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).