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.