Characterization of populations of rapid-cyclingBrassica rapaL. selected for differential waterlogging tolerance

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Abstract
By challenging a heterogenous population of plants (rapid-cycling Brassica rapa L.) with waterlogging stress, we selected plants which differed in their response to root zone hypoxia. These individuals were placed into ‘tolerant’ and ‘sensitive’ populations based on foliage colour after waterlogging and were then mass-pollinated and re-selected over seven generations to produce the stable populations described. 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 dry matter accrual. 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) 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. The results demonstrate that stress-specific differences in population responses to waterlogging can be achieved through recurrent selection.

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