Spore discharge rhythms in pyrenomycetes

[ 355 ] Trans. Br. my col. Soc. 52 (3) , 355- 363 ( 1969) Printed in Great Britain

SPORE DISCHARGE RHYTHMS IN PYRENOMYCETES VI. THE EFFECTS OF CLIMATIC FACTORS ON SEASONAL AND DIURNAL PERIODICITIES By

1.J. HODGKISS*

AND

R. HARVEY

Department of Botany, University College, Cardiff (W ith 5 Text-figures) Seasonal and diurnal p eriodicities of spore discharge have been determined in seven lignicolous pyrenomycetes, using a Hirst spore trap. Hyposylon rubiginosum, H. fragiforme and Lasiosphaeria spermoides had peak discharge periods in October, Eutypa acharii in May, Diatrype disciformis in April-May, Diatrypella quercina in September and Diaporthe sp. in August. No spores were discharged by l lyp oxylonfragiforme during late F ebruary to early April, none by Diaporthe sp, during November-January, and none by Diatrypella quercina in March. Analysis of spore counts and climatic data indicated a positive correlation between spore discharge a nd rainfall excep t in Lasiosphaeria spermoides(di scharge correlated with humidity) an d Eutypa acharii (d ischarge correlated with duration of sunsh ine) . All the sp ecies have diurnal rh ythms with nocturnal peaks exc ept E. acharii, in which the spore discharge rate fluctuates more rapidly producing early morning and afternoon p eaks. Spore discharge rhythms a re considerably modified from time to time by climatic factors, particularly rainfall.

Previously (W alkey & Harvey, 1965) the influence of climatic factors on discharge rhythms in pyrenomycetes was investigated by means of slides placed above stromata on selected logs. Counts of the accumulations of discharged spores obtained in this way ga ve some indication of periodicity, but were not suitable for estimations of spore concentrations. In further investigations the sampling technique has been changed by the use of a Hirst spore trap, which made it possible to estimate both seasonal and diurnal variations in spore concentrations. Selected logs were placed on soil trays immediately adjacent to the Hirst spore trap, on the roof of the Botany Department (approx. 14'3 m above ground level). The trap was locked in position with its orifice directed towards the trays of materials, all of which lay within a distance of 0'9 m, The logs were more exposed than under natural conditions because of the absence of surrounding vegetation. Wind speeds averaged between a-B and 24' I kmJh (3- 15 m .p .h .), butonlya-Bc-S'o km/h (3- 5 m .p .h .) during IS of the total 26 months during which surveys were carried out. Tests with anemometers showed that prevailing winds were much modified by the surrounding buildings which had the effect, over long periods of time, of 'funnelling' air across the materials towards the spore trap. Spore counts obtained indicated that there was no apparent impairment of the efficiency of the trap.

*

Present address : Department of Botany, The University, Hong Kong. 23- 2

Transactions British Mycological Society MATERIALS AND METHODS

The fungi investigated were: Hypoxylon rubiginosum (Pers. ex Fr.) Fr., Hypoxylonfragiforme (Pers. ex Fr.) Kickx, Eutypa acharii Tul., Lasiosphaeria spermoides (Hoffm. ex Fr.) Ces. & de Not., Diatrype disciformis (Hoffm. ex Fr.) Fr., Diatrypella quercina (Pers. ex Fr.) Cooke, and Diaporthe sp. Of these H. rubiginosum was studied previously (Walkey & Harvey, 1966) using a spore train apparatus, and together with H.fragiforme, was further studied under field conditions (W alkey & Harvey, 1968) using fixed slides for the collection of spores. Climatic data were recorded daily at og.oo h and continuously by means of a hygro-thermograph. Incidence and duration of rainfall was recorded by means of a sensor apparatus placed in an exposed position among the logs. This comprised an electrical circuit of interlocking lead strips connected to a Sodeco Printing Impulse Counter, housed in an adjacent greenhouse. Rain falling on the sensor reduces resistance between adjacent lead strips, thus activating a relay which is recorded by the counter. Run-off and evaporation of rain from the surface of the sensor increases circuit resistance. Change from high to low, and low to high resistance operates the impulse counter which records, to the nearest minute, the incidence and duration of rainfall. The spore trap slides were changed daily at 09.00 h. They were scanned, using x 45 or x 100 objectives, across 12 short traverses at z-hourly intervals commencing at 10.00 h on each slide. RESULTS

Seasonal periodicity Spore discharge occurred throughout the year in Hypoxylon rubiginosum, Eutypa acharii, Lasiosphaeria spermoides and Diatrype disciformis (Figs. I, 2). In Hypoxylonfragiforme spores were not discharged for a period of 45 days during late February to early April, whereas in previous surveys (Walkey & Harvey, 1968) discharge was recorded throughout this period for that species, but during October to May only, for Hypoxylon rubiginosum. In Diaporthe sp. no spores were discharged during November to January and in Diatrypella quercina discharge stopped during March. Seasonal variations were more pronounced in the Hypoxylon spp. and Lasiosphaeria spermoides than in Eutypa acharii with peak discharge occurring during October (Figs. I, 2). Both daily and monthly peaks for E. acharii occurred during May when the average daily spore discharge rate was almost three times greater than in any other month; otherwise there was no well-defined seasonal variation in discharge, the rate fluctuating quite rapidly from month to month. The October discharge peaks of the former species coincided with the highest monthly rainfall of 158 mm (6'22 in) and the highest average relative humidity (86 '3 %). The May peak for E. acharii coincided with the highest monthly total of sunshine, 250·6 h (average 8'08 hfday), and a relatively long-day light regime of 16 h light: 8 h dark.

Spore discharge. 1. J. Hodgkiss and R. Harvey

357

All three species investigated during the period June 1967 to May 1968 showed marked seasonal variation in discharge rates (Fig. 2). Diatrype disciformis reached a peak in April and May; Diaporthe sp. showed a rapid increase in spore discharge during June and July, reaching a well-defined

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seasonal maximum in August. Increased rates of spore discharge in Diatrypella quercina were recorded over a longer period of time (June to October) with a maximum in September. Thus, in all three species, the rate of spore discharge was generally increased during the period May to October. There was no obvious correlation between rainfall, sunshine or temperature and the timing of the seasonal peaks.

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The influence ofrainfall was more clearly demonstrated by a comparison of spore counts obtained on days with rain and those obtained on an equal number of days without rain (Table I). In all the species examined there were marked differences in the average rates of spore discharge on wet and dry days, the ratio of 'wet' and 'dry' discharge rates varying from approximately 2: I in Lasiosphaeria spermoides to 177: I in Diatrypella

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quercina. The close link between rainfall and spore discharge was further demonstrated by comparing fluctuations in both factors over short periods of time. The results, illustrated for four of the species (Fig. 3), indicate the close entrainment of spore discharge by rainfall observed on selected days in all seven species. In an attempt to arrive at rather more exact assessments of the influence

Spore discharge. 1. J. Hodgkiss and R. Harvey

359

Table I. Average daily spore counts on days with ( +) and days without ( - ) rain, extending over a period of I yearfor each species H. rubiginosum

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of climatic factors on the discharge mechanisms, spore counts and climatic data obtained over a total period of 12 months for each species have been subjected to statistical analysis using an English Electric computer. Linear and partial correlation coefficients have been determined for spore

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Transactions British Mycological Society

discharge and selected climatic data (temperature, rainfall, sunlight and humidity) over the period of investigation. Bearing in mind the existence of inherent limitations in the statistical analysis of living systems, the results of the analysis indicated certain well-defined relationships: Hypoxylon rubiginosum and H.fragiforme. Spore discharge in both species showed a positive correlation with rainfall and humidity, a result which confirmed previous observations on the nature of the annual peaks in October. Hypoxylon rubiginosum

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Lasiosphaeria spermoides. Spore discharge showed positive correlation with relative humidity, again confirming the conclusion drawn from examination of graphs of seasonal variation. Diatrype disciformis, Diatrypella quercina and Diaporthe sp. Spore discharge showed a positive correlation with rainfall which was not apparent from examination of graphs of seasonal variation. Eutypa acharii. No good correlations were obtained from data recorded over the whole period of the investigation, but monthly averages showed a positive correlation with the number of hours of incident sunlight. To this extent, at least, conclusions based upon graphs of seasonal variations

Spore discharge. 1. J. Hodgkiss and R. Harvey

361

are again confirmed by analysis. E. acharii was also exceptional amongst the species investigated in that no well-defined diurnal rhythm ofdischarge was detectable, even during the peak period of spore discharge.

Diurnal periodicity Diurnal periodicity has been investigated using both the geometric mean values of spore counts during the peak (9- to Is-day) periods of spore discharge (Fig. 4) and total counts obtained throughout the survey periods (Fig. 5) . ."..

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Hypoxylon rubiginosum and H.Jragiforme. Nocturnal species with maximum discharge shortly after midnight. The result for H . rubiginosum confirms a previous observation (W alkey & Harvey, 1966). Lasiosphaeria spermoides. Geometric mean values for the r o-day peak period indicate a nocturnal rhythm increasing to a maximum at 08.00 h. L. ovina was also found to be nocturnal (W alkey & Harvey, 1966) but with a maximum before midnight. Eutypa acharii. No well-defined rhythm of discharge was determined in

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Transactions British Mycological Society

the peak period nor during the remainder of the year. Arithmetical and geometric mean values indicate two to three waves of activity each day, the first during early morning (04.00-06.00 h), the second at 14.00 hand the third at 22.00 h. Diatrype disciformis, Diatrypella quercina and Diaporthe sp. Nocturnal species with maximum rates of spore discharge at 22.00 or 02.00 h, falling to minimal levels between 10.00 and 16.00 h. If diurnal variation on the peak day is compared with that recorded throughout the year (Fig. 5) then it becomes apparent that the longer the period of observation the more 'diffuse' or variable is the recorded discharge rhythm. Thus in Lasiosphaeria spermoides considerable fluctuations in discharge rates by day and night are indicated over a period of one year. In this species arithmetical averages of counts obtained throughout the period indicate an evening maximum at 18.00 h associated with a mainly nocturnal rhythm of discharge. This is interpreted as a measure of the influence of environmental factors on the diurnal rhythm, which is subject to extremes of modification, particularly during the 'off-peak' periods of activity. DISCUSSION

The results obtained for the seven species under investigation confirm, in each case, the existence of both seasonal and diurnal rhythms of spore discharge activity. In some species the peak period of spore discharge can be related to a particular seasonal factor, e.g. Hypoxylon spp. and Lasiosphaeria spermoides produced more spores during the wettest month, October. Similarly, the May peak for Eutypa acharii appears to coincide with the maximum amount of sunlight recorded. Statistical analysis supports these conclusions and also indicates a positive correlation between rainfall and spore discharge in Diatrype disciformis, Diatrypella quercina and Diaporthe sp. Previous work (Walkey & Harvey, 1968) has shown that under natural conditions rainfall has the greatest climatic influence on spore discharge in pyrenomycetes but that its effects are varied according to the nature of the rainfall-in particular its duration and intensity. This has been shown to be true for the present species by comparing rates of discharge during different types of rainfall. Generally short periods of heavy rain produced higher spore counts than longer periods of more continuous light rain. Examination of the duration of rainfall and spore discharge over periods of several days showed a close entrainment of discharge with rainfall. Variations in air temperatures had little apparent effect on discharge, providing temperatures were above freezing-point. In the absence of rain high relative humidity had no apparent stimulatory influence, but was effective in maintaining higher rates of discharge immediately following a period of rain. Where rain periods were followed by dry days with lower humidities (65-70 % r.h.) rates of spore discharge fell much more sharply. Six of the seven species examined proved to have nocturnal peaks in their discharge rhythms. The exceptional species was Eutypa acharii, in which counts of discharged spores showed more frequent fluctuations

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363

throughout the day and night. A comparison of peak-day and peak-period rhythms with those based upon much longer periods of observation (12 months) clearly showed that over long periods the 'normal' pattern of discharge activity is subject to very considerable modifications. The latter are undoubtedly caused by the cumulative effects ofseasonal climatic variations and by short-term fluctuations, as in rainfall. Apart from the complex of interactions between climatic factors and the normal seasonal and diurnal rhythms of discharge there remains the problem of the influence of innate factors. These include the rate of development of the stroma and of the population of perithecia which it bears. Determination of the cycle of development of individual perithecia is possible in culture but is much more problematical in lignicolous stromatic forms. We wish to thank Mrs P. N. Lewis for technical assistance and Mr G. Jones of the Electronics Workshop of this College who devised and constructed the rainfall recording apparatus. One of us (I.J. H.) was supported by a Science Research Council Studentship, which is gratefully acknowledged. REFERENCES

D. G. A. & HARVEY, R. (1966). Spore discharge rhythms in pyrenomycetes, I. A survey of the periodicity of spore discharge in pyrenomycetes. Trans. Br, mycol. Soc. 49, 583-592. WALKEY, D. G. A. & HARVEY, R. (1968). Spore discharge rhythms in pyrenomycetes. IV. The influence of climatic factors. Trans. Br. mycol. Soc. 51, 779-786. WALKEY,

(Accepted for publication 18 November 1968)