Axenic marigold (Tagetes erecta L.) were grown in quartz sand free of organic compounds and one-half strength Hoagland and Arnon nutrient solution plus chelated iron, in individual plant containers, designed for collecting root exudates, inside sterile gnotobiotic flexible plastic isolators. Plant roots were treated with washed conidiospores of Penicillium simplicissimum (Oud.) Thom (7 to 8 x 10⁶ per plant) or Penicillium citrinum Thom (4 x 10⁶ conidiospores per plant), citrinin (0.1, 1.0 and 10 mg/l), and extracts of 80 g axenic or P. simplicissimum-colonized roots. Experiments were harvested 20 days (prior to flowering) and 34 days (flowering) after inoculation. Root exudates were collected at the end of experiments or also weekly obtained for 5 weeks. Inoculated plants were larger in size, contained more dry matter and flowered earlier than the axenic plants. Concentration of 1.0 mg/l citrinin stimulated plant growth and 10 mg/l citrinin had no effect. Extract of P. simplicissimum-colonized roots suppressed plant growth. Roots exhibited 40-60% degradation 34 days after inoculation. Fresh weight of P. citrinum-inoculated plants was lower than the noninoculated. Total water soluble carbohydrates and reducing sugars were significantly higher in the foliage of root colonized plants 34 days after inoculation. Total organic matter and protein were decreased in root exudates 20 days after inoculation. These variations were attributed to reabsorption of organic compounds by the plants and immobilization of those compounds by the mycelial growth. Thirty-four days after inoculation there was increased amounts of organic compounds in the exudates as root colonization progressed. Analyses of periodically obtained exudates indicated a decrease in exudates at flowering from axenic plants only. This phenomenon was not detected in the presence of P. simplicissimum. Percentages of P and K in dried leaves were decreased 20 days after inoculation. However, N, P and K concentrations were not affected 34 days after inoculation. Calcium concentration was significantly increased at the latter stage. Germination of conidiospores occurred in the root hair region, but no direct penetration of intact living cells was observed. There was extensive mycelial growth from infection centers at weakened root hai:rs, damaged epidermal cells and sloughed tissues. Macerating enzyme, polygalacturonase, and cellulase were detected in extracts of P. simplicissimum-colonized roots. / Ph. D.