Environmental (E) plasticity in partitioning between sugar and
starch
Immediate response to stress depends on an organism’s ability to
plastically adjust trait values to accommodate changing environments.
The degree to which an organism can plastically respond will, in many
causes, determine its ability to survive stress. Here, we found
extensive plastic variation due to environmental response in the
proportion of NSC stores in starch versus sugar (Table 2, Figure 3,
solid black lines). In Clatskanie an average of 20% of total NSC stores
were found in starch, while only half that amount was found in Corvallis
(Figure 3). Clatskanie has a coastal climate with rainfall spread
throughout the year and small temperature differences between winter and
summer (Figure 2). In contrast, Corvallis has a continental climate
which regularly experiences extreme temperatures and long periods
without rainfall (Figure 2). Thus, these results may reflect the
differential enzymatic sensitivities of starch degradation and synthesis
to average climatic conditions at these two sites (Pollock & Lloyd,
1987; Aude Tixier et al., 2019). The warmer average temperatures in
Clatskanie could have led to a higher proportion of NSC being left in
starch while the colder, more stressful conditions of Corvallis resulted
in more sugar storage.
The observed plasticity in proportion of starch storage could also be
driven by the weather patterns on the dates of sampling. We happened to
collect woody tissues on the coldest days recorded over the past 38
years in Clatskanie and extremely cold days in Corvallis (Figure 3). Low
temperatures in Clatskanie fell below -10oC some days
and highs never went above 4oC, within the minimum
temperature range that starch synthesis and degradation enzymes can work
(Pollock & Lloyd, 1987). The sharp drop in temperatures due to the
polar vortex may have halted enzymatic activity entirely. Thus, instead
of starch steadily degrading to sugar as temperatures drop, the quick
temperature change may have prevented starch from degrading further.
Conversely, we travelled to Corvallis after sampling in Clatskanie,
where the temperature reached just above 5oC on our
sample dates; just above the minimum temperature range for enzymatic
activity. Thus, starch may have degraded into sugar in trees at this
site. Such a quick change could be possible given that starch synthesis
and degradation have been observed on diurnal scales (A. Tixier et al.,
2018).
It is difficult to pinpoint whether the plasticity in the proportion of
NSC in starch between the two gardens was attributable to the average
climate of the two sites, or the weather at the time of sampling.
However, there is mounting evidence that this plasticity in the
synthesis and degradation of starch in plants is critical for seasonal
signaling in plants (Gibon et al., 2009; Palacio, Gunter, Sala, Korner,
& Millard, 2014; Aude Tixier et al., 2019). Branch NSC stores begin to
synthesize from sugar into starch as temperatures rise in spring (Furze
et al., 2019; MartÃnez-Vilalta et al., 2016), and this process likely
occurs faster in branches than in roots because branches are exposed to
air and not insulated in the soil. Thus, temperature gradients across
the plant may drive the movement of carbohydrates upward in spring to
support leaf flush and stem growth (Sperling, Silva, Tixier,
Theroux-Rancourt, & Zwieniecki, 2017). This synthesis of starch or
movement of carbohydrates could be the signal plants sense in spring to
break dormancy or initiate leafout. Thus, it may be this plasticity in
the conversion of sugar to starch that is in part driving observed
plasticity in phenological timing, a key trait for future tree
adaptation to climate change (Alberto et al., 2011; Hall et al., 2007;
Keller et al., 2011).