Experiment 1: ploidy effects on leaf chemistry and its
response to stress
Ploidy affected concentrations of all chemical compounds when averaged
across genotypes, but differences were subtle (Fig. 2, left panels).
Most pronounced were differences in total salicinoid phenolic glycoside
concentrations, which were 9% higher in triploid than in diploid trees.
Differences in total phenolic glycoside concentrations could be
attributed mainly to variation in tremulacin levels. Triploid trees
exhibited 17% higher tremulacin concentrations and differences in
ploidy explained 54% of the variation in tremulacin (Appendix 2, left
panel). Levels of condensed tannins and foliar nitrogen differed
significantly but only slightly between the two cytotypes.
Total phenolic glycoside concentrations differed between defoliated
triploid and diploid trees when averaged across genotypes, as well as
when accounting for genotypic variation (Fig. 2). These ploidy ×
defoliation interactions were driven by decreasing levels of phenolic
glycoside concentrations in diploid, defoliated trees that were watered
while triploid trees showed no response to defoliation. When not
accounting for genotypic variation, ratio-based follow-up analyses
showed that defoliated triploid trees expressed 6% higher total
phenolic glycoside levels compared with undefoliated trees. In contrast,
diploid, defoliated trees had 3% lower levels compared with
undefoliated trees (p=0.039, η2=0.68). Differences in
stress treatment ratios between cytotypes also remained significant when
accounting for genotypic variation (p=0.049, η2=0.07).
Most other ploidy × defoliation interactions that were observed for
individual phenolic glycosides could not be confirmed in the follow-up
analyses. The only exception was the interaction between ploidy and
defoliation for tremulacin. Drought stress did not affect concentrations
of total phenolic glycosides. It did, however, lead to increased levels
of the less abundant phenolic glycosides, tremuloidin and salicin,
especially in defoliated trees (Appendix 2).
Condensed tannin concentrations increased by 25% under drought stress
and by 56% in response to defoliation. Nitrogen levels increased by
11% under drought stress, but were unaffected by defoliation. Diploid
and triploid trees did not differ in condensed tannin or nitrogen
responses to stress. However, genotypic differences did significantly
affect the variation of both traits (Appendix 6).