Charles M. Leach

ACTION SPECTRA FOR LIGHT-INDUCED SPORULATION OF THE FUNGI PLEOSPORA HERBARUM AND ALTERNARIA DAUCI*,†

Abstract— Action spectra for light induced sporulation were determined for conidia of Alternaria dauci, and conidia and perithecia of Pleospora herbarum (Imperfect stage is Stemphylium botryosum). Only radiation less than 370 mμ induced formation of conidia, and less than 390 mμ formation of perithecia. The action spectra showed increased effectiveness in the 230 and 290 mμ regions, and possibly in the 260–270 mμ regions. Below 280 mμ these action spectra may not be representative of the “true” absorption spectra of the photoreceptors involved because of possible superimposed effects caused by nucleic acid absorption. Though the action spectra were not identical, the number of characters in common were suggestive of a similar photoreceptor. Similarities between action spectra for sexual and asexual reproduction of P. herbarum indicate the possibility of a single photoreceptor for both processes.

An electrostatic theory to explain violent spore liberation by Drechslera turcica and other fungi.

SUMMARY Measurement of voltages associated with sporulating maize leaf lesions revealed electrical charges that appeared to relate to violent liberation of conidia. Under precisely controlled conditions (relative humidity, temperature, light and air flow) maize leaf lesions became electrically charged in saturated air. Violent release of conidia triggered by either decreasing the relative humidity (RH), increasing the RH or exposing specimens to redinfrared radiation, was accompanied by significant voltage changes. Similar electrical changes were associated with healthy maize leaves growing under greenhouse conditions. On the basis of these experiments and other evidence, it is proposed that violent liberation of spores involves an exogenous surface electrical phenomenon whereby spores, sporophores and surrounding surfaces become charged (like charges) during periods of high humidity and this initiates an electrostatic repulsion between spore and sporophore. Lowering of the RH causes the charge to become static and also causes a weakening of the juncture between spore and sporophore. When the anatomical bond is sufficiently weakened the juncture breaks and the electrostatic force propels the spore into the air. Surface charges appear to originate mainly from the surface electrical phenomena associated with evaporation and condensation of water, and also under certain conditions from rain. An exogenous electrostatic mechanism should apply to all fungi having exposed nonmucilaginous spores (conidia, sporangia and basidiospores) borne on sporophores with a constriction between spore and sporophore, or between spores when in chains.

The Qualitative and Quantitative Relationship of Monochromatic Radiation to Sexual and Asexual Reproduction of Pleospora Herbarum

The importance of light in inducing reproduction in fungi, has been demonstrated in many instances (16). The wave lengths of electromagnetic radiation most effective in stimulating sporulation are located mainly in the ultraviolet and blue regions of the spectrum. A few reports (1, 13, 17, 21) have indicated that stimulation of reproduction in some fungi is almost entirely due to ultraviolet radiation. Indeed, a recent study on the qualitative and quantitative relationship of visible and ultraviolet monochromatic radiation to reproduction of Ascochyta pisi Lib. (14) showed conclusively that ultraviolet radiation was most effective in inducing sporulation. Pleospora herbarum (Pers.) Rabenh. was irradiated with monochromatic radiation in a manner similar to that described for Ascochyta pisi (14). The objectives were: 1) to determine the precise qualitative and quantitative relationship of electromagnetic radiation to the induction of both sexual and asexual reproduction in P. herbarum, 2) to determine by comparison whether the light requirements of P. herbarum and Ascochyta pisi indicate a similar photoactivated reproductive mechanism. Pleospora herbarum is known by both its perfect and imperfect (Stemphylium botryosum Wallr.) stages, though S. botryosum is most frequently mentioned in the literature. In this article the fungus will be referred to by its perfect stage name P. herbarum. Ellis (7), while studying the effect of nutrition and temperature on

Chapter XXIII A Practical Guide to the Effects of Visible and Ultraviolet Light on Fungitt

Publisher Summary Visible and near-visible light (200-750 nanometers) profoundly influence many aspects of the growth, development, reproduction, and behavior of fungi. In nature, most fungi are exposed during a portion of their life cycle to a little far-ultraviolet radiation (mainly 290-300 nm), and to considerable near-ultraviolet (300-380 nm) and visible (380-750 nm) radiation. Many effects of light on fungi have been reported by numerous investigators. The chapter presents general practical guide for those interested in the effects of light on fungi. Each light-induced phenomenon is described briefly with emphasis on the responsible wavelengths where these are known. New evidence indicates that visible light may be able to induce mutations in some microorganisms. The effect of far-UV on the survival of fungi is dependent on many factors. Spores for example, are much more resistant to the lethal effects of UV than vegetative mycelium, sensitivity of spores is frequently related to color and pigmented spores often survive longer exposures to far-UV than colorless spores; in addition thickness of spore wall may also be involved in the ability of fungi to tolerate radiation. Age can also influence sensitivity of spores and old spores of Aspergillus melleus are more tolerant to far-UV than young spores. The lethal effects of far-UV have many applications in science, industry, and medicine. The low-pressure mercury vapor lamp or germicidal lamp is an inexpensive, efficient, and widely used source of fungicidal UV.

An Action Spectrum for Light-Induced Sexual Reproduction in the Ascomycete Fungus Leptosphaerulina Trifolii

SUMMARY Perithecia (ascostromata) of the loculoascomycete Leptosphaerulina trifolii (Rost.) Petr. may develop in culture in darkness, but usually they are much more abundant in colonies exposed to light. By selecting and isolating from nonsporulating sectors occurring in dark-reared colonies, an experimental isolate was obtained that rarely sporulated in darkness but which produced abundant perithecia when exposed to light. Only UV wavelengths (<350 nm approx.) stimulated perithecium formation when colonies were exposed to filtered light from a combination of nearultraviolet and daylight fluorescent lamps. Extensive monochromatic radiation studies were conducted using wavelengths from 240-360 nm at 5 nm intervals (5 nm bandwidth). Results of these studies indicated that only wavelengths <370 nm (approx.) stimulated perithecium formation. An action spectrum derived from these results shows prominent peaks of effectiveness at 265, 287 and 300 nm. Increased effectiveness of wavelengths <250 nm was also evident but these results were inconclusive. The action spectrum for L. trifolii is very similar to that previously reported for perithecium induction in Pleospora herbarum (Pers.) Rabenh. Perithecia resulting from a single exposure of vegetative colonies to monochromatic UV (240-360 nm) in all instances developed mature asci following an additional few days of incubation in darkness. There was no evidence that maturation of asci required an additional exposure to light. Optimum temperatures favoring the development of perithecia following exposure to UV, was 24.0-26.6 C which was a slightly narrower range than that favoring linear growth (21.0-24.5 C).

Diurnal electrical potentials of plant leaves under natural conditions

Abstract Surface electrical potentials of leaves of beans ( Phaseolus vulgaris L.), cucumber ( Cucumis sativus L.) and cherry ( Prunus avium L.) were measured continuously and repeatedly over 3–4 day periods, mainly during fine weather. Potentials were monitored with a non-contacting electrostatic voltmeter. Leaves exhibited a diurnal rhythm of potentials with highest voltages after mid-day (+ 120 V max. recorded) and minimum potentials, near zero, at night. On dry, sunny days the polarity of leaves was consistently positive, at night polarity varied between neutral, positive and negative (− 30 V max.). Among the various weather factors examined that might influence leaf potentials, solar radiation and leaf surface temperatures were most intimately associated with electrical changes. Increases in leaf potentials could be detected within a few minutes of sunrise. Potentials were highest on hot, sunny days and lowest on overcast, cool days. Significant potential changes occurred when leaves were severed. Severance of leaves from plants growing in dry soil resulted in a rapid surge of leaf voltage (+) followed by a gradual decline. In contrast, severance of leaves from plants growing in flooded soil resulted in a reversal of polarity and a rapid increase in negative potential. When a severed leaf was electrically reconnected to the plant, its potential would revert back (approx.) to the level before severance. It was also discovered that when plants growing in dry soil were irrigated, leaf potentials increased rapidly. This study has revealed that there are significant patterns of diurnal variations of electrical potential of leaves under natural conditions. The reason for these changes is not understood.

Effect of still and moving moisture-saturated air on sporulation of Drechslera and Peronospora.

Drechslera turcica and Peronospora destructor sporulated in profusion on infected leaves incubated at constant temperature in moisture-saturated air that was still, but not when saturated air was in motion at velocities from 0.3 to 1.5 m/s.

Contact Photomicrography in the Ultraviolet on High-Resolution Plates

Ultraviolet photographs of cells can be made without quartz optics by placing the cells on high-resolution plates capable of resolving more than 2000 lines per millimeter and by passing monochromatic radiation of the desired wavelength through them to the emulsion. Prints can be made by enlarging the resulting negative with a microscope to the magnification desired.