My advice is to remove any yellow leaf and never remove any healthy green leaves, for it is these healthy green leaves that are producing the energy required by your plant to grow. When the leaves go yellow for whatever reason, stored vital elements in the leaves will be "recycled" by the plant to be used for new growth. Premature removal of healthy leaves robs the plant of these stored elements (carbs, sugars, amino acids etc), meaning the plant will have to "re-manufacture" the previously store elements anew, denying energy away from growth/flower production. Defoliation to increase energy going to flowers is the biggest MYTH in the cannabis cultivation world. Removing healthy leaves, lowers available energy production and can lead to smaller flowers for the reasons stated earlier. Energy in a plant can not be "channelled", all energy is used in all areas of growth equally. Cannabis has been evolving for Millenia, it has not evolved to dropping healthy leaves when flowering. In fact, more leaves equals bigger and healthier flowers due to maximum energy production (photosynthesis). Reducing a plants ability to photosynthesize by removing leaves, starves the plant of energy, resulting in smaller and weaker plants. It does no harm to leave the leaves, most of the light will pass through to the lower leaves anyhow. There is no way the small leaves that grow within the flowers can produce enough energy for optimum growth, as compared to just one large and healthy "fan" leaf; the energy makers for the plant. Hope this helps... Organoman.