Is fluctuating asymmetry a reliable biomonitor of stress? A test using life history parameters in soybean

Deviations from a symmetrical state (developmental instability) are often used as biomonitors of stress and are reportedly impacted at concentrations less than those required to impact life history features. Here. we evaluate the use of fluctuating asymmetry (deviations from bilateral symmetry) of soybean leaves across a gradient of salinity ranging from 0 to 9 g NaCl/L. We used a broad gradient to determine if measures of fluctuating asymmetry were more sensitive than survivorship, growth, and reproduction. All plants receiving less than 3 g NaCl/L survived, and reproduced. with no differences among these treatments for any life history feature. In contrast, all plants receiving more than 3 g NaCl/L produced no offspring and died. Thus, we clearly examined the response of plants subjected to sublethal doses. Estimates of fluctuating asymmetry did not differ among the 0 to 900 mg NaCl/L treatments. However the standard error about the regression coefficients of the cumulative internode length vs. node number, an alternative measure of developmental instability, increased significantly under stress. Leaf shape also changed; however, this is not a measure of developmental instability because it does not represent deviations from a developmental invariant. Thus, fluctuating asymmetry failed to identify stress before life history parameters were affected, although alternative measures of developmental instability proved more sensitive in soybean.