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

Two populations of rapid-cycling Brassica rapa L. selected for differe ntial waterlogging tolerance and a commercially available standard pop ulation were compared in their structural responses to waterlogging. R ates of dry matter accrual were similar under drained conditions but w ere lower after 6 days of waterlogging, especially in the sensitive po pulation, Chlorophyll content of leaves from the sensitive population was significantly lower than the tolerant population. Examination of t he foliage by TEM after 4 days of waterlogging revealed large starch g rains that apparently disrupted the grana stacks in the sensitive popu lation. Root morphology also distinguished the populations. After 8 da ys of waterlogging, numerous adventitious roots were visible in the st andard and tolerant populations but not in the sensitive population. N o aerenchyma was detected by examination of root cross sections. Mitoc hondrial morphology was affected by waterlogging, resulting in elongat e, branched organelles. Isozyme analysis of malic enzyme, malate dehyd rogenase, glucose-6-phosphate dehydrogenase, and pyruvate decarboxylas e revealed no differences due to population, treatment, or duration. I sozymes of alcohol dehydrogenase varied with duration of waterlogging, and isozyme forms of phosphoglucomutase were distinct depending on th e population. The appearance of stress-specific structural features in populations of B. rapa is discussed in relation to differences in wat erlogging tolerance.