Impacts of heatwaves and drought on foliar soluble sugars and
inorganic ions
Using an HPLC-RID system, we quantified the monosaccharides: xylose,
arabinose, rhamnose, fructose, mannose, galactose, and glucose; the
disaccharides: sucrose, maltose, and trehalose; and the oligosaccharide
raffinose. However, co-elution of xylose and arabinose, glucose and
galactose, and maltose and trehalose resulted in unresolved peaks and
therefore, data for these sugars are presented as xylose+arabinose,
glucose+galactose, and maltose+trehalose. In both species,
maltose+trehalose was not detected at any of the sampling dates and
raffinose (and rhamnose in spruce) was only detected in trace amounts in
August 2017 (Fig. 4, 5, S4, S5). Additionally, mannose was only present
in the birch tissue. A major difference between the two species was in
the type of sugar they accumulated in greatest concentration.
Prior to the first heatwave in 2016, there were no statistical
differences among treatment groups of either species in soluble leaf
sugars. Soluble sugars in spruce were minimally affected by one season
of heatwave and drought stress where statistical differences in sugar
concentration were only between the HD and H plants in
xylose+arabinose content (Fig. 4b, S4b; p < 0.05HD vs H ). In June 2017, all treatment groups had similar
foliar sugar concentrations. Fructose and glucose+galactose
concentrations were highest at this time whereas sucrose content was
relatively low compared to other sampling dates (Fig. S4e). Two seasons
of HD stress resulted in an > 70% increase in
fructose compared to the C and H treatments (Fig. 4g;p < 0.05 HD vs C ; p <
0.05 HD vs H ) and less xylose+arabinose than the Htreatment (Fig. 4h, S4h; p < 0.05 HD vsH ). In 2017, shifts in foliar sugar concentrations occurred from
fructose and glucose+galactose as the dominant sugars early in the
season to sucrose being most abundant later in the season for all
treatment groups (Fig. S4g). Xylose+arabinose also accumulated in all
the treatment groups by the end of 2017 season (Fig. S4f).
Soluble sugar concentrations in birch were impacted more by the stress
treatments than they were in spruce, especially after the first season
(Fig. 5, S5). Although there were differences among treatment groups in
fructose, glucose+galactose, sucrose, and xylose+arabinose, none of the
stress treatments significantly differed from the C plants (Fig.
5c-d, S5c-d). Instead, differences were found only between the Dand H plants where the D plants produced more fructose and
glucose+galactose (fructose, p < 0.05 D vsH ; glucose+galactose, p < 0.05 D vsH ), but less sucrose and xylose+arabinose than the Hplants (sucrose, p < 0.05 D vs H ;
xylose+arabinose, p < 0.01 D vs H ). At
this time, the combined HD plants exhibited concentrations
between that of the D and H plants (Fig. 5c, d). Similar
to the spruce, the only statistical difference in sugar concentration at
the end of the second season was in xylose+arabinose where the HDplants produced 21% less than the C plants (Fig. 5h; p< 0.05 D vs H ). Similar to spruce, in birch
also, shifts in foliar sugar concentrations occurred from fructose and
glucose+galactose as the dominant sugars early in the season to sucrose
being most abundant later in the season for all treatment groups in 2017
(Fig. S4g).
The most abundant soluble (in 5% PCA) inorganic ions in both spruce and
birch foliage tended to be Ca and K (Fig. S6). At the start of 2016,
there were no differences in the concentration of soluble inorganic ions
among treatment groups apart from K in the birch. By the end of the
first season in spruce, only few statistical differences in inorganic
ion concentration were found among treatment groups. Specifically, theHD plants had nearly 50% more Mg than the H plants (Fig.
6b; p < 0.05 HD vs H ), but no differences
from the C plants were found. However, at the start of the second
season, several statistical differences were found among the spruce
treatment groups, but the only unique response observed in the HDplants was in a 20% reduction in P compared to the C plants
(Fig. 6c; p < 0.05 HD vs C ). Calcium was
reduced in the D plants compared to the H plants (p< 0.05 D vs H ), Mg was 13% lower in Dplants compared to C (p < 0.05 D vsC ), K was elevated in the D plants compared to both theH and HD plants (p < 0.01 D vsH; p < 0.01 D vs HD ), and Fe was reduced
by 32-36% in the D , H , and HD plants compared to
the C plants ( p < 0.05 D vs C ;p < 0.05 H vs C ; p <
0.05 HD vs C ). By the end of the second growing season,
Ca, Mg, and Fe concentrations no longer differed among treatment groups
(Fig. 6d). However, K was elevated by 42% in the H plants
relative to the C and D (p < 0.05H vs C; p < 0.05 H vs D ), P was
elevated in H plants relative to the D and HDplants (p < 0.05 D vs H; p <
0.05 D vs HD ), and Mn was nearly doubled in the Dplants relative to the C and H plants (Fig. 6d, S6d;p < 0.01 D vs C; p < 0.05D vs H ).
Inorganic ion concentrations in birch were largely unaffected by
recurrent heatwave and drought stress (Fig. 6, S6). After the first
season of stress, the D treatment resulted in >150%
increase in Fe compared to the C and H treatment (Fig. 6f;p < 0.01 D vs C; p < 0.05D vs H ) and 50-150% increase in Mn compared to theC , H , and HD treatments (Fig. 6f; p< 0.001 D vs C; p < 0.001 D vsH; p < 0.01 D vs HD ). Potassium
concentrations were reduced by 18% in the H plants and 20% in
the HD plants relative to the C plants at the start of the
second season (Fig. 6g; p < 0.05 H vs C;
p < 0.01 HD vs C ). No other differences among
treatment groups were found at this time or at the end of the season as
well (Fig. 6g, h).