Plant genome size, which affects the functional relationships between cellular and whole plant physiology, plays a key role in determining a plant’s ability to cope with environmental stressors. Genome size may also be a key driver of plant species invasion success and determining range limits in response to climate change. Using the wetland plant Phragmites australis as a model system, we tested the Large Genome Constraint Hypothesis (LGCH) that posits that plants with larger genome size (GS) will be less tolerant of environmental stress than plants with small GS. In a greenhouse experiment replicated in a northern (Rhode Island) and southern (Louisiana) climate, potted plants from 35 source populations of P. australis, varying in GS, latitude of origin and invasion status (native or introduced within the USA) were subjected to one of three salinity treatments (0, 10, 20 ppt). During the peak of the growing season, we measured various plant traits associated with its growth and physiology (stem height, number and growth rate, stomate size and number), nutritional condition (%C, %N) and putative defenses (total phenolics, leaf toughness, silica content). The experiment was terminated at the end of the summer whereupon above and belowground biomass was determined.
Results/Conclusions
Increasing salinity was clearly stressful to P. australis and this was most pronounced in the southern greenhouse. Not surprisingly, plant height, total biomass, and %N decreased with increasing salinity. Using model selection, we found that GS was in the AICc-best model for 14 of 17 traits but the proportion of the variation in a trait explained by GS was always quite low. Overall, we found little support for the LGCH. Plants with small GS did not express higher growth or nutritional condition under salt stress as compared to plants with large GS. Also, there was no evidence that GS interacted with latitude or longitude to affect plant traits which would suggest that plants with different GS respond differently across the geographic range of this species in the USA. Despite a recent study with P. australis suggesting that invasiveness is associated with small GS, this study suggests that factors other than stress tolerance favor invaders with small GS.