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 where by 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 clear latitudinal pattern in the proportion
of starch GxE variation (Figure 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 (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.