eco physiology of niche occupation by two giant rosette plants, lobelia gibberoa

8/8/2019 Eco Physiology of Niche Occupation by Two Giant Rosette Plants, Lobelia Gibberoa

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Ecophysiology of Niche Occupation by Two Giant Rosette Plants, Lobelia gibberoa Hemsland Solanecio gigas (Vatke) C. Jerey, in an Afromontane Forest Valley

U LRICH LU T T G E *{, M A S R E S H A F E T E N E{, M A R K U S L I E B I G{, U W E R A S C H E R{ andE R W I N B E C K}

{Darmstadt University of Technology, Institute of Botany, Schnittspahnstrabe 3-5, D-64287, Darmstadt, Germany,

{Department of Biology, Faculty of Science, Addis Ababa University, P.O Box 1176, Addis Ababa, Ethiopia and

}University of Bayreuth, Department of Plant Physiology, Universitatsstrabe 30, 95440, Bayreuth, Germany

Received: 2 February 2001 Returned for revision: 20 February 2001 Accepted: 9 April 2001 Published electronically 25 June 2001

The niche occupation of two giant rosette plants, Lobelia gibberoa Hemsl and Solanecio gigas (Vatke) C. Jerey, wasinvestigated in a small mountain valley in an afromontane forest, central Ethiopia. Plant distribution, density, life-form, morphology and microsite conditions were related to transpiration, chlorophyll a uorescence and d13Canalysis to explain how ecophysiological traits and morphology determine the niche dierentiation of plants ofsimilar life-forms. L. gibberoa was more abundant at the humid northern ank of the valley while S. gigas was equallydistributed in the northern and drier southern anks; both species occurred in the valley bottom. Photosyntheticcapacity, as determined by chlorophyll a uorescence, was similar for both species. In dry locations, S. gigastranspired somewhat less and had lower leaf conductance for water vapour, g H2 O, than in wet locations. Both specieshad the highest gH2 O and d

13C-isotope discrimination in the wettest locations. Overall for the ecophysiological traits,site dierences were larger than dierences between the two species. L. gibberoa had a well developed vascularcylinder, maintained a large number of rosettes and had an average leaf area index (LAI) of 2 .8, which may restrict itto the more humid locations. S. gigas, with a poorly developed vascular bundle, had many fewer rosettes, an averageLAI of 1.5, and reacted more to the water status of its habitat. It was thus also capable of colonizing drier locations.The role of morphological and anatomical traits and ecophysiological features in niche occupation as revealed byassessment of vegetation is discussed. # 2001 Annals of Botany Company

Key words: Life forms, Lobelia gibberoa, afromontane forest, niche occupation, Solanecio gigas.

INTRODUCTION

A given habitat may be occupied by many dierent plant

species, diering in morphology. However, dierent species

may also be morphologically very similar at sites withsimilar environments (e.g. Scarano et al., 2001), a phenom-

enon known as convergence. If the ecophysiological beha-

viour of morphologically convergent species is also similar,convergence is complete. However, there are also very

dierent physiotypes (i.e. forms of physiological behaviour)between morphologically convergent species. For example,

in the neotropics, some epiphytic bromeliads belonging to

dierent species are morphologically almost indistinguish-

able but perform either C3-photosynthesis or crassulaceanacid metabolism (CAM) (Griths et al., 1986; Smith et al.,

1986). The approximately 250300 known species of Clusia

are of one single morphotype of woody plants with entire,leathery leaves; however, dierent species with C3-photo-

synthesis, CAM and C3/CAM-intermediate behaviour can

occur sympatrically at the same site (Lu ttge, 1999, 2000).

Here we compare two species of the life-form of giant

rosette plants, viz. Lobelia gibberoa Hemsl and Solaneciogigas (Vatke) C. Jerey. The aim was to compare their

morphology and ecophysiology at the same site, and to

trace the diering distribution patterns back to ecophysio-

logical dierences. Particular attention was paid to thequestion: to what extent are dierent niche occupations bythe giant-rosette life-forms of L. gibberoa and S. gigasdetermined by structural and ecophysiological traits?

M A T E R I A L S A N D M E T H O D S

Description of the study site

The study was conducted in Menagesha Natural Forestwhich extends over approx. 2700 ha in central Ethiopia,30 km northwest of Addis Ababa (8856H9800HN, 38832H38856HE). The forest is located on an isolated massif formed

by siliceous volcanic ash, on deep reddish brown soil atlower altitudes (24002700 m) and on shallow light brownsoils at higher altitudes (27003000 m). The forest receivesa mean annual rainfall of about 1250 mm and mean annualtemperatures range from 15 to 24 8C. The rainy season lastsfrom April to September (Demissew, 1988). The study areais a small, deep, narrow valley. Much of the mountain iscovered with a mature Juniperus proceraMyrsine africanaforest community as described by Demissew (1988) andBekele (1993). The deeply incised valley crosses the forest inan east-west direction with a steep north- and a less steepsouth-facing slope. Its bottom is dominated by large standsof Lobelia gibberoa Hemsl and Solanecio gigas (Vatke)

C. Jerey having the same giant rosette form of about thesame height, 34 m. The two species show overlapping but

Annals of Botany 88: 267278, 2001doi:10.1006/anbo.2001.1450, available online at http://www.idealibrary.com on

0305-7364/01/080267+12 $35.00/00 # 2001 Annals of Botany Company

* For correspondence. Fax 49 (0) 61 51 16 46 30, e-mail [email protected]


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not congruent distribution. Both extend up the south- andnorth-faces of the valley. While L. gibberoa mainly inhabitsthe valley bottom and the shady north-slope, S. gigas showsa wider distribution, but has a lower overall density. Sincethe valley does not have a specic name, it will be called`Lobelia-valley' in this communication. The study was con-

ducted in March 1998 towards the end of the dry season.

Plant distribution and density

The frequency of L. gibberoa and S. gigas was deter-mined in a continuous line of plots (5 m 5 m) along theentire transect of the valley. Using a measuring tape, theposition of each species from the valley bottom wasrecorded on both sides. Due to dierences in inclination,the transect was 155 m long on the north-facing slope and115 m long on the south-facing slope.

Measurements of microclimate and transpirationRelative humidity, ambient temperature and leaf

temperature were recorded with Testo-term instruments(Armatherm-Gu nthel, Lemgo, Germany). The temperaturesensor consisted of a fast response NiCr-Ni resistancesensor (15 mm long, diameter 0.5 mm, with a t99 of 9 s).Transpiration (JH2O; see Appendix for list of abbreviations)and leaf conductance to water vapour (gH2O) weredetermined using a diusion porometer (LI-1600 SteadyState Porometer, Li-Cor Inc, Nebraska, USA) and adynamic porometer (AP4, Dynamax Inc, Houston, Texas,USA) on four consecutive days. If not stated otherwise,fully developed leaves with a natural inclination of about30458 were selected. Soil moisture content was determinedusing Time Domain Re ectrometry (TDR) (Model6050 1 TRASE System, Soilmoisture Equipment Corp.,Goleta, California, USA). The metal wave-guides used fordepth measurements were 15 cm long stainless steel rods.The leaf area index of plants was determined using a Li-Cor2000 Leaf Area Meter and the LAI-2050 optical sensor (Li-Cor Inc, Nebraska, USA). The LAI of individual plantswas measured. Values were recorded when repeatedmeasurements gave very close values. Measurements weretaken at noon and plants in the open were measured toavoid the overstorey eect.

Measurement of chlorophyll a uorescence

Chlorophyll a uorescence was measured with a portablepulse-amplitude modulated photosynthesis yield analyser(Mini-PAM, Heinz Walz GmbH, Eeltrich, Germany)equipped with a standard 2030-B leaf clip holder (Bilgeret al., 1995; see also Rascher et al., 2000). Measurements ofphotosynthetically active radiation, PAR (l 400700 nm), close to the leaf surface were taken by themicro-quantum sensor on the leaf clip calibrated against aLi-Cor quantum sensor (Li-Cor Inc, Nebraska, USA). Leaftemperatures were recorded simultaneously with a Ni/NiCr-

thermocouple of the leaf clip holder appressed to the lowerabaxial surface of the leaves.

Potential quantum yield of photosystem II (PS II) wasmeasured in the very early morning (Fv/Fm-predawn) andduring the day after darkening the leaves for 30 min(daytime Fv/Fm), where Fv is maximum variable uor-escence and Fm is maximum uorescence of the dark-adapted leaf under a light saturating ash of 800 ms and

approx. 3000 mmol m

2

s

1

(Fv Fm Fo ; where Fo isground uorescence of the dark-adapted leaf). Eectivequantum yield of PS II (DFaFm

H) was calculated asFm

H FaFmH, where Fis uorescence of the light-adapted

leaf and FmH is the maximum light-adapted uorescence

when a saturating light pulse of 800 ms duration issuperimposed on the prevailing environmental light ux(Genty et al., 1989; Schreiber and Bilger, 1993). Apparentrates of photosynthetic electron transport (ETR) wereobtained as 05 084 DFaFm

H PAR, where thefactor 0.5 accounts for the excitation of both PS II andPS I and the factor 0.84 assumes a reection of PAR of16 % at the leaf surface. Non-photochemical quenching

(NPQ) was calculated by the Stern-Volmer equation asNPQ Fm FmHaFm

H, where Fm is the value of thepredawn measurements (Bilger et al., 1995). To maintainbasic uorescence (F) in the optimal range of detection,internal gain and measuring light intensity of the Mini-PAM were adjusted according to plant species, sites andtime of day. Each time, a calibration was made using theuorescence standard of Walz so that all values of F andFm

H could be normalized.Using the Mini-PAM, daily courses of chlorophyll

uorescence parameters were measured across the valleytransect on four consecutive days using ve plants each ofL. gibberoa and S. gigas. Measurements were taken

separately for leaves oriented in the four directions of thecompass on each plant and three readings were taken ateach time. Since data analysis showed no trends related todirection, all readings taken at one time were averaged(n 12).

Light-response curves of DFaFmH and ETR were

obtained using the light-curve programme of the instru-ment. Leaves were darkened for 30 min and then actiniclight intensity was increased during 4 min in eight stepseach 30 s apart. Due to this short time, leaf photosynthesiswas not in steady state during these measurements.However, using the eld data reported here, Rascher et al.(2000) showed that corrections can be made if both

measurements taken under ambient conditions with photo-synthesis in the steady state and instant light-responsecurves are available; this was applied to the present data.

Carbon isotope analysis

Carbon isotope ratios were determined by mass-spec-trometry as relative deviations to Pee Dee belemnitestandard [d13C (-)]. Several leaves were collected fromeach plant; these were dried, nely powdered and thepowders mixed. These composite samples were representa-tive of the plant. The precision of the d13C-determinationwas +0.1-.

Carbon isotope ratios (d13

C values) in the leaves of C3plants indicate the relations between intercellular CO2

268 Luttge et al.Ecophysiology of Two Giant Rosette Plants in Africa


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partial pressures (ci) and CO2 assimilation rates asdetermined by the average degree of stomatal opening(stomatal conductance) during photosynthesis over the life-time of the sample material, with more negative valuesindicating higher average conductance. Carbon isotopediscrimination, which is related to average conductance and

water use eciency over time was calculated as:

D d13ca d

13cp

1000 d13cp 103% 1

where d13cp is the value measured for the plant material andd13ca is the value of CO2 in the ambient atmosphere. Weused the value of 8.00 - for d13ca which is the valuegenerally applied when d13ca has not been measured

(Farquhar et al., 1989a). The average ratio of internal toexternal (atmospheric) CO2-partial pressures during photo-synthesis over the life-time of the leaf sample was obtainedas:

ciaca D ab a

2

where a gives 13C-discrimination due to CO2-diusion in air(4.4 -) and b the net fractionation caused by carboxyla-tion in C3-photosynthesis (27 -) (Farquhar et al., 1989a,b; Broadmeadow et al., 1992).

Statistical analysis

Errors reported here are standard deviations (s.d.). Thestatistical signicance of data-pair comparisons was testedusing Mather's t-test. Two plants each of L. gibberoa and

S. gigas were studied on the north face and the valleybottom with the denser vegetation of giant rosette plantswhile one plant of each species was studied on the southface where coverage was more sparse.

R E S U LT S A N D D I S C U S S I O N

Plant density and growth characteristics

Figure 1 shows Lobelia-valley with its south-facing(Fig. 1A) and north-facing slopes (Fig. 1B). The treecanopy was denser on the north-facing slope (crown density

475 % was 56 %) than on the south-facing slope (475 %crown density 33 %). S. gigas was almost equally dis-tributed on the valley bottom and the northern andsouthern faces of the valley, while L. giberroa predominatedon the valley bottom and the northern face (see Table 1,Fig. 2). Soil moisture was highest in the valley bottom,

somewhat less on the north face and much lower on thesouth face of the valley (Table 1). In the wetter sites, i.e.both the northern face and the valley bottom, L. gibberoahad a signicantly higher LAI than S. gigas. On the muchdrier southern face, L. gibberoa still tended to have a higherLAI that S. gigas but the dierence was not statisticallysignicant. S. gigas produced considerably fewer giantrosette leaves (eight12: Fig. 3A) than L. gibberoa (2035:Fig. 3B). The overall mean LAI for S. gigas and L. gibberoawas 1.5 and 2.8, respectively (Table 1). Both plants had asimilar diameter at breast height (DBH) and plant height.

Stem anatomy and transpirationStem anatomy is presented in Fig. 4. While L. gibberoa

exhibits a well developed, although narrow-pored, xylemcylinder, the vascular cylinder of S. gigas is barelydeveloped. In contrast, the pith of S. gigas dominates thecross-section and shows all the features of a succulenttissue. The pith of L. gibberoa is disrupted, forming a densesequence of air-lled chambers separated by thin layers ofdead tissue. Thus, anatomical prerequisites for a high waterconductance in the stem ofL. gibberoa are obvious and maybe able to maintain the water supply to the many leaves.S. gigas, on the other hand, with its poorly developedxylem, appears to depend on water conductance by the

living cells of the pith and their water capacitance, as foundpreviously for other giant rosette plants, e.g. Esplenetia(Meinzer et al., 1985). As a consequence, the rate of watersupply to the leaves may be limited which might explain (1)the conspicuous wilting observed in S. gigas at noon, incontrast to L. gibberoa, and the early closure of stomata inresponse to drought, and (2) the smaller number of giantrosette leaves which can be maintained by each stem.

Measurements of transpiration (JH2O) and leaf conduc-tance to water vapour (gH2O) were made and werecompared to these anatomical features. All measuring

TA B L E 1. Percent soil moisture, density, plant height, leaf area index and diameter at breast height (DBH) of mature non- owering plants of L. gibberoa and S. gigas at the valley bottom and south- and north-faces of the Lobelia-valley in

Menagesha montane forest

SitePercent soil moisture

(n 5) SpeciesPlant density per plot

(25 m2)Plant height (m)

(n 8)Leaf area index

(n 8)DBH (m)

(n 8)

Valley bottom 29.6 (2.4)a L. gibberoa 0.78 (0.03)a 2.50 (0.3)ab 2.2 (0.4)a 0.13 (0.012)a

S. gigas 0.52 (0.20)a 4.00 (0.5)a 1.3 (0.6)b 0.11 (0.015)a

South face 15.4 (2.3)b L. gibberoa 0.07 (0.03)b 3.00 (0.4)a 2.2 (0.4)a 0.22 (0.013)b

S. gigas 0.34 (0.20)a 4.00 (0.5)a 1.8 (0.6)a 0.13 (0.016)a

North face 23.5 (3.0)c L. gibberoa 0.63 (0.20)a 3.00 (0.3)a 4.0 (0.5)c 0.15 (0.010)a

S. gigas 0.25 (0.20)c 2.00 (0.3)b 1.5 (0.3)ab 0.09 (0.013)a

Data are means (+ s.d.).Dierent superscripts indicate statistically signicant dierences at the P 0.05 level.

Luttge et al.Ecophysiology of Two Giant Rosette Plants in Africa 269


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days were sunny with occasional clouds. Relative air

humidity (RH) in the valley was 6065 % around 0900

1000 h, declining to approx. 45 % at 1100 h and recovering

to 5055 % in the afternoon from 1400 to 1800 h.

Integrated JH2O over a daily course of 7 h from 1000 to

1700 h was 75 mol m2 and 51 mol m2 for L. gibberoa

and S. gigas, respectively, on the south-facing slope (oneplant each) and 47 mol m 2 and 74 mol m2 for L. gibberoa

and S. gigas, respectively, on the north-facing slope (two

plants each). Average values for gH2O are given in Table 2.

This limited information clearly shows that S. gigas tends to

have a lower JH2O and gH2O on the drier south face than on

the wetter valley bottom and north face. Its JH2O is

somewhat lower than that of L. gibberoa on the south

face but higher on the north face, and its gH2O is also loweron the south face. This may be related to the poorly

F IG . 1. `Lobelia-valley' showing the less steep south-facing (A) and the steeper north-facing slopes (B).



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developed xylem of S. gigas compared with that of

L. gibberoa restricting JH2O and gH2O more on the drier

southern face in S. gigas than in L. gibberoa. For

L. gibberoa, there are no dierences in gH2

O on the south-

and north-facing slopes. Both species have the highest gH2Oin the wet valley bottom. Overall, it appears that for the two

giant rosette plants, dierences between sites are larger than

interspecic dierences.

70

70

60

50

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30

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10

0

40

30

20

10

60

50

North face South face

Numberofindividuals

Distance from valley bottom (m)

50

40

30

20

10

0Solaneciogigas

Lobeliagibberoa

F I G . 2. Transect across Lobelia-valley and frequency (given by theheight of the columns with scale in the lower right) of L. gibberoaand S. gigas along the north and south faces of the valley. Plotswere 5 m 5 m. Distances are in m from the middle of the valley

bottom.

F I G. 3. Plants of Solanecio gigas (Vatke) C. Jerey (A) and Lobelia gibberoa Hemsl (B).

TA B L E 2. Average values of leaf conductance for watervapour (gH2O ) of Lobelia gibberoa and Solanecio gigas onthe south (S) and north (N) faces and the valley bottom (V)

of Lobelia valley

gH2O (mmol m2 s1)

S V N

L. gibberoa 111 170 116S. gigas 77 170 127

Values were obtained from daily courses of measurements for oneplant each on the south face and the valley bottom and two plants onthe north face.



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This trend is corroborated by carbon isotope discrimi-nation (D) and the derived average ratio of internal toexternal CO2 partial pressures (ci/ca) during photosynthesisover the life-time of the leaves, which are proportional toaverage stomatal conductance of the leaves and water useeciency of C3-plants over time (Farquhar et al., 1989a, b;Broadmeadow et al., 1992). For both L. gibberoa and

S. gigas, values of D and ci/ca are higher in the valley

bottom and on the north-exposed face than on the south-exposed face (Table 2).

Daily courses of photosynthesis given by chlorophyll auorescence parameters

Eective quantum yield of photosystem II, DFaFmH, and

non-photochemical quenching, NPQ, measured on 2 d,

together with PAR and leaf temperature are shown in

Figs 5 and 6. DFaFmH showed the usual inverse relation to

PAR. There were no obvious dierences between the two

species and, thus, PAR was the dominant factor determin-ing the instant performance traced in these measurements.

F I G. 4. Details of stem anatomy of S. gigas (A) and L. gibberoa (B). C, Cork; Cx, cortex; H, hollow; Ld, latex ducts; Ls, leaf scar; P, pith; Ph,phloem; V, vascular leaf strands; Vs, vascular strands; Vt, Vascular tissue; X, xylem.



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PAR(molm

2s

1

)

17.03.98 18.03.98

Lobelia gibberoa

Time of day (h)

0800 1000 1200 1400 1600 0800 1000 1200 1400 1600 1800

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NPQ

F I G . 5. Daily courses of incident PAR, leaf temperatures, DFaFmH and NPQ of L. gibberoa on two consecutive days in Lobelia-valley. d, South-

facing slope (S) (averages for one plant; n 12 leaves); D, valley bottom (V) and h, north-facing slope (N) (averages for two plants each; n 24

leaves).



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17.03.98 18.03.98

Solanecio gigas

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F I G. 6. Daily courses of incident PAR, leaf temperatures, DFaFmH and NPQ of S. gigas on two consecutive days in Lobelia-valley. d, South-

facing slope (S) (averages for one plant; n 12 leaves); n, valley bottom (V) and h, north-facing slope (N) (averages for two plants each; n 24

leaves).



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Both species show a high capacity for photoprotection bynon-photochemical quenching; values for NPQ of 3 to 4

were reached when DFaFmH declined to 0.2 at high PAR

(approx. 1500 mmol m2 s1). NPQ tended to be a little

higher in L. gibberoa than in S. gigas but the dierences

were not statistically signicant. The summarizing overviewpresented in Fig. 7, where all the individual readings are

plotted against PAR, conrms these assessments. The usualexponential decline ofDFaFm

H and the curvilinear increase

of apparent photosynthetic electron transport rate (ETR)with increased PAR are evident. The maximum uor-

escence yield of leaves (FmH) adapted to the incident PAR,

as given on the abscissa, decreased markedly with PAR and

to a similar extent as PAR. This shows that the PAR-dependent decline of DFaFm

H was largely due to variable

uorescence (DF FmH F) and not to changes in F. This

was also veried by plotting normalized F vs. PAR (data

not shown). The increase of NPQ related to PAR( proportional to ETR and inversely related to Fm

H and

DFaFmH) is also evident, and these plots of individual

readings also conrm the conclusion drawn from Figs 5

and 6 that L. gibberoa potentially attains a marginallyhigher NPQ than S. gigas.

To evaluate whether NPQ was due to photoprotective orphotodestructive mechanisms, potential quantum yield of

photosystem II of dark-adapted leaves (Fv/Fm) was alsomeasured (Fig. 7). The debate on photoprotective and

photodestructive mechanisms of photoinhibition iscurrently open and controversial. Short-term photoinhibi-tion is thought to be due to the build-up of an electrical

gradient across the thylakoid membranes. Medium-termphotoinhibition involves xanthophylls (Schindler and Lich-tenthaler, 1996); zeaxanthin may divert chlorophyll exci-

tation from the reaction-centre chlorophyll a in the lightharvesting complex of PS II (Horton et al., 1994) or the

xanthophyll cycle may serve to dissipate energy as heat(Demmig-Adams, 1990; Demmig-Adams and Adams,

1992; Pfu ndel and Bilger, 1994). Long-term photoinhibi-tion is related to turnover and destruction of the D1 and D2

proteins of PS II (Critchley and Russel, 1994), but thisdestructive mechanism is also photoprotective. Values of

Fv/Fm below 0.830.80 generally indicate photoinhibition

(Bjo rkman and Demmig, 1987). For the present eld study

we followed Thiele et al. (1998) where a depression of Fv/Fm , reversible within approx. 10 min, is considered to result

from a build-up of an electrical gradient across thethylakoid membranes. Depressions reversible within60 min presumably result from energy dissipation as heat,while low values reversible overnight are due to damage ofthe D1 protein of PS II. Thus, measurements were taken inthe early morning (Fv/Fm-predawn) and during the course

of the day after darkening the leaves for 30 min (daytimeFv/Fm). Predawn values of Fv/Fm were 50.80 in both

L. gibberoa and S. gigas, while daytime Fv/Fm was alwaysless than 0.80 but still well above 0.70. This was interestingbecause leaves of S. gigas regularly showed severesymptoms of turgor loss (wilted appearance) at midday.Thus, protective mechanisms must have been operating,minimizing the photoinhibition but reversible within30 min.

Overall, dierences in photosynthetic performancebetween the two giant rosette plants are negligible andsite dierences in the valley transect are more importantthan species dierences.

Cardinal points of photosynthetic light-response curves

The miniaturized pulse amplitude modulated uorometerallows instant light-response curves to be obtained to assessthe potential performance of plants. This adds importantinformation to that on the actual performance given underthe prevailing environmental conditions (especially withrespect to PAR) as presented above. Although during suchmeasurements of light-response curves, with instant arti-cial changes of PAR, the leaves are not in steady state, asshown by Rascher et al. (2000), corrections can be madeleading to acceptable values for the cardinal points of

photosynthesis (see also Materials and Methods). Theapproach has advantages for comparative purposes. Severallight-response curves can be obtained rapidly within a shortperiod (4 min for each curve), and thus comparisons can bemade for dierent plants under a given environmentalsituation.

A comparison of species vs. sites in Lobelia-valley(Table 4) shows that for L. gibberoa, ETRmax is lower onthe northern face than in the valley bottom and thesouthern face (signicant at P 0.01). These lower electrontransport rates suggest that there is a tendency for plants tobe more like shade-plants on the northern face. There areno site dierences for the other cardinal points; PARsat is

somewhat, but not signicantly, lower on the north-facingslope, consistent with the above conclusion regarding amore shade-plant nature on this site. For S. gigas, there areno signicant dierences in cardinal points between thesites. A comparison of sites vs. species shows that ETRmax ,PARsat and

12 PARsat are not signicantly dierent for

L. gibberoa and S. gigas. DFaFmH-sat and 12DFaFm

H-sat aresimilar at all sites for both species.

CONCLUS IONS

The present study shows that ecophysiological traits play animportant role in niche occupation of the two giant rosette

species studied in Lobelia-valley. In life-form and generalmorphology, the giant rosette plants L. gibberoa and

TA B L E 3. Carbon isotope discrimination (D) and ratios ofinternal to external CO2-partial pressures during photosyn-thesis (ci/ca) of leaves harvested from plants on the south face

(S), the north face (N) and the valley bottom (V)

Parameter

L. gibberoa S. gigas

S V N S V N

D (-) 20.2 21.8 22.4 20.2 22.7 21.1

ci/ca 0.70 0.77 0.80 0.70 0.81 0.74

For N and V, values are averages of two plants, the smaller value ofthe two was always larger than the value for S.



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40L. gibberoa S. gigas

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1750

PAR (mol m2 s1)

NPQ

Fm

DF/Fm

Leaftemp(C)

ETR(molm

2s

1)

F I G. 7. All individual readings of leaf temperature and chlorophyll uorescence parameters made on 4 d and at the three sites in Lobelia-valley for

L. gibberoa and S. gigas, plotted against PAR.



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S. gigas are quite similar; however, there are some more

subtle morphological and some pronounced anatomical

dierences. A clear dierence between the two species is in

the number of leaves per rosette, leading to large dierences

in LAI between the two species. With many more leaves in

its giant rosettes, L. gibberoa probably requires a greater

water supply than S. gigas, and therefore is more conned

to the moister sections ofLobelia-valley. S. gigas appears to

react more directly to soil moisture and is thus more

exible, having a higher density on the south-facing slope

and a lower density on the north-facing slope. Moreover,

the structure of the stem supporting the giant leaf rosettes in

the species is quite dierent. On the basis of this

observation, one might propose that water is supplied to

the leaves in dierent ways: via high stem water conduc-tance in L. gibberoa, allowing higher transpiration than in

S. gigas even at drier sites, and from reserves due to water

capacitance of the pith in S. gigas, allowing occupation ofthe drier sites. This leads to greater biomass production by

L. gibberoa than by S. gigas, with the latter appearing lessdemanding with respect to continuous water supply andthus more capable of colonizing drier sites.

A comparison of the ecophysiological data ofL. gibberoaand S. gigas suggests dierent strategies to control water

relations. Transpiration (JH2O) and water vapour conduc-tivity (gH2O) of leaves were generally similar in both speciesalthough responses to soil moisture diered. S. gigas had alower JH2O and gH2O on the drier south face of the valley;both species had the highest gH2O on the wet valley bottom.Carbon isotope discrimination, D, was higher for bothspecies on the wetter north face and valley bottom than onthe drier south face. The dierent strategies to control water

relations may lead to a similar use of PAR (as shown inTable 4). At the end of the dry season, the photosyntheticperformance of leaves of both species was very similar, withonly a slight tendency for a higher apparent quantum yieldof L. gibberoa. Overall site dierences are larger thandierences between the two species. The present integratedstudy on two giant rosette species illustrates how combinedmorphological and anatomical traits and ecophysiologicalfeatures may contribute to niche occupations.

ACKNOW LEDGEM ENTS

This investigation was supported by the German AcademicExchange Service (DAAD) as part of a joint eld studywithin the Partnership Program between the Universities ofBayreuth, Darmstadt, and Marburg and Addis AbabaUniversity. Students of the universities of Bayreuth,Darmstadt and Addis Ababa who took part in datacollection are gratefully acknowledged. We thank W.Schulze for the d13C-measurements, and the management

of Menagesha Natural Forest for allowing us to carry outthe study and for the facilities they provided.

L I T E R AT U R E C I T E D

Bekele T. 1993. Studies on remnant Afromontane forests in the CentralPlateau of Shewa, Ethiopia. Acta Phytographica Suecica 79: 159.

0.8

Day time

Pre-dawn Morning Midday Afternoon

Fv

/Fm

0.6

0.4

0.2

0.0

0.8

Fv

/Fm

0.6

0.4

0.2

0.0

S. gigas

L. gibberoa1200

900

600

300

0PAR(molm

2s

1)

1200

900

600

300

0PAR(molm

2s

1

)

F I G . 8. Potential quantum yield of photosystem II measured on dark-adapted leaves in the very early morning (Fv/Fm-predawn) and after30 min of dark-adaptation during the day (day-time Fv/Fm , h)together with the prevailing ambient PAR (G columns) at the

respective times (morning, midday, afternoon). The dashed horizontallines indicate an Fv/Fm of 0

.83.

TA B L E 4. Cardinal points of photosynthetic light-response curves

Parameter

L. gibberoa S . gigas

S V N S V N

ETRmax (mmol m2 s1) 136+ 31 128+ 37 80+21 107+ 31 103+33 101+45

PARsat (mmol m2 s1) 767+ 163 719+ 175 483+193 711+ 226 742+182 680+263

12 PARsat (mmol m

2 s1) 209+ 53 207+ 61 141+39 174+ 59 166+56 158+63

DFaFmH-sat 0.36+ 0.06 0.37+ 0.09 0.36+0.13 0.33+ 0.07 0.31+0.11 0.31+0.10

12 DFaFm

H-sat 0.68+ 0.06 0.66+ 0.07 0.62+0.08 0.67+ 0.07 0.66+0.14 0.66+0.13

ETRmax is apparent photosynthetic electron transport rate at saturating photosynthetically active radiation, (PARsat);12 PARsat is half-saturating

radiation; DFaFmH-sat is eective quantum yield of photosystem II at saturating radiation and 12 DFaFm

H-sat is eective quantum yield at half-saturating radiation. S, V and N refer to the south-facing slope, the valley bottom and the north-facing slope of the valley, respectively. Data aregiven as "x+ s.d.; n 8 for sites S and V, and 6 for N (n refers to the total number of light curves obtained on dierent days and for dierent leavesof one given plant each).



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AP P ENDIX

List of abbreviations

ci/ca Ratio of internal to ambient CO2-partial pressuresd13C Carbon isotope ratioD Carbon isotope discriminationDBH Diameter at breast heightETR Apparent rate of photosynthetic electron transport

(ETRmax at light saturation)F Fluorescence of the light-adapted leafFm Maximum uorescence of the dark-adapted leafFm

H Maximum uorescence of the light-adapted leaf

Fo Ground uorescence of the dark-adapted leafFv Maximum variable uorescenceFv/Fm Potential quantum yield of PS IIDFaFm

H Eective quantum yield of PS II (DFaFmH-sat at light

saturation and 12 DFaFmH-sat at half light saturation of ETR)

gH2 O Leaf conductivity to water vapourJH2O TranspirationLAI Leaf area indexNPQ Non-photochemical quenchingPAR Photosynthetically active radiation

(PARsat , saturating and 1/2 PARsat , half saturating ETR)PS PhotosystemRH Relative air humidity


eco physiology of niche occupation by two giant rosette plants, lobelia gibberoa

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