Genotype-by-environment (GxE) plasticity in total stores, but not
in proportion of stores in starch
Genetic variation in environmental response could provide a key
mechanism through which populations can evolve a more adaptive response
to future environmental stress. Here, we found that plastic variation in
total storage is almost entirely comprised of genotype-by-environment
interactions. The average total NSC storage concentrations between the
two gardens only differ by 0.5 mg•g-1, but individual genotypes differ
by -3.4 to 3.3 mg•g-1 between the two gardens. This flexibility amongst
genotypes may be the result of intrinsic differences in response to
environment (Des Marais et al., 2013) and could explain why previous
studies have failed to converge on a consistent response to stress to
date (Adams et al., 2017; Thalmann & Santelia, 2017). These works have
found that NSC concentrations may decrease, increase, or remain stable
in response to stressors such as drought and extreme temperature. Thus,
our work underscores the importance of controlling for genetics when
conducting any NSC manipulations, as the high heritability and GxE of
the trait can confound interpretation of results.
In contrast, plastic variation in the proportion of NSC stores in starch
is almost entirely attributed to differences in environment and not GxE
interactions. This finding further supports a model whereby the amount
of NSC residing in starch is driven by intrinsic enzymatic environmental
limits. While GxE for this trait is lower than the amount of GxE for
total NSC stores, there is a slight latitudinal pattern in the
proportion of starch GxE variation (Figure 4, S3). The trending
correlation between latitude and the proportion of starch RDPI suggests
that some genotypes may be more plastic than others in their ability to
move between sugar and starch, however it is not significant (Figure 5).
In particular, genotypes from northern populations appear to have more
flexibility between the two gardens. Put another way, northern genotypes
are more responsive to environmental differences between the two sites.
One possible explanation for this trend is that their starch degradation
enzymes may be more sensitive to temperature fluctuations or are able to
continue to act at slightly lower temperatures than those from southern
populations. This may be beneficial to trees in northern latitudes
experiencing extreme temperature lows and large temperature swings
throughout the day. Although the amount of variation explained by GxE
interactions may be small, the importance for immediate environmental
response may be very important and is worth further investigation.