Responses of Representative Fungi to Certain Growth-Regulating Substances

Abstract

1. Four filamentous fungi, Phycomyces blakesleeanus (+), Aspergillus candidus, Schizophyllum commune (haplont), and Neurospora tetrasperma, were grown in the presence of a range of concentrations of four growth-regulating substances, indole-3-acetic acid (IAA), alpha-naphthaleneacetic acid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). 2. Growth responses were measured by determining linear growth rates of these fungi in the presence of the different substances and in untreated controls. Effects of the substances on the different fungi were of two classes, stimulating and inhibiting, and were not identical for the four test organisms. As a general rule inhibition occurred within the concentration range 10-2 to 10-3 molar; stimulation within the concentration range 10-3 to 10-4 molar. 3. The effects of these substances on growth were not constant throughout the test period. Three types of stimulation relative to time were observed: (a) stimulation during the early phases of growth, (b) prolonged stimulation over the entire period of growth, and (c) stimulation during the late growth phases. 4. P. blakesleeanus (+) showed (a) a 7% increase over controls in early growth rates and a 6% increase in late growth in the presence of 2,4-D, (b) an 11% increase in early growth with IAA, and (c) an 8% increase in average growth and a 21% increase in late growth in the presence of NAA. Effects of 2,4,5-T were inhibitory. 5. Stimulated responses occurred in the late phases of growth with A. candidus in the presence of 2,4-D, 2,4,5-T, and NAA. The latter substance stimulated growth rates continuously from the beginning of growth and exerted a maximum stimulation (22%) in the late growth phases. 6. Effects of the four substances on the primary mycelium of S. commune were inhibitory, the degree of inhibition increasing in proportion to the concentrations. The growth curves for this fungus were not similar to growth curves of the other three fungi. 7. Increases in the early growth rate of N. tetrasperma in the presence of 2,4-D and IAA were obtained in final, but not in preliminary, tests. At higher concentrations both 2,4,5-T and NAA were inhibitory. 8. The percentages of spores of A. candidus germinating in 24 hours on a series of concentrations of IAA, 2,4-D, and 2,4,5-T were inversely related to concentration. Germination of spores was increased in the presence of NAA in the 0.001-0.00018 M range. 9. Conidiospores of A. candidus were treated for different time intervals with concentrations of NAA and IAA which were completely inhibiting to vegetative growth. These substances caused decreases in the percentage of germination in proportion to the length of treatment and the concentration used. Similar treatment with completely inhibitory concentrations of 2,4-D and 2,4,5-T effected a 95-100% reduction in germination percentage of the spores within 24 hours after removal from the solutions as compared with controls. When the duration of treatment with the latter two substances was shortened, results were similar.