Influences of ploidy level and population of origin on trembling aspen growth and physiology

Background/Question/Methods: Polyploidy is an important phenomenon in plant ecology, and polyploids are typically formed rarely in natural populations. However, quaking aspen (Populus tremuloides) populations in the south-central Intermountain West of the United States have a high proportion of naturally occurring triploids (>50% in some locations). In a different population of aspen, located in the Great Lakes region of the U.S, triploids are present at substantially lower frequencies (approx. 2%). Aspen triploid ramets are known to have larger mass than diploid ramets, but factors contributing to a difference in growth rate between ploidy levels are not known. We performed multiple growth studies on diploid and triploid aspen originally from both the Intermountain West and Great Lakes populations in three settings: greenhouse, field, and outdoor cold frames. We measured tree growth, photosynthesis, respiration, and biomass allocation.

Results/Conclusions: Overall, triploids tended to grow faster than diploids. For example, by the end of the field-based study, triploid genotypes were 50% larger than diploids genotypes when controlled for initial plant volume. However, in the greenhouse study ploidy was insignificant to mass acquisition, and in the cold frame study ploidy was only marginally significant. In some studies, triploids had higher photosynthetic rates than diploids and allocated more biomass to leaves than diploids. These patterns were not consistent among all studies and measurement windows. Although triploids grew faster than diploids, ploidy had less impact on plant final size than did population of origin. Compared to those from the Great Lakes, genotypes from the Intermountain West grew more slowly, had lower leaf mass ratios, thicker leaves, lower foliar nitrogen and condensed tannin concentrations, and higher phenolic glycoside concentrations in one or more studies (p<0.05 for all effects). Overall, these studies indicate that a triploid growth advantage may be explained by physiological factors such as allocation to leaves or gas exchange rates, but influences of ploidy level on tree growth were less pronounced than those associated with population of origin.