STRUCTURAL-CHANGES IN RAPID-CYCLING BRASSICA-RAPA SELECTED FOR DIFFERENTIAL WATERLOGGING TOLERANCE

Two populations of rapid-cycling Brassica rapa L. selected for differential waterlogging tolerance and a commercially available standard population were compared in their structural responses to waterlogging. Rates of dry matter accrual were similar under drained conditions but were lower after 6 days of waterlogging, especially in the sensitive population, Chlorophyll content of leaves from the sensitive population was significantly lower than the tolerant population. Examination of the foliage by TEM after 4 days of waterlogging revealed large starch grains that apparently disrupted the grana stacks in the sensitive population. Root morphology also distinguished the populations. After 8 days of waterlogging, numerous adventitious roots were visible in the standard and tolerant populations but not in the sensitive population. No aerenchyma was detected by examination of root cross sections. Mitochondrial morphology was affected by waterlogging, resulting in elongate, branched organelles. Isozyme analysis of malic enzyme, malate dehydrogenase, glucose-6-phosphate dehydrogenase, and pyruvate decarboxylase revealed no differences due to population, treatment, or duration. Isozymes of alcohol dehydrogenase varied with duration of waterlogging, and isozyme forms of phosphoglucomutase were distinct depending on the population. The appearance of stress-specific structural features in populations of B. rapa is discussed in relation to differences in waterlogging tolerance.