Are tetraploid roses better resistant to stress compared to diploids?

Ploidy levels in rose species vary from diploid to octoploid. Examples of diploid species are R. arvensis, R. banskiae, R. bracteata, R. nitita, R. rugosa and R. wichurana. Tetraploids are for example R. foetida, R. gallica and R. spinosissima. A special case is found in the pentaploid dogroses (section Caninae) with their very specific asymmetric meiosis. In most rose species only one ploidy level is observed. However, some can have different ploidy levels. For example R. chinensis can be di-, tri- or tetraploid, while R. acicularis can be tetra-, hexa- or octoploid. Interspecific hybrids between different ploidy levels have occurred spontaneously in wild species, but also in the breeding history of roses different ploidy levels were used. Now, most garden roses are diploid, triploid or tetraploid. Typically cut roses are tetraploid. In this study we aimed to compare biotic and abiotic stress resistance in diploid versus tetraploid roses. Therefore ten diploid rose genotypes were artificially chromosome doubled in tissue culture using the antimitotic agent oryzalin. Morphological differences could be observed between plants of the two ploidy levels. In general the tetraploids grew slower and had lower dry matter content compared to the diploids. Four diploid rose genotypes and their polyploidized counterparts were evaluated for biotic stress resistance towards the fungal pathogen, powdery mildew (Podosphaera pannosa). Two pathotypes of rose powdery mildew were used for artificial inoculation and disease indexes were calculated for the diploids and tetraploids. For all four genotypes the tetraploids exhibited slightly higher resistance towards powdery mildew compared to the diploids. Ten diploid rose genotypes and their tetraploid counterparts were submitted to drought as abiotic stress inducer. Biomass, relative water content, stem diameter, leaf water potential, stomatal conductance, carbohydrates etc. were measured. Results were genotype dependent, on the tetraploid level three genotypes became more tolerant to drought stress, while five were less tolerant. In two genotypes there was no difference in drought stress tolerance between the two ploidy levels. Nevertheless, strong drought stress has led to metabolic adaptations, both in the level of stress related hormones (abscisic acid) as osmotic adaptation (soluble sugars and proline content). Also the type of adaptation was genotype depended.