Temperature-Sensitivity of Light Inhibition of Uredospore Germination inMelampsora Medusae

Abstract

The germination of uredospores of Melampsora medusae, deposited on cover glasses and incubated in sealed humidity chambers (ca. 100%) at 15 and 25 C in darkness and at five levels of increasing light intensity (range 100-500 ,tE m-2 s-1), was recorded at 12 and 24 h. At both temperatures, germination (per cent) decreased with increasing light intensity, being a maximum in continuous darkness and zero at 25 C coupled with light intensities of 400 and 500 ,LE m-2 s-1. The latter inhibition of germination was reversible and normal germination occurred when these uredospores were subsequently incubated in the dark. The inhibitory effect of comparable light intensities was temperature sensitive being more pronounced at 25 than at 15 C. While reports of the effects of temperature on uredospore germination are frequent, those for effects of light intensity are much less common (Givan and Bromfield, 1964a, b; Tollenaar and Houston, 1966). Further, such studies rarely report the effects on germination of concurrent variation in these variables, hence extrapolation from such laboratory studies to field environments is very restricted. Spiers (1978) reported that light (intensity and quality unspecified), when compared with continuous darkness, had no apparent effect on the germination of uredospores of Melampsora medusae Thiim. Preliminary investigations (Singh, unpublished data) suggested that cool fluorescent light, even at intensities less than 500 ,uE m-2 s-1, was inhibitory to germination of uredospores of this organism. This paper reports a factorial experiment in which the effects of concurrent variation in incubation temperature (two levels), light intensity (six levels) and time (two periods) on the germination of uredospores of M. medusae were examined. The temperatures and light intensities employed are within the range of those occurring adjacent to the abaxial surface of leaves of Aigeiros poplars in the field in summer/autumn in Southeast Australia.