The effects of bars on abundance gradients has recently been studied extensively in particular by Martin, Roy, Walsh, Belley, and Julien. The bottom three panels of Fig. 2 show the tendency for barred spirals to possess shallower gradients. The paper by Martin & Roy (1994) further relates gradient slope to bar strength, a quantity which measures bar ellipticity. They find direct relations between the slope of the oxygen abundance gradient of a barred spiral and the galaxy's bar strength (ellipticity) and length. This empirical result is consistent with radial flow models of chemical evolution in which the presence of a bar enhances large-scale mixing over the galaxy's disk, damping radial abundance variations.
A negative vertical gradient in O/H in the Milky Way is suggested by planetary nebula studies. Abundance data compiled by Kaler (1980) for PNe ranging in height above the disk from less than 0.4 kpc to greater than 1 kpc show a decrease in O/H with increasing height above the plane. A comparison of more recent studies of PNe close to the plane (Perinotto 1991), greater than 300pc above the plane (Cuisinier et al. 1996), and in the halo (Howard & Henry 1997) shows averages of 12+log(O/H) for these three samples of 8.68, 8.52, and 8.02 respectively, qualitatively consistent with Kaler.
Thorough tests for azimuthal gradients in spiral disks have yet to be carried out. One example of apparent O/H asymmetry is discussed by Kennicutt & Garnett (1996) in their study of M101. They find that H II regions located along a spiral arm southeast of the major axis have a lower oxygen abundance by 0.2-0.4 dex compared with H II regions on the opposite side.
Global metallicities in low surface brightness galaxies are generally found to be subsolar by roughly a factor of three, according to McGaugh (1994), indicating that these galaxies evolve very slowly and form few stars during a Hubble time. Apparently, they also lack detectable gradients. This, despite the fact that these objects are similar in mass and size to prominent spirals defining the Hubble sequence. McGaugh suggests that a galaxy's environment and surface mass density are more relevant to galaxy evolution than gross size.
Effects of cluster environment on the chemical evolution of galaxies has been investigated by Skillman et al. (1996), who studied oxygen profiles in several Virgo spirals representing a range in H I deficiency (taken as a gauge of cluster environmental interactions). Their results imply that global metal abundances in disks tend to be higher in stripped galaxies, presumably because reduced infall of metal-poor H I gas means less dilution of disk material. Henry et al. (1996 and references therein) investigated metallicity and heavy element abundance ratios (N/O, S/O) in three cluster spiral disks with normal H I and found no clear signatures of environmental effects. Thus, cluster environment alone is apparently not a sufficient condition for altered chemical evolution.
The mathematical form of abundance profiles in spiral disks, has been investigated recently by Henry & Howard (1995), who fit line strength behavior over the disks of M33, M81, and M101 using photoionization models. Their best fits for O/H versus galactocentric distance were produced using exponential profiles, although power law forms could not be ruled out. However, linear profiles poorly reproduced the observations. Henry and Howard also concluded that there is currently no strong observational case for gradient flattening in the outer parts of some disks, although such flattening has been proposed by several authors (see Mollà et al. 1996).