Synthetic allohexaploid Brassica are able to express best-parent traits in response to drought

Many species are allopolyploid, containing sets of chromosomes from multiple different species. Ploidy increase may also confer resistances to specific stresses, but few studies have specifically tested allopolyploid hybrids in comparison to their exact parent species genotypes for abiotic stress response. However, the impact of drought-induced stress on the productivity, germination, and chlorophyll levels of Brassica crops have been extensively studied. Here, we aimed to test whether novel allohexaploid hybrids between Brassica species show better drought tolerance than their parent allotetraploids. We tested two allohexaploid lines and each of their three parent genotypes of Brassica species ( B. carinata , B. juncea , and B napus ) for photosynthetic response and phenotypic traits under drought stress (approximately 20% water field capacity) and well-watered control conditions. Physiological parameters including chlorophyll fluorescence, normalized vegetative index, chlorophyll content, Stress Tolerance Index (STI) and leaf wilting were assessed at four time-points during stress endurance. Chlorophyll fluorescence was the physiological trait that showed the most significant variation between drought and control treatments. Out of the two synthetic allohexaploids N6C2J2 performed better than the best parent J2 for most of the physiological traits and for leaf wilting, although not for number of seeds per plant. By contrast, N1C1J1 showed poor performance but was still better than the worst parents J1 and C1 for most traits. Our study suggests that allohexaploid Brassica shows promising potential to contribute in drought stress tolerance breeding, but that genotype-specific effects likely play a major role in any observed phenotypic advantages of novel allopolyploids.