Limited Growth in Fungi

SUMMARY

Current concepts of fungal growth hold that fungi should grow indefinitely if sufficient nutrients are present and if staling substances are not produced in concentrations high enough to limit growth. The data in this report do not support the idea of such indefinite growth but rather indicate that an age dependent, growth regulating mechanism exists in at least some fungi and is responsible for restricted growth. The size of surface grown mycelial pads of Rhizoctonia solani Kühn in liquid media was always limited to approximately the same diameter. This restriction occurred even when the fungus was grown under constant flow conditions to replenish nutrients and remove exotoxins. The growth rate of R. solani increased to a maximum at 4–5 days then decreased until growth ceased at 8–9 days. Similar changes in growth occurred in 2 types of media, and with 6 different isolates. The growth limitation is a function of the entire thallus and is not a result of the permanent impairment of the peripheral hyphae. The respiration of the peripheral hyphae of mycelia that have ceased growth is greater than that in older parts of the thallus. Electron microscope studies of the peripheral hyphae of young, vigorously growing cultures and those of old colonies whose growth had stopped revealed no morphological differences. An ethanol extract of old cultures is inhibitory to the growth of R. solani from new inocula. Helminthosporium carbonum Ullstrup, Sclerotinia sp., Gibberella zeae (Schw.) Petch, Pellicularia chordulata Rogers, P. flavescens (Bon.) Rogers and Phaeocytosporella zeae Stout also had limited growth. Sclerotium bataticola Taub, and H. turcicum Pass, had growth curves similar to R. solani but did not entirely cease growth before reaching the walls of the culture chamber.