With basic shading in place, now add the rougher texture by drawing small dark areas that represent the "valleys" on the skin surface. You don't have to be exactly accurate to every valley on the lemon photo, just pick out the major valleys that you see. As you add "valley shadows", you may have to darken the main shadowed area of the lemon. Don't make the valleys too dark, because that will make the valley look deeper than it is. The end result will make your shaded lemon skin look rougher and more "lemon-like".