Higher ploidy coincides with inferior performance and no difference in drought tolerance in a mesocosm experiment with common reed (Phragmites australis)

Climate change leads to more extreme weather events. Therefore, high stress tolerance is becoming increasingly critical for plants. A higher ploidy level has been reported to lead to higher stress tolerance in plants. Phragmites australis is a species well known for its many ploidy levels and is a target species for paludiculture, i.e. the wet use of peatlands. We expected octoploid genotypes of P. australis to outperform tetraploid ones in a 15-month mesocosm experiment including a gradient of 0 to 100 days of drought. We used pairs of genotypes differing in ploidy from three different geographic regions. Increasing drought length led to a decrease in growth, biomass, morphological and ecophysiological traits in both ploidy levels. A ploidy level of 4x outperformed 8x in almost all traits under constant water supply, leading to more than doubled biomass production, and up to moderate drought (approximately 50 days). Under severe and prolonged drought, both ploidy levels performed equally poorly. This study suggests that lower ploidy levels can outperform higher ploidy levels of P. australis . In this species, ploidy alone may not explain performance, but the genotype can be as or more important than ploidy. • 4x outperforms 8x P. australis up to moderate drought (0-50 days drought) • 4x and 8x P. australis perform equally poorly at prolonged drought (50-100 days) • A higher ploidy level does not outperform lower ploidy of P. australis under stress • In addition to ploidy, genotype determines P. australis performance