THE SPORULATION OF HELMINTHOSPORIUM ORYZAE AS AFFECTED BY EXPOSURE TO NEAR ULTRAVIOLET RADIATION AND DARK PERIODS

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Abstract
Investigated was the interrelation of near ultraviolet radiation (3100–4000 Å) and darkness to the sporulation of Helminthosporium oryzae. When H. oryzae was grown on potato dextrose agar at 70–75° F and irradiated at intensities of 76–470 μw per cm2: (1) Near ultraviolet stimulated sporulation while visible light did not. (2) Conidiophore development was initiated under irradiation but not in darkness. Both continuous irradiation and intermittent irradiation (2-hour ultraviolet, 2-hour dark cycle) caused formation of conidiophores. (3) Conidia developed only when a period of irradiation was followed by a period of darkness. No conidia developed under continuous irradiation or in continuous darkness. (4) Under a single cycle treatment, the minimum dark period necessary for conidial development following an extended period of continuous irradiation was [Formula: see text] hours. (5) Under a single cycle treatment, the minimum exposure to ultraviolet followed by an extended dark period necessary to cause moderate to profuse conidial development was 4 hours, although conidia did form sparsely after shorter exposures. (6) Under alternating cycles of dark and irradiation of 2, 4, 6, 8, and 12 hours (e.g. 2 hours dark, 2 hours ultraviolet, etc.), conidia developed under the 8- and 12-hour cycles, but not under the shorter cycles. Conidia developed under shorter cycles when the cyclical exposures were followed by a dark period of 12 hours. (7) Continuous irradiation of nutrient broth cultures caused a significant increase in dry weight growth over cultures kept in darkness.

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