J. Phytopathology 130, 235—240 (1990)© 1990 Paul Parey Scientific Publishers, Berlin and HamburgISSN 0931-1785

Division of Biological Sciences, University of Lancaster, U.K.

The Effects of Temperature on the Respiratory Responsesof Groundsel {Senecio vulgaris)

to Infection by Rust {Puccinia lagenophorae)

DIANE JENNINGS and PETER G. AYRES

Authors' address: Division of Biological Sciences, Institute of Environmental and Biological Sciences,University of Lancaster, Bailrigg, Lancaster LAI 4YQ, U.K.

With one figure

Received Fehruary 6, 1990; accepted April 12, 1990

Abstract

Groundsel {Senecio vulgaris) was grown in either a warm (20'^C) or a cool (8°C) controlledenvironment and infected with Puccinia lagenophorae. Dark respiration, tneasured over the range 6 to18°C, was higher in leaves of healthy plants grown under low temperatures than in those of plantsgrown under high temperatures. Infection increased the rates of dark respiration in the region ofsporulating lesions in both sets of plants, but the greater increase in plants grown under warmconditions resulted in both sets having similar respiration rates across the range 6 to 18°C. Theconclusion that the tnagnitude of the respiratory increase following rust infection depends upon theconditions under which plants were grown is supported by literature on other rust diseases and hasimphcations for the utilization of carbohydrate reserves and the survival of both rust and hostpopulations over winter.

Zusammenfassung

Der Einflufi der Temperatur auf die Atmungsreaktiotien des gemeinen Kreuzkrautes(Senecio vulgaris) nach einer Rostinfektion (Puccinia lagenophorae)

Das gemeine Kreuzkraut {Senecio vulgaris) wurde entweder in einer warmen (20'^C) oderkiihlen (8°C) Umgebung herangezogen und mit Puccinia lagenophorae inokuliert. Die Atmung imDunkeln, gemessen liber den Temperaturbereich 6 bis 18°C, war hoher in Blattern von gesundenPflanzen, die bei der kiihleren Temperatur heranwuchsen, ais in den Blattern von Pflanzen nach einerhoheren Anzuchttemperatur. Eine Rostinfektion erhohte die Rate der Atmung im Dunkeln in derRegion um die sporuherendet! Lasionen in beiden Pflanzengruppen. Durch die grofiere Erhohung inPflanzen, die der warmeren Anzuchttemperatur ausgesetzt waren, hatten beide Pflanzengruppenahnliche Atmungsraten liber den Temperaturbereich 6 bis 18 °C. Die Schlufifolge, da£ die GroEe derAtmungserhohung nach einer Rostinfektion von den Anzuchtbedingungen der Pflanzen abhangt,

U.S. Copyrigh, Clearance C.n.er Code Sutement: 093 1 -1 7 8 5 / 9 0 / 3 0 0 3 - 0 2 3 5 $ 0 2 . 5 0 / 0

236 JENNINGS and AYRES

wird durch andere Literaturzitate iiber andere Rostkrankheiten bestatigt. Diese Abhangigkeit hat eineBedeutung beziiglich der Verwertung von Kohlenhydratreservoiren und auf die Uberlebenschancender Rost- und Wirtspflanzenpopulationen im Winter.

Survival over winter is a potential problem for obligately biotrophic patho-gens of annual hosts, both wild and crop plants. In practice, however, crops sownin autumn form a 'green bridge' between growing seasons and in many wildannual plant species there is a small cohort that survives winter vegetatively, thus,offering pathogens of that species an opportunity to bridge successfully the gapbetween the main growing seaons. Groundsel (Senecio vulgaris L.) is such a plant;it is capable of surviving winter in Britain and growing intermittently whenconditions are favourable. Although it is often difficult to find lesions of rust(Puccinia lagenophorae Gooke) on plants in late winter, direct observationindicates that a few survive. Moreover, the rapid increase in frequency of rustlesions on seedlings in spring suggests that infection arises from surviving localsources.

In susceptible hosts, rust fungi cause prolonged increases in rates of darkrespiration, which are usually centred on infection sites and reach a peak at or justbefore sporulation commences (BUSHNELL 1984, FARRAR and LEWIS 1987). How-ever, measurements of respiration have always been made in the range 20—25 °G,i.e. under conditions typical of a temperate European summer. The object of thisstudy was to measure dark respiration under conditions more tj'pical of winter,since enhanced respiration at low temperatures could seriously deplete the energyreserves of plants whose photosynthetic activity was curtailed by the short, dulldays typical of winter. If reserves were exhausted, the result could be the death ofthe pathogen and, even, of the host. Respiration was compared in plants grownunder cool and warm conditions since the former would represent more realisti-cally plants that occur in winter. The effects of growth conditions on therespiratory responses of plants to infection have not been investigated previously.

Materials and Methods

Plants and inoculation

Groundsel seeds were germinated on moist Shamrock potting compost (Bord na Mona, Dublin)and incubated at 20 ± 1 ''C. After one week the seedlings were transplanted into individual potscontaining John Innes No. 2 potting compost and transferred to growth rooms at either 20 or 8 ±1 "C; in both rooms irradiance of 190 ̂ E m"̂ s~' was provided for 16 h per day.

After 35 or 14 days for plants grown at low and high temperatures, respectively, plants wereinoculated with an aqueous suspension of aeciospores of P. lagenophorae, collected from stock plants,as described previously (PAUL and AY5!ES 1984). Plants were incuhated in the dark for 24 h at 15°C,under polythene bags to maintain a high humidity, and then returned to the separate growth rooms.Measurements of respiration were made after sporulation commenced, which was approx. 21 days and10 days after inoculation for plants grown under low and high temperatures respectively.

Respiration

Rates of dark respiration were determined by measuring O2 uptake by excised tissues in aGilson respirometer. Leaf discs, 3 mm in diameter, were cut with a cork borer, using leaves ofcomparable age from plants grown at the same temperature. Discs avoided the midrib and in infected

The Effects of Temperature on the Respiratory Responses of Groundsel 237

tissues centred on freely sporulating lesions. Fourteen discs were fitted into each respirometer flask,the bottom of which was lined with foam rubber soaked with 2 cm^ of distilled water to tnaintam asaturated atmosphere in the flask. The side-arm of each flask contained 1 ctn' of 2 M KOH to absorbCOi. A fresh set of discs was prepared for each temperature at whicb respiration was measured andeach treatment (growth temperature/infection) was replicated three times.

Results

In healthy and infected plants grown at both temperatures, dark respirationincreased as the temperature at which it was measured increased (Fig. 1). Inhealthy plants, the rate at each temperature was higher in plants grown at 8°Cthan in plants grown at 20°C, but plants had similar respiration rates when theywere compared at their own growth temperatures, as is commonly found in suchcircumstances (MACIEJEWSKA et al. 1984, MCNULTI' and CUMMINS 1987).

Fig. 1. Dark respiration in leaves of groundselgrown at either 20°C ( ) or S°C ( ) andhealthy (O) or infected by rust ( • ) . Bar shows

L.S.D. at P = 0.056 10 14 18Temperature at which measured (°C)

Infection stimulated respiration across the temperature range of measure-ment; the increase was greater in plants grown at 20 °C than in plants grown at8°C, but in each set of plants appeared unaffected by the temperature at which itwas measured (Table 1).

Table 1Effects of piatit growth temperature on the rate of dark respiration m groundsel after mfection by rust

Growthtemperature,

Respiration, infected/healthy

Measurement temperature, °C

10 12 14 16 18

20 2.411.56

2.251.06

3.001.38

3.091.46

2.771.47

3.191.46

2.651.20

23 S JENNINGS and AYRES

Discussion

The contribution that fungal respiration makes to the total respiration ofrust-infected tissues cannot be determined directly, since that of host and funguscannot be separated and, consequently, it is a matter of continuing debate(OwERA et al. 1981). It is possible that the greater increase in respiration thatoccurred in warm-grown than in cool-grown plants after infection was the resultof there being greater fungal biomass, i.e. more infection, in the former.However, this seems unlikely since visible symptoms of infection were similar atthe time the two sets of plants were sampled (at different times after inoculation)and each small disc was selected so that it was centred on a prominent lesion.Whatever the contribution of host and fungus, the rate of total respiration afterinfection was as high in cold-grown plants as in warm-grown plants.

The conditions used here effected proper cold-acchmation, as reflected incarbohydrate accumulation. In related studies under similar conditions (N. D.PAUL, unpublished results from this laboratory), the carbohydrate concentrationsof cold- and warm-grown healthy groundsel were 21.9 and 8.4 mg g"' freshweight, respectively. Rates of tissue respiration are not linked by any simplerelationship to the carbohydrate content of the tissue (FARRAR 1985), but respira-tion is a drain upon carbohydrate reserves. Thus, in cold-grown plants, rustreduced the carbohydrate concentration to 18.7 mg g"' fresh weight {F = 5.50with 19 df, P = 0.05). In the field such reserves would be less readily replaced inwinter than in summer suggesting that exhaustion of reser\'es may be one reasonwhy the frequency of rust colonies decreases over winter and, even, why hostmortality is exceptionally high (PAUL and AYRES 1986).

The relative increase in respiration after infection in cold-grown plants wassmaller than in warm-grown, which may be seen as a means of conservingcarbohydrate. Examination of recent Hterature on the respiration of rust-infectedplants reveals that, apart from 10-fold, or more, increases in the respiration rate ofwheat infected by stem rust (BUSHNELL 1970), increases in a range of plants grownand studied at warm temperatures have remarkably consistent peaks (Table 2) andsimilar to those noted here. This is in spite of the fact that healthy tissues are oftenincluded and that some values are based on O2 uptake and others on CO2 output.Therefore the two could be differently affected if rust infection changed theengagement of the alternative (cyanide-resistant) pathway of respiration as doespowdery mildew infection in barley (FARRAR and RAYNS 1987, MCAINSH et al.1989). We can find only one previous instance of respiration being measured onplants grown at low temperatures; bluebells were collected from the field inspring and their respiration was measured at 20°C (ScHOLES and FARRAR 1985).The small increase in respiration in that instance, 1.4, is remarkably similar to thatnoted here for cold-grown plants. It is clear that the conditions under whichplants are grown have a marked effect on their respirator)^ responses to infectionand both the temperature of growth and that at which respiration is measuredshould be carefully considered in any assessment of the consequences of increasedrespiration, as, for example, for the survival of rust colonies and host plants overwinter.

The Effects of Temperature on the Respiratory Responses of Groundsel 239

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240 JENNINGS and AYRES, The Effects of Temperature on the Respiratory Responses

We thank Dr. NiGEL PAUL for many helpful discussions in the course of this work and Mr. S.G. HALLETT and Mr. P. SMITH for technical assistance.

Literature

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, 1984: Structural and physiological alterations in susceptible host tissue. In: BUSHNELL, W. R.,and A. P. ROELFS (eds). The Cereal Rusts. Vol. I. Origins, specificity, structure andphysiology, pp. 477—507. Academic Press, London.

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AiTiES (eds). Fungal Infection of Plants, pp. 92—132. Catnbridge University Press, Cam-bridge.

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