Higher ploidy coincides with inferior performance and no difference in stress tolerance in reed
(1) Climate change leads to more extreme weather events. Therefore, a high stress tolerance is becoming more critical for plants, with higher ploidy being reported to lead to higher stress tolerance. Phragmites australis (P. australis) is a target species for paludiculture, i.e. the wet use of peatlands, and well known for its many ploidy levels. (2) 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. (3) Increasing drought length led to a decrease in growth, biomass, morphological and ecophysiological traits in both ploidy levels, but 4x outperformed 8x in almost all traits under constant water supply (e.g., 2.5-fold more biomass production) and up to moderate drought (about 50 days). Under severe and prolonged drought, both ploidy levels performed equally poorly. (4) Our study suggests that higher ploidy levels do not necessarily outperform lower ploidy levels of P. australis under stressful conditions. In this species, ploidy alone may not explain performance, but the genotype can be as or more important than ploidy.