A lack of aerenchyma and high rates of radial oxygen loss from the root base contribute to the waterlogging intolerance of Brassica napus

The morphology and physiology of the response of two cultivars of Brassica napus to an anaerobic root medium was investigated. The cultivars Chikuzen and Topas showed a large reduction in growth rate when their roots were exp osed to a de-oxygenated stagnant nutrient solution containing 0.1% w/v agar . Older seedlings (11 d old) were more sensitive to stagnant agar, expresse d as biomass accumulation, than younger ones (5 d old). Brassica napus was characterized by a constitutively low root porosity (3-5%), typical for pla nt species with a low tolerance to waterlogging. A hypoxia pretreatment (16 h; 2.25% O-2) before exposure to de-oxygenated stagnant agar had no effect on the final number or length of lateral roots and adventitious roots. Bra ssica napus cv. Chikuzen is characterized by radial oxygen loss being most at the basal portion of the root, when a strong oxygen sink surrounds the r oot. Oxygen profiles through laterals of Brassica napus cv. Chikuzen show a typical pattern with low oxygen concentrations in the stele and somewhat h igher levers in the cortex. Despite the continuum of intercellular air spac es in the root cortical tissue the lack of aerenchyma and therefore low rat es of internal oxygen diffusion restricts root growth in anaerobic media an d presumably contributes to the sensitivity of Brassica napus to waterloggi ng.