Growth Rhythms

Several fungi growing on solid media develop a series of regularly spaced alternating thick and thin mycelium density, or alternating zones of sporulating and nonsporulating hyphae, called zonations (Jerebzoff, 1965). For example, cultures of Sclerotinia fruticola show zona-tions of decreasing intensity every 24 hours after dark-grown cultures are exposed to light (Figure 11.1). In dark-grown cultures of Fusarium discolor var. sufureum, Trichothecium roseum and Verticillium lateritium (Fungi Anamorphici), daily illumination of 1000 to 3000 lux for a few seconds suffices to induce zonations. The rhythmic zonations can continue for some time after the fungus is transferred to complete darkness and is almost certainly

Figure 11.1 Growth rhythms in Sclerotinia fruticola. Culture maintained in dark for two days received a first photoperiod of 12 hours at 250 lux, and a second after five days. After each stimulus, four zonations of decreasing intensity occurred, one every 24 hours. (From Jerebjoff (1965). With permission of Elsevier.)

due to the diurnal periodicity of alternating light and darkness. This rhythmic growth pattern is a manifestation of an endogenous clock—they have a period of approximately 24 hours and are independent of temperature. In some, fungi zonation is stimulated by the supplementation of growth media by factors present in yeast, malt or potato extract (Jerebzoff, 1965), suggesting they need excitation by certain chemical factors for the manifestation of rhythms. The endogenous rhythms persist for a long time and continues without attenuating for three weeks in Sclerotinia fruticola (Ascomycotina), one month in Aspergillus ochraceus and A. niger and 70 days in Alternaria tenuis (Fungi Anamorphici).

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