779

T. S. GUNASEKERA, N. D. PAUL* AND P. G. AYRES

Division of Biological Sciences, Institute of Environmental and Biological Sciences, Lancaster University, Lancaster LA1 4YQ , U.K.

The sensitivity to UV-B (290–320 nm) radiation of common phylloplane yeasts from two contrasting UV-B environments was

compared in the laboratory using mixtures of white light (PAR: 400–700 nm) and UV-B radiation from artificial lamp sources.

Sporidiobolus salmonicolor, Rhodotorula mucilaginosa and Cryptococcus sp., the dominant yeasts on leaves of tea (Camellia sinensis),

were isolated in Sri Lanka (SL), while Sporidiobolus sp. and Bullera alba, dominant on faba bean (Vicia faba), were isolated in the

U.K. Dose responses were determined separately for each yeast. UV-B reduced colony forming units (due to cell mortality or

inactivation) and colony size (due to reduced multiplication) of all yeasts. The LD&!

values and doses causing 50% reduction of cells

per colony were higher for SL isolates than U.K. isolates. Results indicated 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 the 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. mucilaginosa 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 colony size.

The spectral responses of Sporidiobolus sp. (U.K.) and of B. alba (U.K.) were broadly similar. Wavelengths longer than 320 nm had

no measurable effect on colony forming units. However, colony survival was significantly reduced at 310 nm and all shorter

wavebands. No colonies were counted at 290 nm or below.

Leaves are displayed to intercept maximum incident solar

radiation. Leaves are characteristically colonized by epiphytic

communities of bacteria, yeasts and filamentous fungi (Last &

Price, 1969 ; Andrews & Kenerly, 1979 ; Campbell, 1985 ;

Thompson et al., 1993 ; Dix & Webster, 1995) so, at least on

unshaded surfaces, this particular microbial habitat is subject

to relatively high doses of solar radiation (Lindow, 1991). The

ultraviolet component of solar radiation is, in general,

damaging to living organisms, so it is often assumed that the

shorter wavelengths that reach the Earth’s surface (UV-B,

290–320 nm) have a major role in regulating both the size and

balance of the microbial populations (Ayres et al., 1996). There

is, however, little empirical evidence to support this

assumption.

UV fluxes reaching the Earth’s surface broadly increase with

decreasing latitude and increasing elevation. Superimposed on

this pattern are daily and, especially in temperate latitudes,

seasonal fluctuations. We tested the hypothesis that the leaf

surface microbes are adapted to their local environment by

examining the response to UV-B of phylloplane organisms

isolated from contrasting environments, that is from leaves, of

tea, growing at an elevation of 1400 m in the tropics (latitude

7°N) and from those, of faba bean, growing at sea level in

* Corresponding author.

temperate NW Europe (latitude 54°N). Experiments were

carried out in vitro so that doses of UV-B could be

precisely regulated. Yeasts were chosen for the study because

of their ubiquity on leaf surfaces and the relative ease with

each they can be quantified. We studied the types, three from

tea and two from faba bean, dominant after leaf washings were

plated on malt extract agar.

Since there is evidence that longer wavelengths of solar

radiation (white or visible light, 400–700 nm), even at

extremely low fluxes, may protect organisms from damage by

UV-B, the effect of white light on UV-B responses of the test

organisms was examined.

MATERIALS AND METHODS

Isolation of yeasts

Yeasts were isolated from leaves of tea (Camellia sinensis L.)

and bean (Vicia faba L.) in Sri Lanka and the U.K. respectively.

Leaf samples were excised and immediately brought to the

laboratory. Leaf discs were cut out using a 10 mm diam. cork

borer and shaken for 15 min in 20 cm$ sterile water containing

0±05% (v}v) Tween 80. The washings were serially diluted in

sterile water and 0±25 cm$ of washings were transferred to

9 cm diam. Petri dishes containing suitable culture medium.

Plates were incubated for 72 h at 22 °C in darkness and yeasts

Mycol. Res. 101 (7) : 779–785 (1997) Printed in the United Kingdom

Responses of phylloplane yeasts to UV-B (290–320 nm)radiation: interspecific differences in sensitivity

UV-B and phylloplane yeasts 780

were isolated. Three dominant yeasts from leaves of tea and

two from faba bean were isolated and were maintained on 5%

(w}v) malt extract agar (MEA). Subcultures were identified by

the International Mycological Institute, Bakeham Lane, Egham,

Surrey, U.K., as, respectively Sporidiobolus salmonicolor Fell &

Tallman, Rhodotorula mucilaginosa (A. Jo$ rg.) F. C. Harrison and

Cryptococcus sp., from tea and Sporidiobolus sp. and Bullera alba

(W. F. Hanna) Derx, from bean.

Dose–response curves

Subcultures less than 48 h old, in an exponential growth

phase, were used. Irradiation was carried out in a dark room,

where temperature was maintained at 20³2°. Suspensions of

yeast cells of 1500–2000 cm−$ were made up in sterile

distilled water, and 0±2 cm$ was spread over the surface of

MEA Petri dishes, using a sterile glass spreader, before being

irradiated.

Broad band UV-B (Phillips TL UV-B 40W}12) and daylight

fluorescent tubes (Osram L58W}23) were used to provide

UV-B and white light respectively. UV-B sources were filtered

with 0±1 mm thick cellulose diacetate film (‘Clarifoil ’, supplied

by Courtaulds Ltd, Derby, U.K.) to remove wavelengths

below approximately 292 nm. UV irradiance was measured

using a double monochromator scanning spectroradiometer

(Macam SP991-PC, Livingstone, U.K.) and irradiances were

weighted according to the DNA action spectrum of Setlow

(1974) normalized to 1 at 300 nm. The spectroradiometer was

calibrated for wavelength using the spectral lines from a

mercury arc lamp (LOT Oriel, Leatherhead, Surrey, U.K.) and

for spectral irradiance against tungsten and deuterium sources

(Macam SR903, Livingstone, U.K.) referable to National

Physics Laboratory Standards. To provide a range of fluxes,

the distance between the Petri dishes and uv tubes was varied

by hanging the tubes at an angle approximately 30° to the hori-

zontal surface on which plates were arranged. Irradiation was

carried out against both dark and white light (400–700 nm¯250 µmol m−# s−") backgrounds in order to investigate the

possibility of photoreactivation.

After exposure to uv radiation, plates were further

incubated for up to 66 h in the dark at 20° and visible

macrocolonies were counted. To determine the number of

cells per colony, 5–10 colonies were carefully picked off into

20 cm$ water containing 0±05% (v}v) Tween 80 and shaken

for 15 min before cells were counted using a haemocytometer.

Dose–response studies were carried out separately for each

organism.

Relative sensitivity to UV-B radiation

To compare the yeasts directly all five isolates (two U.K. and

three SL) were simultaneously exposed to the same dose of

UV-B (4±98 kJ m−# UV-BDNA

). A suspension of 1500–2000

cells cm−$ was prepared as above and 0±2 cm$ was spread

onto five replicate Petri dishes (9 cm) containing 5% (w}v)

MEA. Irradiations were carried out under a white light

(400–700 nm¯ 250 µmol m−# s−") background for 6 h. Cells

were subsequently incubated for up to 66 h in the dark at 20°and visible macrocolonies were counted. Controls were

covered with a polyester filter supplied by Lee Filters,

Andover, Hants, U.K. (spectrally equivalent to Mylar, which

removes wavelengths shorter than approximately 320 nm, i.e.

removes most of the UV-B) so that they were exposed only

to white light and whatever UV-A radiation (" 320 nm) was

emitted from the UV-B sources.

Effect of white light on colony forming ability of yeast

isolates

In the experiments above, white light was provided in order

to test the possible occurrence of photoreactivation, but white

light alone might influence yeast cell growth. To test this,

each of the five yeasts was dispersed onto 9 cm Petri dishes

containing 5% (w}v) MEA and then exposed to 6 h white

light (approximately 250 µmol m−# s−"). Samples were incu-

bated for a further 60 h in the dark at 20° and numbers of

visible macrocolonies were counted. Results were compared

with those for unexposed (dark) controls. The experiment was

repeated three times and each had five replicates.

Effectiveness of different wavebands within the UV

spectrum

A concentration of approximately 500 cells cm−$ was prepared

with sterile distilled water, and 100 µl of the suspension was

spread onto 4±5 cm Petri dishes containing 5% (w}v) MEA,

with a glass spreader. Cells were then exposed to different

wavebands of uv radiation within the range of 270–380 nm.

To provide defined ranges of narrow band uv, two 75 W

xenon arc lamps, each in a ‘Q arc ’ housing with four outlet

ports, were used. Each of the four output beams was

collimated, and visible and infrared radiation was attenuated

using dicroic beam turners. Narrow band interference filters

(10 nm width at half peak height) were placed in between the

specimen and the beam to obtain the required waveband. The

distance of the colonies from the light source was adjusted

until all treatments received 1±1 Wm−#. All components were

supplied by LOT Oriel (Leatherhead, Surrey, U.K.).

Statistical analysis

Dose–response curves. Analysis of variance (ANOVA) and

non-linear regressions for mortality and cells per colony

(colony size) were made using SPSS (SPSS Inc., Michigan Av.,

Chicago, U.S.A.) statistical software. Two regression models

were used ; Y¯ 1}(1}u­b!(b

"

t)) and Y¯ b!­b

"t­b

#t# for

mortality and cells per colony respectively, where b!, b

", b

#¯

constants ; u¯ upperbound value for the logistic model ; t¯a time value or the value of an independent variable : e.g. UV-

B dose. LD&!

values and the dose causing 50% reduction of

cells per colony were calculated from fitted curves.

Response of different yeast isolates to UV-B radiation.

Analysis of variance and Tukey–HSD multiple range tests

were used to detect significant differences among treatments

using SPSS statistical software. Percentages were arcsin

transformed before applying statistical tests.

T. S. Gunasekera, N. D. Paul and P. G. Ayres 781

Effect of white light (PAR) on colony forming ability of

yeast isolates. Student’s t test and analysis of variance

(ANOVA-general factorial) were used to compare the means

of dark and light grown colonies of each isolate.

RESULTS

Dose–response curves

The effect of increasing UV-B dose on cell mortality and cells

per colony of S. salmonicolor (SL) is shown in Fig. 1. Dose

responses for the other organisms are not presented in such

detail, since their form was similar in each case. However, their

LD&!

values and the dose causing a 50% reduction of cells per

colony (colony size) were calculated from the fitted curves,

and these are summarized in Table 1. The values estimated for

regression models for dose–response curves, R# (coefficient of

determination), and P (significance) values are also summarized

in Table 1.

In general, increasing UV-B dose had a highly significant

(P! 0±001) effect, increasing mortality and decreasing cells

per colony of all yeasts, except B. alba which showed a less

significant (P! 0±01) effect on cells per colony under a light

background.

There was variation between yeasts of similar origin but

LD&!

values were higher for SL isolates than U.K. isolates,

irrespective of background irradiation. Similarly, higher UV-B

doses were required to reduce cells per colony (colony size) by

50% for SL isolates than for U.K. isolates, both under light and

dark backgrounds (Table 1). For S. salmonicolor (SL), Crypto-

100

9080

7060

5040

30

20

10

00 2 4 6 8 10 12 14

R2 = 0·7756

0·00E + 000 2 4 6 8 10 12 14

R2 = 0·8638

1·00E + 06

2·00E + 06

3·00E + 06

4·00E + 06

5·00E + 06

6·00E + 06

7·00E + 06

Cel

ls p

er c

olon

y

Mor

tali

ty (

%)

100

90

80

70

60

50

40

30

20

10

00 2 4 6 8 10 12 14

R2 = 0·67480

0·00E + 000 2 4 6 8 10 12

R2 = 0·88117

1·00E + 06

2·00E + 06

3·00E + 06

4·00E + 06

5·00E + 06

–10

(b)

(d )

(a)

(c)

Does (kJ m–2 d–1)

Fig. 1. Response of Sporidiobolus salmonicolor (SL) to UV-BDNA

dose : (a) colony number (per cent mortality) in the dark, (b) colony size

(cells per colony) in the dark, (c) colony number in white (PAR: 400–700 nm) light and (d ) colony size (cells per colony) in white light.

R#, coefficient of determination for the fitted curve. Each point represents data for a single Petri dish or, in the case of colony size, the

mean of 5–10 colonies per plate.

coccus sp. (SL) and B. alba (U.K.), the inhibitory effect of UV-

B on survival and colony size was reduced by concomitant

irradiation with white light, i.e. there was evidence of

photoreactivation. In the U.K. isolate of Sporidiobolus photo-

reactivation was less apparent, but colony size was increased

under a light background. In contrast, in R. mucilaginosa the

reduction in both survival and colony size by UV-B was

greater under a light background than under a dark background

(Table 1).

Relative sensitivity to UV-B radiation

Sri Lankan (SL) isolates were significantly (P! 0±05) less

susceptible to damage than U.K. isolates when all five were

exposed simultaneously to UV-B (4±98 kJ m−# UV-BDNA

)

radiation (Fig. 2). Susceptibility did not differ significantly

(P! 0±05) between the three SL yeasts but the two U.K.

yeasts showed different sensitivity ; Sporidiobolus sp. was

significantly (P! 0±05) less susceptible than B. alba (Fig. 2),

which did not survive at this dose.

Effect of white light (PAR) on number of colonies of

yeast isolates

White light alone had no significant (P! 0±05) effect on the

number of colonies of any of the five yeast isolates. However,

S. salmonicolor from SL showed some slight sensitivity to

white light (Fig. 3).

UV-B and phylloplane yeasts 782

Table 1. Dose of UV-BDNA

required to cause 50% cell mortality (LD&!) and 50% reduction in cells per colony (ED

&!) of five phylloplane yeasts cultured

with white light or dark background

Source Phylloplane yeast Fitted dose–response curve

r# and P

value

LD&!}ED

&!†

(kJ m−# d−" UV-BDNA

)

(a) Mortality

White light background

SL S. salmonicolor 1}((1}100)­(1±15¬0±58D)) 0±67*** 8±64³0±42SL Cryptococcus sp. 1}((1}100)­(4±78¬0±45D)) 0±80*** 7±92³0±72SL R. mucilaginosa 1}((1}100)­(0±29¬0±70D)) 0±55*** 7±25³0±63U.K. Sporidiobolus sp. 1}((1}100)­(2±87¬0±23D)) 0±55*** 3±94³0±42U.K. B. alba 1}((1}100)­(52±4¬0±02D)) 0±85** 2±23³0±18

Dark background

SL S. salmonicolor 1}((1}100)­(0±83¬0±54D)) 0±67*** 7±29³0±39SL Cryptococcus sp. 1}((1}100)­(0±66¬0±51D)) 0±75*** 6±24³0±47SL R. mucilaginosa 1}((1}100)­(0±09¬0±69D)) 0±48*** 9±49³0±88U.K. Sporidiobolus sp. 1}((1}100)­(0±85¬0±30D)) 0±55*** 4±06³0±43U.K. B. alba 1}((1}100)­(8±27¬0±22D)) 0±86*** 1±78³0±13

(b) Colony size (cells per colony)

White light background

SL S. salmonicolor (6±74¬10')®(5±64¬10&¬D)­(3701¬D#) 0±86*** 6±24³0±35SL Cryptococcus sp. (1±43¬10()®(1±78¬10'¬D)­(54636¬D#) 0±87*** 4±68³0±41SL R. mucilaginosa (1±01¬10()®(1±42¬10'¬D)­(52038¬D#) 0±82*** 4±30³0±37U.K. Sporidiobolus sp. (5±70¬10')®(2±20¬10'¬D)­(204354¬D#) 0±80*** 1±50³0±12U.K. B. alba (1±17¬10')®(5±37¬10&¬D)­(57736¬D#) 0±53*** 1±27³0±09

Dark background

SL S. salmonicolor (3±27¬10')®(5±52¬10&¬D)­(23634¬D#) 0±88*** 3±49³0±37SL Cryptococcus sp. (1±25¬10()®(1±78¬10'¬D)­(23460¬D#) 0±87*** 4±12³0±39SL R. mucilaginosa (9±53¬10')®(9±60¬10&¬D)­(62733¬D#) 0±82*** 5±79³0±40U.K. Sporidiobolus sp. (3±40¬10')®(1±61¬10'¬D)­(185119¬D#) 0±83*** 1±22³0±15U.K. B. alba (3±52¬10&)®(2±66¬10&¬D)­(47141¬D)# 0±83*** 0±77³0±09

LD&!

and ED&!

values and their standard errors were calculated from dose–response curves (see Fig. 1 for example). The equations of the fitted dose–response

curves, their associated coefficients of determination r# and statistical significance are also given for each yeast. D¯UV-B dose, ** and ***, significant at

P! 0±01 an P! 0±001 respectively.

† LD&!, mortality ; ED

&!, colony size (cells per colony).

0

5

10

15

20

25

30

35

40

45

50

a

a

a

b

SLPY SLRY SLWY BPY BWY

c

Yeast isolate

Sur

viva

l (ar

csin

tran

sfor

med

) (%

)

Fig. 2. Effects of UV-B radiation (4±98 kJ m−# UV-BDNA

) on survival

of colonies of five yeasts. Irradiated colonies are compared with dark

controls. SLPY: Sporidiobolus salmonicolor (SL) ; SLRY: Rhodotorula

mucilaginosa (SL) ; SLWY: Cryptococcus sp. (SL) ; BPY: Sporidiobolus sp.

(U.K.) ; BWY: Bullera alba (U.K.). Different letters indicate significant

(P! 0±05) difference. Bars indicate standard error of mean of five

replicates. The experiment was repeated with similar results.

0

0·2

0·4

0·6

0·8

1

1·2

SLRY SLWY BPY BWY

Yeast isolate

cfu

(lig

ht/d

ark)

SLPY

Fig. 3. Effects of white light on five yeasts. Ratios of colony forming

units (cfu) in light and dark are shown. SLRY: Rhodotorula mucilaginosa

(SL) ; SLPY: Sporidiobolus salmonicolor (SL) ; SLWY: Cryptococcus sp.

(SL) ; BPY: Sporidiobolus sp. (U.K.) ; BWY: Bullera alba (U.K.). Bars

indicate standard error of three repeated experiments, each had five

replicates.

The effectiveness of different bands within the UV

spectrum

The spectral responses of Sporidiobolus sp. (U.K.) (Fig. 4a) and

of B. alba (U.K.) (Fig. 4b) were broadly similar. Wavelengths

T. S. Gunasekera, N. D. Paul and P. G. Ayres 783

120

100

80

60

40

20

0270 280 289 300 310 320 330 340

Wavelength (nm)

Sur

viva

l (%

)

(b)

120

100

80

60

40

200

270 280 289 300 310 320 330 340

(a)

Fig. 4. Effects of different wavebands of UV radiation on the survival

of colonies of (a) Sporidiobolus sp. and (b) Bullera alba. Irradiated

colonies are compared with dark controls. Bands transmitted by the

filters were 10 nm wide at half peak height. Four measurements were

made with each filter and are plotted at the band centres. The

histogram shows the mean values for each waveband.

longer than 320 nm had no significant effect on the number of

colonies. However, colony survival was significantly reduced

at 310 nm and all shorter wavebands. No colonies were

counted at 290 nm or below.

DISCUSSION

Both colony number and size were examined here because, as

elsewhere, the highest doses of UV-B killed cells, probably

through direct damage to DNA (Zo$ lzer & Kiefer, 1983).

Biphasic survival curves similar to those reported here have

been found for the yeast community of the apple phylloplane

and interpreted as evidence for ‘multiple-hit ’ inactivation

kinetics (Pennycook & Newhook, 1982). UV-B at lower doses

may kill only part of any population and, also, may reduce

colony size by affecting rates of cell division. Thus, James &

Nasim (1987) found uv caused an immediate post-irradiation

delay in the division of Saccharomyces cells, followed by a

further delay in the production of second-generation buds but,

thereafter, a stimulation in the rate of cell division which

persisted through several generations. Liefer et al. (1986)

found both budding and division were equally delayed in

Sacchomyces cerevisiae Meyen ex E. C. Hansen by radiation at

313 nm while radiation at 303 nm affected budding signi-

ficantly less than division. Since yeast cells growing in the

exponential phase were used in the present study, emergence

of buds and DNA synthesis may have been affected by

exposure to UV-B.

When filtered with cellulose diacetate, the spectrum of UV-

B lamps (Philips TL UV-B 40W}12) does not include the

wavelengths shorter than 292 nm. Since polyester does not

sharply separate longer wavelengths of UV-B (! 320 nm)

and shorter wavelengths of UV-A (" 320 nm) a little radiation

at wavelength" 320 nm was presented. However, the

spectral responses of two yeasts, Sporidiobolus sp. (U.K.) and B.

alba (U.K.), were examined in detail (Fig. 4) and both showed

that UV-A caused little damage. Even within the UV-B range,

shorter wavelengths were much more effective than longer

wavelengths approaching 320 nm. Our studies were not

designed to establish action spectra and we do not exclude the

possibility that the higher irradiance of UV-A present in

natural sunlight may have some deleterious effect on

phylloplane yeasts. However, our data clearly indicate that it

is the UV-B component of solar radiation that is primarily

effective in regulating populations of phylloplane yeasts.

Confirmation comes from field studies which consistently

show that polyester screens, which remove UV-B from

sunlight, significantly increase populations of phylloplane

yeasts (Ayres et al., 1996 ; Gunasekera, 1996).

Both dose–response studies and simultaneous irradiation

(using the same dose) showed the isolates from Sri Lanka were

less sensitive to UV-B than were the U.K. isolates. Five is a

small sample on which to base a firm conclusion other than

that there are large and significant differences between species

in their sensitivity to UV-B, and the extent of intra-specific

variability remains to be investigated, but the results do

suggest that the incidence of UV-B-tolerant organisms may

increase with decreasing latitude. Among higher plants similar

differences are well known. Sullivan & Teramura (1988)

showed that species native to higher elevations, where UV-B

doses are relatively high, were less susceptible to UV-B than

those from lower elevations. Similarly, plants native to high

latitudes are often more sensitive to UV-B injury than those

originating from lower latitudes (Bachelet et al., 1991). Isolates

of the same filamentous fungus, Septoria tritici Roberge, from

geographical regions with contrasting UV-B climates had

different tolerances of UV-B (Rasanayagam et al., 1995). When

the extension of germ-tubes was compared, isolates from

Tunisia, where doses of UV-B are relatively high, were more

tolerant of UV-B than isolates from the U.K. where doses are

relatively low, although a later survey (Paul et al., 1996)

showed significant variation in UV-B response between

isolates from a single site in the U.K.

The maximum mid-day irradiance at Lancaster under clear

sky conditions predicted by the model of Bjo$ rn & Murphy

(1985) is 0±08 W m−# s−", and the maximum integrated dose

is 1±7 kJ m−# d−" UV-BDNA

. The corresponding maximum

value predicted for Sri Lanka is 3±8 kJ m−# d−" UV-BDNA

during March. Most importantly, and supporting the

hypothesis that this work tested, survival and}or colony size

of all isolates are predicted to be affected in vivo, to greater or

lesser extent, by doses of UV-B occurring at their own site of

origin (Table 2). The two U.K. isolates would be differently

affected, so B. alba and Sporidiobolus sp. are predicted to show

11±7 and 3±8% mortality, respectively. Similarly, the three SL

isolates would also show contrasting responses ; Cryptococcus

sp. would show no mortality, while S. salmonicolor and R.

mucilaginosa would show 6±4 and 20±2% mortality, re-

spectively. Colony size would be reduced for both U.K. and

SL isolates at ‘ local ’ doses, but the effect would be greater

with the U.K. isolates than with the SL isolates (Table 2).

None of the yeasts showed total insensitivity to its ‘ local ’

UV-B environment. This suggests there is intra-specific

variation in response and that insensitivity to (or tolerance of)

UV-B probably has a cost, or disadvantage. Thus, natural

UV-B and phylloplane yeasts 784

Table 2. Estimated mortality and reduction of cells per colony (colony

size) for yeasts under UV-B doses representative of their natural habitat

Source Yeast

Dose

(kJ m−# d−"

UV-BDNA

)

Mortality

(%)

Reduction

of cells

colony−"

(%)

SL S. salmonicolor 3±78 6±35 30±80SL Cryptococcus sp. 3±78 0±11 41±65SL R. mucilaginosa 3±78 20±18 45±48U.K. Sporidiobolus sp. 1±7 3±81 54±96U.K. B. alba 1±7 11±72 63±54

Values derived from the dose–response curves (Fig. 1). UV-B doses of

1±7 kJ m−# d−" UV-BDNA

(equivalent to mid-summer Lancaster level under

clear sky) and 3±8 kJ m−# d−" UV-BDNA

(maximum theoretical value for Sri

Lanka) were used for U.K. and Sri Lankan isolates respectively.

populations may be comprised of cells that collectively have

a range of tolerances. The UV-B environment is not uniform;

the more sensitive cells may occupy sites that provide some

protection against radiation, such as depressions between

cells, substomatal cavities, or the base of trichomes.

White light provided relatively little protection for the U.K.

isolate of Sporidiobolus sp. but, excepting R. mucilaginosa, all

yeasts demonstrated decreases in UV-B damage in the

presence of white light, i.e. evidence of photoreactivation or

other light mediated repair mechanisms (PR). The capacity for

PR is not a universal characteristic among yeasts, thus S.

cerevisiae is photoreactivable while Schizosaccharomyces pombe

Lindner is not (Harm, 1980), and may be related to habit, e.g.

leaf or animal surface, as well as taxonomy (the yeast growth

form occurs in taxonomically diverse groups). The absence of

PR in R. mucilaginosa does not prove an absence of that

capacity. Though PR is usually induced by wavelengths

ranging from 310 to 490 nm, some wavelengths may be

particularly effective. For example, PR occurs between 320

and 440 nm with a peak around 375 nm in S. cerevisiae (Jagger,

1985). The quantity and spectral balance of white light

provided here may not have been enough to induce PR in

R. mucilaginosa. The inhibitory effect of white light, in the

presence of UV-B, on R. mucilaginosa is problematical.

However, somewhat similar results have been reported for

human limphoblastoid cells (Tyrell & Amaudruz, 1984), where

UV-A and visible wavelengths sensitized those cells to the

lethal action of UV-B.

Apart from the capacity for PR, tolerance of UV-B may be

related to pigmentation, which for micro-organisms living on

aerial plant surfaces is believed to play a protective function

against injurious solar radiation (Dickinson, 1986). In many

instances, it has been suggested that pigments in fungi are the

main protective mechanism against uv radiation (for example,

Maddison & Manners, 1973 ; Dickinson, 1986 ; Asthana &

Tuveson, 1992). It is not known whether UV-B promotes

carotenoid synthesis in Sporidiobolus sp., but in Rhodotorula

minuta the action spectrum for photoinduced carotenogenesis

has a prominent, broad peak centred on 280 nm (Tada et al.,

1990). Of the two U.K. isolates, B. alba (U.K.) was more

sensitive than Sporidiobolus sp. (U.K.), possibly because B. alba

lacks particular carotenoid pigments possessed by Sporidiobolus

sp. (Simpson et al., 1971). The same relative sensitivities were

noticed in the field when Sporidiobolus sp. and B. alba were

inoculated on young bean leaves and exposed to depleted or

full sunlight (Ayres et al., 1996).

The results show that spatial and temporal variations in

current UV-B doses may have a major role in regulating

populations. The ramifications of climate change, in which

UV-B doses are increased, are intriguing because these micro-

organisms typically grow in microbial communities and often

interact with each other. Thus, since B. alba is more sensitive

than Sporidiobolus sp. to UV-B, a change of UV-B climate

would give B. alba advantage over Sporidiobolus sp. and could

alter the balance between these two organisms which occupy

the same ecological niche. Similarly, since leaf surface yeasts

are known to interact with plant pathogens and affect disease

development in the field (Bashi & Fokkema, 1977 ; Fokkema et

al., 1979 ; Fokkema, 1984), increased doses of UV-B could

affect the balance between saprotrophs and parasites and

influence the incidence of disease in the field.

We thank the Tea Research Institute of Sri Lanka (TSG) and

the U.K. Department of Environment (Contract PECD

7}12}21 : NDP) for financial support.

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