THE QUANTITATIVE AND QUALITATIVE RELATIONSHIP OF ULTRAVIOLET AND VISIBLE RADIATION TO THE INDUCTION OF REPRODUCTION IN ASCOCHYTA PISI

Abstract

Irradiation under either daylight fluorescent lamps or diffuse sunlight stimulated pycnidial development in 40 isolates of Ascochyta pisi. Sporulation varied among these same isolates when grown in darkness, and ranged from none to profuse. Irradiation of selected isolates through filters showed the importance of ultraviolet radiation in stimulating reproduction. The precise relationship of quality, intensity, and length of exposure of radiation to induction of sporulation was studied with monochromatic radiation at 21 °C. Treatment of colonies with monochromatic radiation of wavelengths 2378 Å to 5461 Å (band width 132 Å) at the same intensity of 100 μw/cm2 and with exposures ranging from 1/50 to 20,000 seconds proved conclusively that only ultraviolet radiation induced sporulation. Two wavelengths, 2378 Å and 3131 Å, were notably effective in inducing pycnidial formation over a wide range of exposures. The dosage effective in inducing sporulation decreased with decrease in wavelength. Similarly, the minimum dosage which caused inhibition of sporulation was also least at the shorter wavelengths. Onset and abundance of sporulation differed significantly when the intensity of radiation was varied. The higher the intensity, the shorter was the exposure necessary to stimulate sporulation, or cause inhibition of sporulation. Pycnidia were stimulated by near ultraviolet radiation over a wide range of temperatures (10° to 30 °C), over a wide range of pH values (4.5 to 8.3), and on four commonly used media. Growth (dry weight) under continuous exposure to near ultraviolet radiation did not differ from growth in darkness.The region of mycelium most sensitive to effects of radiation on sporulation at 21 °C was a peripheral zone of young mycelium approximately 1.5 to 2 mm in width. In colonies incubated at a constant temperature of 21 °C, the older mycelium was practically insensitive to radiation. A photoactivated sporulation precursor was able to move small distances from the irradiated peripheral mycelium to newly formed non-irradiated mycelium. Movement of the percursor appeared to be by translocation within hyphae rather than by diffusion through the medium.The location of pycnidia relative to the surface of the medium was dependent on length of exposure and quality and intensity of radiation; absorption of radiation by medium; incubation temperature; and age of mycelium.The size of pycnidia, and the size and shape of conidia both in culture and on the epicotyls of Vicia villosa, were significantly influenced by the dosage and quality of radiation.