Leaf fluctuating asymmetry is not a reliable indicator of stress

Stress experienced during development in organisms with bilateral
structures could result in developmental instability, which is expressed
as subtle non-directional deviations from perfect symmetry, known as
fluctuating asymmetry (FA). As such, FA has been proposed, and extensively
used, as a trait indicating stress for many organisms with bilateral
structures and many types of stress. However, while this concept may apply
to animals, the evidence for plants' main vegetative structures,
i.e., leaves, remains equivocal, and a comprehensive synthesis on this
topic is still missing. We designed observational field and controlled
greenhouse studies, combining different growth and leaf forms across
multiple stress gradients, comprising 21 species and 80 populations. We
measured FA as the difference between the left and right area of the leaf,
an approach that accommodates diverse leaf forms. We used high-precision,
blind, single-person measurements and tested for other forms of symmetry
and the effect of leaf size. We further complemented our study with a
systematic literature review of FA in plant leaves, compiling 51 studies
comprising 72 species, 23 stress types, and 131 unique entries (species ×
stress type).  We consistently found no effect of stress on leaf
FA in any of the studied species in both our field and experimental
gradients. In the systematic literature review, only 39% of the unique
entries showed the expected increase in FA with stress, 53% showed no
effect, 9% showed an opposite trend of a decrease in FA with stress, and
1% showed an unimodal relationship. Importantly, only 40% of all entries
fulfilled the crucial step of controlling for a high-precision
measurement, and of these 49% reported the expected increase of FA with
stress.  Both the results of our observational and experimental
approaches and the systematic literature review failed to support a clear
relationship between stress and FA in plant leaves. These results clearly
show that FA in plant leaves cannot be used as a reliable trait indicating
stress during development.