Experimental Evidence Questions the Relationship between Stress and Fluctuating Asymmetry in Plants

The eco-evolutionary theory of developmental instability predicts that small, non-directional deviations from perfect symmetry in morphological traits (termed fluctuating asymmetry, FA) emerge when an individual is unable to buffer environmental or genetic stress during its development. Consequently, FA is widely used as an index of stress. The goal of the present study was to experimentally test a seemingly trivial prediction derived from the theory of developmental instability—and from previous observational studies—that significant growth retardation (which indicates stress) in plants is accompanied by an increase in FA of their leaves. We induced stress, evidenced by a significant decrease in biomass relative to control, in cucumber (Cucumis sativus), sweet pepper (Capsicum annuum), and common bean (Phaseolus vulgaris) by applying water solutions of copper and nickel to the soil in which plants were grown. Repeated blind measurements of plant leaves revealed that leaf FA did not differ between stressed and control plants. This finding, once again, demonstrated that FA cannot be seen as a universal indicator of environmental stress. We recommend that the use of FA as a stress index is discontinued until the scope of the developmental instability theory is clarified and its applicability limits are identified.