The balance between yeast and lactobacilli (LAB) influences the sourness. More LAB means more acid producers, so a more sour bread.
When feeding the motherdough, influence the balance with:
Maturity is about when to use the mother culture for the dough. A dough should be active when used for baking. Use it at it's peak rise for less sourness, or let it shrink again to develop more LAB and increase sourness.