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Journal of Experimental Botany, Vol. 50, No. 335, pp. 845±852, June 1999 Pattern of respiration by intact inflorescences of the thermogenic arum lilyPhilodendron selloum

Roger S. Seymour

1 Department of Environmental Biology, University of Adelaide, Adelaide, SA 5005, Australia

Received 5 October 1998; Accepted 25 January 1999

Abstractodours that are attractive to insect pollen vectors (Meeuse

and Raskin, 1988). Some species are able to raise in¯or-Inflorescences of the neotropical arum lily,escence temperature as much as 35°C above the temper-Philodendron selloum, are strongly thermogenic forature of the air, and consume oxygen at prodigious rates2 d during anthesis. Continuous measurements of(Seymour, 1997; Seymour and Schultze-Motel, 1997). Aspadix temperature (Ts) and rate of oxygen consump-few species are extraordinary because they regulatetion (VÇO2) were made outdoors in whole inflorescencesin¯orescence temperature by varying the rate of heatattached to the plants. Some inflorescences wereproduction inversely with ambient air temperature.exposed to uncontrolled ambient temperature (Ta)The ®rst documented thermoregulating plant was thewhile others were enclosed in clear water-jacketsarum lily,Philodendron selloum, a native of Brazil (Nagythat produced nearly constant ambient conditions.et al., 1972). This study involved measuring temperatureA repeatable, diphasic pattern of heat productionand rate of oxygen consumption of the spadix after itappeared, most clearly in water-jacketed inflores-had been cut from the plant and placed into a cabinet atcences, and it comprised a short peak phase at sunsetselected ambient temperatures. Maximum spadix temper-followed by a plateau phase that lasted until the follow-atures varied little (38±46°C) within a broad ambienting sunset. Regulation ofTsoccurred in both phases,temperature range (4±39°C). However, each severedbut at different levels. Peak phaseTswas regulated inspadix produced only one intense thermogenic episodethe region of 38±42?C, but plateau phaseTswas usu-that ended in less than 2 h and never achieved steady-ally in the range of 25±36?C. BothVÇO2and total heatstate. A subsequent study showed that thermogenesis byproduced throughout anthesis increased at lowerTa.intact (uncut) in¯orescences lasted almost 2 d and wasThe data imply that the short peak phase is related tocharacterized by two episodes of heatingÐthe ®rst pro-the enhancement of odour production that attracts aduced a large temperature elevation that was associatedsingle species of large scarabaeid beetle in its nativewith receptivity of female (pistillate) ¯orets at the baseBrazil, and regulation of maximumTsmay prevent over-of the spadix, and the second produced a smaller elevationheating. Thermoregulation in the long plateau phasethat occurred before pollen was shed from male (stamin-produces equable temperatures inside the inflores-ate) ¯orets at the top (Seymouret al., 1983). However,cence that may facilitate the resident beetles' activit-neither study included measurements of oxygen consump-ies as a direct energetic reward.tion of in¯orescences attached to the plants, and there

were no respiratory measurements during the secondKey words: Thermoregulation, thermogenesis,Philo-phase, even from cut spadices. Levels of heat productiondendron, arum lily, heat production, oxygen consumption,and extent of temperature regulation remained unknownAraceae.throughout the natural sequence of ¯owering. Therefore,the current study includes continuous measurements of

oxygen consumption throughout anthesis by in¯ores-Introduction cences attached to the plant.

The in¯orescences of several species of the arum lily Thermoregulation is mediated by small changes in

temperature of the sterile male ¯orets on the spadixfamily (Araceae) warm up during anthesis and volatilize

1Fax:+61 8 8303 4364. E-mail: rseymour@zoology.adelaide.edu.au

© Oxford University Press 1999Downloaded from https://academic.oup.com/jxb/article/50/335/845/582141 by guest on 26 October 2023

846Seymour

by Seymour and Schultze-Motel (1998). In each channel, (Seymouret al., 1983). Rising ambient temperature atmospheric air was continuously drawn by an individual pump reduces the rate of heat loss, causing spadix temperature through a plastic hood that covered the in¯orescence (spathe to increase. An inverse relationship between respiration and spadix) and through a mass ¯ow meter at about and temperature in the sterile male ¯orets causes heat

400 ml min

1 . A separate pump sampled each channel in turn, and diverted the sample into a paramagnetic oxygen analyser. production to decrease steeply with small rises in spadix Temperatures were measured with PVC-coated thermocouples temperature. Conversely, when ambient temperature in the core of the spadix (Ts), at the level of the sterile male decreases, in¯orescence temperature declines slightly and ¯orets, and in the ambient air (Ta) outside of the spathe but heat-production rises. The e V ect of changes in ambient inside the hood. Barometric pressure and instrument temper-

temperature are not immediate, however; it requiresatures were also measured. Output signals from the ¯owmeters,

oxygen analyser and thermocouples were recorded every 2 min periods of up to 2 h for the ¯orets to recover from throughout the ¯owering sequence. However,VÇO2was calculated exposure to high ¯oret temperature (Seymouret al., every 24 min, because the system cycled through four respirome-

1983), and presumably even longer to respond fully from

try channels (three in¯orescences and one empty reference) at a change in ambient temperature, because of thermal

6 min intervals during this period. The respirometry calculations

and their assumptions are published (Seymour and Schultze- inertia of the~150 g spadix. Slow regulatory responses

Motel, 1998).

to ambient temperature change are a characteristic of All of the in¯orescences were shaded by vegetation and other thermoregulatory ¯owers, for example, Eastern umbrellas to avoid heating by the sun. Ambient temperature skunk cabbageSymplocarpus foetidus(Knutson, 1979; was not controlled in most cases, but six in¯orescences were surrounded by a water-jacket that consisted of a double-walled Seymour and Blaylock, unpublished data) and the sacred wine cooler made of clear styrene to admit light. Construction lotusNelumbo nucifera(Seymour and Schultze-Motel, of the jacket and details of the water circulation system are

1996, 1998; Seymouret al., 1998). Because the responses

described elsewhere (Seymouret al., 1998). The water bath was ofP.selloumin¯orescences lag behind changes in ambient set at 25°C, but the temperature inside the jacket was slightly temperature, the pattern of warming witnessed in outdooraVected by spadix heating. Total energy expenditure over the course of anthesis was plants may be in¯uenced not only by changes in ambient measured in two ways, as integrated temperature excess in temperature over time, but also by their thermal history. degree-days (

Cd), and as total heat produced in Joules (J).

In an attempt to eliminate these eVects, this study also Integrated temperature excess was calculated by summing all includes measurements from outdoor in¯orescences under of the di V erences betweenTsandTafor a heating period and dividing by 720 (records per day). Total heat produced was nearly constant ambient temperature conditions. calculated by summing allVÇO2(ml min-1) records during the Continuous recording of oxygen consumption enables heating period and multiplying by 24 (minutes per record), and an estimate of total energy expenditure throughout anth- by 20.4 J ml

1(the heat equivalence of oxygen consumption in

esis. This was attempted previously by integrating the P .selloum; Seymouret al., 1983).

temperature elevation throughout the period and estimat-Statistics involved calculation of 95% con®dence intervals

(CI) around means, model 1 least square regressions, and two- ing total heat lost (Seymouret al., 1983). The result was tailedt-tests, assuming unequal variance. that the sterile male ¯orets, which produce most of the heat, expended a total of about 5.5 kJ g 1 . This estimate was the only one possible at the time, but it was of

Results

questionable validity because (1) it applied only to thePatterns of heatingsterile male ¯orets, not the entire in¯orescence, (2) it was

based on a set of unveri®ed assumptions, mostly relating The complete ¯owering sequence inP.selloumhas been to thermal conductance, and (3) it was derived from described as occurring over 4 d (Seymouret al., 1983; Gottsberger and Amaral, 1984), and the present analysis temperatures of outdoor plants under uncontrolled ambi- uses the same de®nitions of days. On day 1, the spathe ent conditions. This study approaches the measurement loosens, but does not open, and heat production is so more directly. low that spadix temperature (Ts) rises only a few degrees above ambient air temperature (Ta). On day 2, the spathe opens widely and warming increases greatly to a max-

Materials and methods

imum in the early evening. On day 3, heat production remains moderate and the spathe closes in the afternoon. Specimens ofPhilodendron selloumwere studied in gardens in In that evening, the spathe reopens partially and heating suburban Adelaide, South Australia, during three ¯owering

begins to decline. During the night between days 3±4, theseasons, in December and January 1995 to 1998. [Note:P.

selloumis been synonymous withP.bipinnati®dumaccording spathe once again closes around the spadix, and heating to Mayo (1991), but the two are thought to be clearly distinct virtually ceases. by Gottsberger and Amaral (1984), because of marked Temperature data for each in¯orescence were quanti®ed di V erences in morphology, pattern of heating, and insect according to a uniform protocol that identi®ed six points visitors.] during the thermogenic episode (Fig. 1): The beginning Rates of oxygen consumption (VÇO2) were measured with a four-channel, open-¯ow respirometry system described in detail

of the episode was considered to be the onset of an abruptDownloaded from https://academic.oup.com/jxb/article/50/335/845/582141 by guest on 26 October 2023

Philodendrontemperature regulation847

Times of peakVÇO2and peakTsdid not diVer signi®cantly. Similarly, elevations ofTsduring the plateau phase were closely matched byVÇO2elevation. Although the period between points 1 and 6 was considered to be the entire thermogenic episode, small elevations inVÇO2were evident before it, and, to a lesser extent, after it; these rates were considered `basal' (Table 1). The origin of this consump- tion is not known, but it is likely to be sum of respiration by the developing spadix and spathe, as well as photosyn- thesis in the green outside surface of the spathe. Peak V ÇO2was about 11 times higher than mean basal, and plateauVÇO2about 4.7 times higher. In general, the responses of water-jacketed and hooded

in¯orescences were similar, but there were some quan-Fig. 1.Pattern of heating in intact in¯orescences ofP.selloum. Upper

curves are central spadix temperatures and lower ones are ambient titative di V erences. Water-jacketed in¯orescences were temperatures just outside of the spathe. Six records from water-jacketed exposed to an averageTathat was 3.8°C higher than the in¯orescences are superimposed in time to show the similarity of

mean for hooded ones, which accounts for all signi®cantresponses and the temperature oscillations in the plateau phase.

Numbers refer to points in the sequence (see text). di V

erences between the two groups (Table 1). MeanTsand plateauTswere signi®cantly higher in water-jacketedrise inTsabove the previously prevailing level (point 1).in¯orescences. BasalVÇO2was higher in the water-jacketedThe next de®nitive point was the `peak'Ts(2). After aones becauseTswas higher before and after the thermog-severe dip inTs(3), it regained a second crest that markedenic episode. On the other hand, net oxygen consumedthe beginning of the `plateau' phase (4). This phase ended(and net heat produced) during the episode was higherwhenTsbegan a precipitous drop (5), and the entirein the hooded in¯orescences because they required moreepisode ended whenTsapproachedTa(6). The times ofheat to maintainTsat prevailing lowerTa. Total oxygeneach of these points are provided in Table 1 as Centralconsumed during the episode was not signi®cantly dif-Australian summer times. Sunrise was between 05:55 hferent in the two groups, partly because of oppositeand 06:34 h and sunset was between 20:15 h and 20:34 h.eVects ofTaon basal and thermogenic respiratory rates.For the six water-jacketed in¯orescences, the patternApparently a diVerence in meanTaof only 3.8°C wasof heating was remarkably similar (Fig. 1). Heating beganalso insuYcient to produce signi®cant diVerences in eitherbetween 07:35 h and 11:12 h on day 2, eventually reachingpeakTsor peakVÇO2in the two groups, although thea peakTsof 41:4±1.8°C between 19:49 h and 20:40 h.means diVered in the expected directions.Then a period of rapid cooling occurred on the morning

of day 3, with the ¯owers reaching a minimumTsofEffects of ambient temperature30.2±1.4°C between 23:31 h and 02:20 h, before warming

to a plateau beginning between 01:09 h and 09:32 h. ThisData for peakTswere not signi®cantly diVerent fromplateau remained relatively constant at 34.2±0.2°C,those obtained earlier (Seymouret al., 1983), so theythroughout day 3, although most in¯orescences showedwere combined (Fig. 3, top). PeakTsappeared to decreaseslow oscillations inTsduring this period. The in¯ores-slightly at lowerTain 39 in¯orescences, but the regressioncences started to cool between 13:19 h and 20:58 h in theequation (Ts=0.190Ta+35.4) had a non-signi®cant slopeevening of day 3 and approachedTabetween 01:49 h and(r2=0.08). However, two points were outliers, with08:17 h on day 4. It was possible to discern points 1, 2,unusually low peakTsvalues. One of these was associated3, and 6 in hooded in¯orescences (Table 1). Althoughwith lowVÇO2, suggesting an abnormal, but valid, result,hooded ones started heating a little earlier, the timing ofand the other had normalVÇO2, suggesting that the thermo-the peak and dip, and the total duration of the thermog-couple may have been displaced by the opening spathe.enic period did not diVer signi®cantly. Because of variableWhen these points were removed, the regression becameTain hooded in¯orescences, the limits of the plateauTs=0.145Ta+36.9, with a signi®cant slope (r2=0.11).phase were not clear. The timing of these events corre-VÇO2at peakTswas quite variable, but tended to increasesponded reasonably well to those of pattern I in¯ores-at lowerTa, with a signi®cant regression ofcences of Seymouret al. (1983); pattern II was neverVÇO2=-0.488Ta+26.54 (r2=0.15) (Fig. 3, bottom).evident.It was diYcult to determine the starting and ending

times of the plateau phase in hooded in¯orescences,Patterns of respirationbecause of variation inTa. Therefore the period observed

in water-jacketed in¯orescences (04:49 h±17:04 h) wasRates of oxygen consumption (VÇO2) re¯ected the pattern

of warming in water-jacketed in¯orescences (Fig. 2). applied to the hooded ones to analyse the e V

ect ofTaonDownloaded from https://academic.oup.com/jxb/article/50/335/845/582141 by guest on 26 October 2023

848Seymour

Table 1.Characteristics of thermogenic episodes ofPhilodendron selloumin water jackets and respirometry hoods

VariableWater jacketHoodAllt-testa

n

6)(n=21)(n=27)P=

Mean 95% CI Mean 95% CI Mean 95% CI

Timing(hours5minutes)

Beginning (1)9527 0541 4556 1542 5556 1531<0.01 S

Peak (2)20526 0515 20557 0540 20550 0532 0.18 NS

Dip (3)0543 0546 23557 1558 0507 1532 0.48 NS

Plateau beginning (4)4549 2512

Plateau end (5)17504 2525

End (6)5527 2509 3528 1549 3555 1531 0.19 NS

Total heating episode44500 2533 46532 2504 45558 1544 0.15 NS

Plateau phase12514 3526

Temperatures

MeanTa(°C)26.00.5 22.21.5 23.11.3<0.01 S

MeanTs(°C)32.40.6 28.91.2 29.71.1<0.01 S

PeakTs(°C)41.41.8 39.11.9 39.61.60.10 NS

Taat peakTs(°C)27.40.6 24.01.8 24.81.5<0.01 S

Ts-Taat peakTs(°C)14.11.5 15.12.2 14.91.80.46 NS

DipTs(°C)30.21.2 24.31.6 25.61.6<0.01 S

PlateauTs(°C)34.20.2 31.21.5 31.81.2<0.01 S

Taat plateauTs(°C)26.30.6 23.41.6 24.01.3<0.01 S

Ts-Taat plateauTs(°C)7.90.77.81.47.81.10.93 NS

IntegratedTs-Ta(°Cd)11.92.0 13.02.5 12.82.00.47 NS

Oxygen consumption

BasalVÇO2before (ml min-1)2.20.11.40.2b1.60.2b<0.01 S BasalVÇO2after (ml min-1)1.50.50.90.21.00.20.06 NS Mean basalVÇO2(ml min-1)1.80.21.10.21.30.2<0.01 S VÇO2at peakTs(ml min-1)13.11.3 14.82.4 14.41.90.22 NS VÇO2at plateauTs(ml min-1) 6.10.76.10.86.10.60.95 NS Total O2consumed (l)13.90.9 14.51.7 14.41.40.54 NS

Basal O2consumed (l)4.80.43.10.53.50.5<0.01 S

Net O2consumed (l)9.11.2 11.41.4 10.91.20.02 S

Heat production

Total heat produced (kJ)2841929735294280.54 NS

Net heat produced (kJ)1862423329222240.02 S

a

2-tailed, unequal variances, water jacketed versus hooded in¯orescences.

bn-2. betweenTsandTaincreased at lowerTa. The relation- ship was linear in the region involving four or more

plants (Ts=0.516Ta+19.1;r2=0.99).VÇO2increased asTadecreased, apparently reaching a maximum of about

8 ml min

1whenTawas 15°C. An individual in¯ores-

cence that happened to bloom during a cool period inexplicably showed lowerTsandVÇO2than predicted by

the other data (single low points on Fig. 4). WhenVÇO2was plotted againstTswithin the plateau phase of hooded

in¯orescences, an inverse relationship appeared (Fig. 5). MaximumVÇO2occurred whenTswas about 28°C and it decreased asTsapproached 40°C. For all 27 in¯orescences that provided complete temper-

Fig. 2.Pattern of oxygen consumption rate in the same in¯orescencesature records, the diVerences betweenTsandTawere

ofP.selloumshown in Fig. 1. integrated, yielding an index of total heat that averaged 12.8

2.0°Cd (Table 1). No signi®cant diVerence in total

TsandVÇO2. To summarize the data, values forTsandintegrated temperature diVerence was found between

VÇO2were collected in 2°CTaintervals (e.g., 12.1±14.0°C)water-jacketed and hooded in¯orescences. The total

and individual means calculated for each of 21 hooded energy liberation (Htot) for all in¯orescences averaged in¯orescences. Grand means of these independent samples 294

28 kJ and the net heat produced (Hnet=Htot-Hmean

basal) was 222±24 kJ (Table 1). There was a relationshipare presented in Fig. 4. In the plateau phase, the diVerenceDownloaded from https://academic.oup.com/jxb/article/50/335/845/582141 by guest on 26 October 2023

Philodendrontemperature regulation849

Fig. 5.Relationship between oxygen consumption rate and spadix temperature during the plateau phase. The data and statistics are from Fig. 4. betweenHnetand meanTaover the entire heating epis- odes (Fig. 6). The regression equation wasHnet=

11.122

Ta+480.8 (r2=0.35), and the line extrapolated

to zero heat production atTa=43.2°C. There was also a signi®cant inverse relationship betweenHtotand meanTa. Fig. 3.EVects of ambient temperature on spadix temperature (top) and Masses of the parts of 22 in¯orescences are given oxygen consumption rate (bottom) at the peak temperature elevation in Table 2. in hooded in¯orescences outdoors. Each point represents an individual in¯orescence. Spadix temperatures from Seymouret al. (1983) are indicated as open symbols. Solid lines are linear regressions and the dashed line is isothermal.

Discussion

Temperature regulation in the peak and plateau phases This study shows that there are two phases of thermogen- esis inP.selloumand that temperature regulation exists at two distinct levels. PeakTsis regulated in the region of 38±42°C (Fig. 3), but plateau phaseTsis usually in the range of 25±36°C (Fig. 4). HighTsin the peak phase is usually a single spike lasting only 2±3 h, while plateau Tsoscillates around a mean for about 12 h (Fig. 1). Thus the peak represents only regulation of maximumTs, while the plateau phase demonstrates true regulation around a mean.

Fig. 4.EVects of ambient temperature on spadix temperature (top) andFig. 6.EVect of mean ambient temperature on the net amount of heat

oxygen consumption rate (bottom) during the plateau phase in hooded released during the entire episode of thermogenesis. Heat production in¯orescences outdoors. Grand means and 95% con®dence intervals are has been calculated from oxygen consumption (20.43 kJ l

1), and mean

given for data collated in 2°C intervals ofTa. The number of individualbasal heat production has been subtracted from total heat production

in¯orescences is indicated above each symbol. The dashed line is to yield net heat production. Each point represents an individual isothermal.

in¯orescence and the solid line is a linear regression.Downloaded from https://academic.oup.com/jxb/article/50/335/845/582141 by guest on 26 October 2023

850Seymour

Table 2.Mass of the parts ofPhilodendron selloumin¯orescences used for respirometry

MassWater-jacketHoodAll

(g)(n=4)(n=18)(n=22)

Mean95% CIMean95% CIMean95% CI

Spathe355.249.4277.834.7291.832.0

Spadix225.321.9150.217.9163.919.4

Male ¯orets58.617.741.66.244.76.5

Sterile male ¯orets46.03.230.74.833.54.7

Female ¯orets24.45.319.22.020.22.0

It is tempting to speculate that the oscillations inTsin directly, however. Respiration rate depends immediately

the plateau phase may be similar to those of an engineered on changes inTs, which in turn depends onTa(Nagyet

thermoregulatory system in which heat production is not al ., 1972; Seymouret al., 1983). TheVÇO2during the proportional to the di V erence between current temper- plateau phase decreased with risingTs(Fig. 5), as it does ature and the set-point. In such a system, temperature in the peak phase (Seymouret al., 1983), but the plateau may approach the set-point, over-shoot it because of data were o V set toward the lower left compared to the thermal inertia, change direction and under-shoot, and peak phase. MaximalVÇO2occurred at aTsof 28°Cin so on, with the amplitude of oscillations gradually the plateau phase (Fig. 5), but it is near 37°C in the peak decreasing. Several in¯orescences appeared to do this phase (Seymouret al., 1983). The biochemical mechanism (Fig. 1). After the peak phase of intense heat production, behind the thermal inhibition is not known, but is prob-

Tswas considerably above the set-point, and heat produc-ably related to changes in activity of the alternative

tion was inhibited to such an extent thatTsdropped welloxidase (Seymouret al., 1983). Whatever the mechanism,

below the set-point and then recovered. Such over- and the shift in set-points between the peak and plateau under-shooting of ¯ower temperature was also apparent phases clearly indicates that the level of heat production

in a study involving quickTachanges in the thermoregula-is related not only toTa(henceTs), but also to the stage

tory sacred lotus,Nelumbo nucifera(Seymouret al.,of anthesis.

1998). In this species, the oscillations are based on a

marked lag in the biochemical regulatory mechanism,Respiratory rates and energeticswhereby changes in heat production follow changes in

Sterile male ¯orets consume oxygen at a rate equivalent¯ower temperature by as much as 2 h (Seymour and

to about 2

3, and fertile male ¯orets about 1

3, of theSchultze-Motel, 1998). However, biochemical lag may

entire spadix (Nagyet al., 1972). Assuming that eachnot be the entire explanation for the oscillations during

spadix had 33.5 g of sterile males and 44.7 g of fertilethe plateau phase inP.selloum, because small oscillations

males (Table 2), and the meanVÇO2at the peak wassometimes occurred as the in¯orescence was warming up

14.4 ml min-1(Table 1), the estimate for mass-speci®ctoward the main peak (Figs 1, 2).

VÇO2becomes 15.5 ml g-1h-1for the sterile males. ThisThe pictures of temperature regulation in the two

is about 75% of the maximumVÇO2of 20 ml g-1h-1byphases diVer markedly, not only in the level ofTsmain-

isolated sterile male ¯orets cut from the stalk and incub-tained, but also in the relationship betweenTsandTa.

ated at 40°C (Nagyet al., 1972; Seymouret al., 1983).The precision of thermoregulation can be measured as

The integrated temperature excess over the entire ther- the slope of the regression line relatingTsandTa, a slope

mogenic episode was 12.8°Cd, similar to the ®gure ofof 1 indicating no regulation and a slope approaching 0

10.6°Cd calculated by Seymouret al. (1983). With con-showing perfect regulation. For the peak phase, the slope

tinuous measurement ofVÇO2it is possible to estimate thewas 0.19 (Fig. 3), indicating extremely precise regulation

entire energy expenditure by the in¯orescences throughout and similar to the value of 0.18 of the initial study ofP. anthesis. The average in¯orescence consumed 14.4 l of selloum(Nagyet al., 1972). On the other hand, the slope oxygen and released 294 kJ of heat in total (Table 1). during the plateau phase was 0.516 (Fig. 4), indicating If 33.5 g of sterile male ¯orets (Table 2) contained less precise regulation and a value similar to that in the

22.77 kJ g-1(Seymouret al., 1983), then just these ¯oretsarum liliesSymplocarpus foetidus(Seymour and Blaylock,

held 763 kJ prior to anthesis. The entire spadix was likely unpublished data) andDracunculus vulgaris(Seymour to hold over 3000 kJ of energy, so the heat generated was and Schultze-Motel, unpublished data). less than 10% of the energy it contained. Nevertheless, it

Temperature regulation in both phases involves an

is apparent that some energy must have been imported inverse relationship betweenVÇO2andTa, butVÇO2in the into the ¯orets during thermogenesis. If the sterile males plateau phase is considerably lower than in the peak

phase (Figs 3, 4). In¯orescences do not react toTaproduced 196 kJ (=2/3 of 294), then their mass-speci®cDownloaded from https://academic.oup.com/jxb/article/50/335/845/582141 by guest on 26 October 2023

Philodendrontemperature regulation851

energy release was 5.85 kJ g-1. This is similar to the value An hypothesis for the diphasic pattern that is consistent

of 5.54 kJ g

1calculated by Seymouret al. (1983) fromwith the behaviour of the beetles is that the peak phase

values ofVÇO2, thermal conductance and total integratedof thermogenesis is associated with enhanced odour pro-

temperature excess. However, the decrease in energy duction to attract beetles and that the plateau phase content of the sterile male ¯orets, as measured by bomb provides a warm, stable environment during their resid-

calorimetry, was 3.35 kJ g-1, only 57% of the estimatedence. Regulation of maximumTsmay prevent thermal

total (Seymouret al., 1983).damage to the beetles or the in¯orescence itself, while Regulation of heat production was also re¯ected in the regulation of plateauTspermits the beetles to carry out total amount of heat released during anthesis; in¯ores- their activities (e.g. eating, digesting, mating, preparing

cences produced more heat at lowerTa(Fig. 6). Thisfor ¯ight) at appropriate body temperatures within the

suggests that the energy for thermogenesis was not lim- spathe.

ited, at least atTaabove 15°C, and the in¯orescences didThe evidence that supports this hypothesis indicates

not simply exhaust their energy and then cool down. One that body temperatures above about 42°C are avoided wonders whether energy is ever limited in the natural by beetles in general, but temperatures between roughly

environment whereTacan drop below 10°C (Gottsberger30°C and 40°C are often required for activity (Heinrich,

and Silberbauer-Gottsberger, 1991).

1981, 1993). For example, African scarabaeid dung beetles

generally remain below 42°C and always below 46°CRole of thermogenesis in pollination(Bartholomew and Heinrich, 1978), and Namib Desert

tenebrionid beetles avoid the sand surface when summerStudies of pollination biology ofP.selloumin Brazil byconditions cause body temperatures to rise much aboveGottsberger and colleagues reveal a strong nexus bet-40°C (Henwood, 1975). Body temperatures of scarabaeidween thermogenesis and activities of the pollen-carryingbeetles engaged in locomotion and competition forinsects (Gottsberger and Amaral, 1984; Gottsberger andresources are typically in the region of 34±42°CSilberbauer-Gottsberger, 1991). They reported that the

(Bartholomew and Casey, 1977; Bartholomew andsequence of ¯owering was `amazingly precise in thisHeinrich, 1978). The thoracic temperatures required forspecies, and all individuals follow exactly the same

¯ight in large scarabaeid beetles are in the range ofrhythm'. According to their detailed observations, the

27±34°C (Chappell, 1984; Heinrich and McClain, 1986;only pollinators are large dynastine scarabaeid beetles,

Morgan, 1987). Temperatures for activity are not knownErioscelis emarginata. It is thought that the in¯orescence

forE.emarginata, but Gottsberger and Silberbauer-is the only food source and the sole location for mating

Gottsberger (1991) noted that the beetles were activeof the beetles. They ¯y to the in¯orescences, often in

within warm in¯orescences or ¯ying between them, andgreat numbers, in the evening of day 2 (between 18:55 h

that when ambient temperature dropped as low as 6°Cand 19:10 h), when odour and heat production by the

during the ¯owering season, the beetles were unable to ¯y.spadix are maximal and the female ¯orets are receptive

Beetles can reach activity temperatures by endogenousto pollination. Approximately 10±15 beetles can be

heat production through contractions of thoracic muscles,accommodated in the ¯oral chamber within the base of

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