  We investigate the evolution of hydromagnetic perturbations in a small
section of accretion disks. It is known that molecular viscosity is negligible
in accretion disks. Hence, it has been argued that Magnetorotational
Instability (MRI) is responsible for transporting matter in the presence of
weak magnetic field. However, there are some shortcomings, which question
effectiveness of MRI. Now the question arises, whether other hydromagnetic
effects, e.g. transient growth (TG), can play an important role to bring
nonlinearity in the system, even at weak magnetic fields. Otherwise, whether
MRI or TG, which is primarily responsible to reveal nonlinearity to make the
flow turbulent? Our results prove explicitly that the flows with high Reynolds
number (Re), which is the case of realistic astrophysical accretion disks,
exhibit nonlinearity by best TG of perturbation modes faster than that by best
modes producing MRI. For a fixed wavevector, MRI dominates over transient
effects, only at low Re, lower than its value expected to be in astrophysical
accretion disks, and low magnetic fields. This seriously questions (overall)
persuasiveness of MRI in astrophysical accretion disks.
