This study tested whether leaf elongation rate (LER, mm h−1) and its components – average relative elemental growth rate (REGRavg, mm mm−1 h−1) and leaf growth zone length (LLGZ, mm) – are related to phosphorus (P) concentration in the growth zone (PLGZ, mg P g−1 tissue water) of Lolium perenne L. cv. Condesa and whether such relationships are modified by the arbuscular mycorrhizal fungus (AMF) Glomus hoi. Mycorrhizal and non-mycorrhizal plants were grown at a range of P supply rates and analysed at either the same plant age or the same tiller size (defined by the length of the sheath of the youngest fully expanded leaf). Both improved P supply (up to 95%) and AMF (up to 21%) strongly increased LER. In tillers of even-aged plants, this was due to increased REGRavg and LLGZ. In even-sized tillers, it was exclusively due to increased REGRavg. REGRavg was strictly related to PLGZ (r2 = 0.95) and independent of tiller size. Conversely, LLGZ strictly depended on tiller size (r2 = 0.88) and not on PLGZ. Hence, P status affected leaf growth directly only through effects on relative tissue expansion rates. Symbiosis with AMF did not modify these relationships. Thus, no evidence for P status-independent effects of AMF on LER was found.