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

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.