Characterization of populations of rapid-cycling Brassica rapa L. selected for differential waterlogging tolerance.
B. rapa [B. campestris var. rapa] plants were placed into 'tolerant' and 'sensitive' populations based on foliage colour after waterlogging and were then mass-pollinated and re-selected over 7 generations to produce stable populations. To assess responses to root zone hypoxia in the selected populations, plants were grown for 1 week after germination under normal watering conditions and then subjected to waterlogging stress for up to 8 d. Under control conditions, no differences were found between the tolerant and sensitive populations in any of the parameters studied. Chlorophyll concentrations in the tolerant population were significantly greater than the concentrations in the sensitive population when plants had been waterlogged. A similar stress-specific difference was found in root and shoot DM production. As soil redox values (and hence, available oxygen) decreased, an increase in soluble carbohydrates and starch occurred in the leaves of waterlogged plants. Changes in soluble carbohydrates were noted as early as 12 h after waterlogging in the sensitive plants, and starch concentrations were significantly higher for this population 24 h after waterlogging. Under waterlogged conditions, activities of alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC) in roots increased, phosphoglucomutase and malate dehydrogenase decreased, and malic enzyme and glucose 6-phosphate dehydrogenase did not change. The sensitive population exceeded the tolerant population in activities of ADH and PDC after 18 and 48 h of waterlogging, respectively. It was concluded that stress-specific differences in population responses to waterlogging can be achieved through recurrent selection.