Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant Pathology and Crop Physiology

First Advisor

Mary E. Musgrave


Challenging a heterogenous population of plants (rapid-cycling Brassica rapa L.) with waterlogging, we selected plants which differed in their response to rootzone hypoxia. Based on visual criteria, individuals from the waterlogged wild-type population were separated into "tolerant" and "sensitive" groups and mass pollinated to produce the next generation. This selection protocol was continued for seven generations. Under waterlogged conditions, chlorophyll concentrations in the tolerant population were significantly greater than in the sensitive population. After eight days of waterlogging, tolerant plants had significantly greater dry root and shoot weights than the sensitive plants. However, when tolerant and sensitive populations were grown under control conditions chlorophyll concentrations and dry weights were not significantly different. Soluble carbohydrates and starch concentrations steadily increased in the leaves of waterlogged plants, but carbohydrate concentrations of control plants maintained a constant level. Examination by transmission electron microscopy showed distorted thylakoid membranes in the chloroplasts of waterlogged sensitive leaves. Activities and electrophoretic profiles of six root enzymes: alcohol dehydrogenase (ADH), glucose 6-phosphate dehydrogenase (G6PDH), malate dehydrogenase (MDH), malic enzyme (ME), phosphoglucomutase (PGM), and pyruvate decarboxylase (PDC) were examined in the populations subjected to rootzone hypoxia. ADH and PDC activities increased, PGM and MDH activities decreased, and ME and G6PDH activities did not change. There were significant differences in ADH activities in the populations after 18 hours of waterlogging. The isozyme patterns of ADH and PGM were also different among the populations. Adenylate energy charge ratios measured in the populations were similar after 18 hours of waterlogging. There were swollen cristae and electron dense matrices in the mitochondria of roots from all plants grown under waterlogged conditions. Enzyme activities, energy charge, carbohydrate status, and ultrastructure differences were considered to be indicators of rather than causes of differences in population responses to rootzone hypoxia.