On the Blue Light Inhibition of Sporulation in Helminthosporium oryzae

Abstract

Experiments were carried out to understand the inhibition of sporulation of H. oryzae (isolate HA2) by the continuous light in examining the sensitive growth stage in inhibition of sporulation by the irradiation of blue light and the interaction between blue light and near-ultraviolet radiation (BLB).Sporulation of isolate HA2 is dependent upon the length of exposure time rather than the amount of dosage of BLB irradiation. The sporulation process of HA2 seems to be composed of three steps: (1) the initiation of conidiophore formation under BLB irradiation, (2) the maturation of conidiophore progress under darkness and suppressed by blue light and BLB irradiation, and (3) the conidium formation not suppressed by blue light. The blue light inhibits the maturation of conidiophore, and it is most effective when the fungus is exposed to blue light between 4 and 10hr after the termination of inductive BLB irradiation. A small amount of blue light exposure (1, 250erg/cm2/sec) for 10 minutes is enough to suppress the sporulation, when the light is given at the second step of the sporulation process. There seems to exist some interaction between blue light and BLB radiation. This is suggested by the fact that the inhibition of spo-rulation by blue light is counteracted by BLB irradiation.From these results, it is suggested that the inhibition of sporulation by the continuous exposure with BLB is due to the inhibiting action of short wave region of visible light contained in BLB radiation. The less sporulation of the dark period shorter than 8 hr in the alternation of light and dark or a steep decrease in sporulation under the day length longer than 14hr is able to be under stood from the result that the inhibition by blue light of sporulation is most prominent when the blue light is applied between 4 and 10hr after the termination of inductive BLB irradiation.