Fluctuating asymmetry in plants: Is it a general indicator of stress
We normally expect most plant leaves to be bilaterally symmetric. That is the ‘idealized’
phenotype. Developmental asymmetry is the result of deviations from the ‘normal’ developmental
program that results in ‘errors’ in the symmetrical expectation in the structures and dimensions of
leaves (or other plant parts, and can also be observed in animals at both molecular and organ structural
levels). We expect that development involves various levels of feedback that ‘normally’ produces
symmetry between left and right halves of bilaterally symmetric plant parts, e.g. leaves. Stress of
almost any sort (drought, various chemical stressors, potentially even severe competition during
development) may disrupt those feedbacks that maintain parallel development, and lead to asymmetry.
The asymmetry is not likely to be consistent from leaf to leaf, since it results from a disruption of
normal pattern. The end result is called fluctuating asymmetry, since it varies from leaf to leaf over the
stressed plant. There may be disruptions at higher levels, e.g. plant life histories, but the stress that
would alter plant structure and development at these higher levels must be far more severe.
Measurement of leaf asymmetry should be a more sensitive indicator, evident at much lower levels of
stress.
Methods
We will measure dimensions of leaves that ought to indicate asymmetry if present. For each
species we should measure at least three leaves on each of five plants (minimum numbers both for leaf
and plant) of a few species in areas (habitat conditions, a measure of soil moisture unlikely after
Tuesday’s rainfall) that can be designated as having higher stress and also in areas having lower stress.
Since, so far as I know, developmental asymmetry has not been assessed in any of these species, it is
hard to predict what we will find. Initial suggestions for species to measure are: 1) a morning glory or
bindweed species, either an Ipomoea or a Convolvulus, 2) an oak (Quercus) and 3) a thistle species,
most likely is Canada thistle, Cirsium arvense. Each has leaves with structural patterns that make
measurements of asymmetry relatively straightforward. For each species different specific
measurements will be used appropriate to the leaf ‘design’.
For the morning glory there is a history of asymmetry measurement of a different species
common in the southeastern United States (Freeman, et al. 2005). The measurements used in the
earlier study are shown on the figure below. They were left and right widths 1 cm from the leaf tip (B
and C) and left and right lobe depth (E and F). Convolvulus is a much more common plant, and very
similar measurements can be made.
For oak leaves different measures look promising to measure fluctuating asymmetry. Here the lengths
of right and left hand lobes appear to indicate asymmetry. To avoid keying on one set of lobes, I
suggest measuring the lowest and highest complete left and right hand lobes, as indicated in the
scanned oak leaf shown below.
For Cirsium arvense again measuring the lengths of lobes appears to offer the greatest potential to
detect fluctuating asymmetry. In scanning this leaf, there was some difficulty getting its many lobes to
all lie flat simultaneously. The suggestion is to measure the length of the lowest complete lobes (and
possibly the lobes just below the tip region) along the axis defined by the midline vein on both the left
and right sides of the leaf. Below is a scan to indicate these measurements:
Remembering that the idea is that fluctuating asymmetry should increase with increasing
level/intensity of stress or disturbance, we need some independent measure of stress. This does not
need to be a quantitative measure; it can be a scale measure. I suggest we use a classification into
higher stress and lower stress (though exactly how we decide what constitutes higher and lower levels
of stress is not clear).
Analysis
We’ll use the methods developed by Freeman et al. (2005). They first note that leaves
contribute to their own grown and expansion. Therefore any size-related measure needs to be corrected
for size itself. This correction is done by taking natural logs of measures. What we therefore calculate
as fluctuating asymmetry is:
FA = |ln(left) – ln(right)|
(The vertical lines mean that we use an absolute value, as opposed to a measure which could be
positive or negative). Since this measure will comprise only half a normal distribution, there is a
standard technique in statistics to fix that. The method is called a Box-Cox transformation. The
calculation is:
FA’ = (FA + .00005)1/3
We can then perform an ANOVA of fluctuating asymmetry as the dependent variable and an index of
disturbance level/intensity as the predictor variable.
Bibliography
D. Carl Freeman, M. L. Brown, J. J. Duda, J. H. Graham, J. M. Emlen, A. J. Krzysik, H. Balbach, D.
A. Kovacic, J. C. Zak. (2005) Leaf fluctuating asymmetry, soil disturbance and plant stress:
a multiple year comparison using two herbs, Ipomoea pandurata and Cnidoscolus stimulosus.
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D. Carl Freeman, M. L. Brown, J. J. Duda, J. H. Graham, J. M. Emlen, A. J. Krzysik, H. Balbach, D.
A. Kovacic, J. C. Zak. (2004) Developmental instability in Rhus copallinum L.: Multiple
stressors, years, and responses. Int. J. Plant Sci. 165:53-63.
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Anne Premchand, F. Mawri, S. Gladstone, D. Carl Freeman (1998) Is fluctuating asymmetry a reliable
biomonitor of stress? A test using life history parameters in soybean. Int. J. Plant Sci. 159:559565.