Research paperFluctuating asymmetry of birch leaves did not increase with pollution and drought stress in a controlled experiment

- Fluctuating asymmetry (FA) is widely used as indicator of environmental stress.
- We experimentally studied impacts of environmental stressors on birch seedlings.
- Drought and heavy metals decreased seedling growth and photosynthetic efficiency.
- Both these stressors had no statistically significant effect on leaf FA.
- Leaf FA may be less sensitive to stress than plant growth or photosynthesis.

Fluctuating asymmetry (FA), defined as small, non-directional deviations from perfect symmetry in morphological characters, is often recommended as a handy indicator of environmental stress. A reliance on observational data to provide empirical evidence for the effects of different stressors on FA, in combination with increasing attempts to use FA in environmental research, underscore the need for careful examination of the relationships between environmental stress and FA. We experimentally tested the hypotheses that (i) heavy metal and drought stress increase the leaf FA in plants, and that (ii) plants persisting in heavily polluted sites possess greater stress tolerance and therefore show smaller increases in leaf FA in response to heavy metals than do plants from unpolluted sites. We collected mountain birch, Betula pubescens subsp. czerepanovii, seeds from eight polluted and ten unpolluted sites and reared the seedlings in a sophisticated greenhouse (phytotron). We compared leaf FA between control seedlings, seedlings irrigated with water containing copper and nickel sulphates, and seedlings exposed to drought. Leaf FA showed no response to either heavy metals or drought, despite significant impacts of these treatments on seedling height, leaf size and photosynthetic efficiency. This FA result was independent of the level of pollution at the site of seed origin and consistent for FA values based on low and high accuracy measurements of leaf width, as well as for FA values based on measurements of the widths and areas of leaf halves. Our findings add to accumulating evidence regarding inconsistent relationships between FA and abiotic stress, thereby questioning the indicatory value of FA. We strongly recommend that the use of FA as an indicator of environmental stress be limited to study systems for which the existence of cause-and-effect relationship between the stressing impact and the changes in FA is confirmed by controlled, blinded experiments.

Drought and heavy metals had no statistically significant effect on leaf FA in mountain birch, Betula pubescens subsp. czerepanovii, despite significant impacts of these treatments on seedling height, leaf size and photosynthetic efficiency.65