Ploidy level of white ash trees influences water relations in a common garden
Different degrees of ploidy within plants can produce morphological and physiological differences in response to local environments including larger xylem vessels and larger stomata due to increased cell size. It is predicted that larger vessels among polyploids may allow for higher hydraulic conductance relative to diploid progenitors under wet conditions, yet this could lead to enhanced embolism with subsequent blockage of water transport under dry conditions. Furthermore, if water-use strategies differ among cytotypes within a species, then this should be reflected in leaf-level physiological responses under common conditions. Unfortunately, the physiological characteristics of different cytotypes are not well understood at the leaf level. Moreover, because ploidy is often closely correlated with geographical distribution, the effects of genome duplication versus population origin on physiological functioning can be confounding, and few studies have been able to tease apart these variables. Here, we report on the physiological characteristics of diploid, tetraploid, and hexaploid white ash trees, whereby all cytotypes are represented within populations originating from diverse locations. Additionally, all measurements are made within a common garden in Lawrence, KS. Using this unique approach we were able to control for between-population level variation and isolate the effects of ploidy level on key physiological traits.
We observed significant differences in midday leaf water potential (Ψw) across cytotypes nested within three white ash populations (Overton, TN; Jackson, IL; Hopkins, KY). Average Ψw became increasingly more positive as ploidy level increased. For example, average Ψw for individual trees sourced from Hopkins, KY was -2.8, -2.4, and -1.9 MPa for diploids, tetraploids, and hexaploids, respectively. These results suggest that there may be differences in water stress among cytotypes that should be reflected in leaf-level physiology. Interestingly, we found that this pattern does not hold for leaf-level gas exchange, including photosynthetic rate, stomatal conductance to H2O, and transpiration. Additionally, leaf δ13C and the concentration of proline did not vary significantly across cytotypes nested within populations. The pattern of decreasing Ψw was observed during an above-average wet year at this site and this pattern was not reflected in leaf-level physiological traits; thus, these results suggest that differences in Ψw among cytotypes are likely driven by underlying hydraulic mechanisms rather than strictly leaf-level responses. Our future results will incorporate responses during a dry year. In conclusion, we show that ploidy level can influence the level of water stress within plants, although this does not appear to be driven by leaf-level processes.