Foliar DMSO:DMSP ratio and metal content as potential indicators of stress in Spartina alterniflora

Physiological responses can occur long before there are visible signs of stress. This study evaluated two potential indicators of stress (the ratio of dimethylsulfoxide to dimethylsulfoniopropionate (DMSO:DMSP) and foliar metals) in Spartina alterniflora collected from areas affected by 4 different disturbances (sudden marsh dieback, horse grazing, increased snail densities, wrack deposition) across 20 marshes in GA.  The DMSO:DMSP ratio was a stronger and more consistent indicator of stress than either DMSP or DMSO concentrations alone, with significantly higher ratios occurring in leaves and stems collected from affected as compared to healthy areas at all four disturbance types.  Foliar metal concentrations also differed in affected as compared to healthy areas: concentrations of 19 of 20 metals evaluated were increased in leaves collected from edge and affected areas.  Multidimensional scaling using the entire suite of metals showed clear separation between plants from affected and healthy areas, but no difference among disturbance types.  In contrast, chlorophyll a concentrations were not significantly different between affected and healthy areas, and did not correlate with variation in either of the two indicators.  These results suggest that the DMSO:DMSP ratio and foliar metal suite are sensitive indicators of sublethal stress in Spartina, capable of identifying stress before there are visible signs such as chlorophyll loss.  The fact that both indicators were consistent across a variety of disturbance types suggests that they may be primarily responsive to general oxidative stress and thus, broadly useful tools for evaluating the health of salt marsh habitat in the field.