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).