RESPONSES OF PHYLLOPLANE YEASTS TO UV-B (290-320 NM) RADIATION - INTERSPECIFIC DIFFERENCES IN SENSITIVITY

The sensitivity to UV-B (290-320 nm) radiation of common phylloplane y easts from two contrasting UV-B environments was compared in the labor atory using mixtures of white light (PAR: 400-700 nm) and UV-B radiati on from artificial lamp sources. Sporidiobolus salmonicolor, Rhodotoru la mucilaginosa and Cryptococcus sp., the dominant yeasts on leaves of tea (Camellia sinensis), were isolated in Sri Lanka (SL), while Spori diobolus sp. and Bullera alba, dominant on faba bean (Vicia faba), wer e isolated in the U.K. Dose responses were determined separately for e ach yeast. UV-B reduced colony forming units (due to cell mortality or inactivation) and colony size (due to reduced multiplication) of all yeasts. The LD50 values and doses causing 50% reduction of cells per c olony were higher for SL isolates than U.K. isolates. Results indicate d that each yeast is somewhat vulnerable to UV-B doses representative of its natural habitat. The relative insensitivity of SL isolates was shown when SL and U.K. isolates were irradiated simultaneously with th e same dose of UV-B. Of the two U.K. yeasts, B. alba was significantly more sensitive than Sporidiobolus sp. to UV-B. Except for R. mucilagi nosa from SL, all yeasts demonstrated some photorepair in the presence of white light. White light provided relatively little protection for the U.K. isolate of Sporidiobolus sp. although it allowed increased c olony size. The spectral responses of Sporidiobolus sp. (U.K.) and of B. alba (U.K.) were broadly similar. Wavelengths longer than 320 nm ha d no measurable effect on colony forming units. However, colony surviv al was significantly reduced at 310 nm and all shorter wavebands. No c olonies were counted at 290 nm or below.