I think that your observations regarding the growth defect of
a leu- mutant is due to the fact that the nitrogen source used in the SD medium
is ammonium sulfate. The major pathway of the ammonium assimilation in yeast
is mediated by NADP-dependent glutamate dehydrogenase whose expression is
strongly decreased in the presence of leucine in external medium (you might
want to look at paper by Yuanming Hu et al, MCB, 15:52-57 (1995) on activation
of GDH1 expression by Leu3p activator).
When the medium is supplemented with leucine (I assume that your
strains are Leu- auxotroph), you can observe a limited growth. More the
external concentration of leucine is high, more cell growth is affected.