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Journal of AridEnvironments

Journal of Arid Environments 66 (2006) 635–647

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doi:10.1016/j

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A study on functional and structural traits of thenocturnal flowers of Capparis spinosa L.

S. Rhizopouloua,�, E. Ioannidia,b, N. Alexandredesa,A. Argiropoulosa

aDepartment of Botany, Faculty of Biology, University of Athens, Panepistimioupolis, Athens 157 84, GreecebLaboratory of Pharmacognocy, Department of Pharmacy, Aristotle University of Thessaloniki,

541 24 Thessaloniki, Greece

Received 19 April 2004; received in revised form 10 November 2005; accepted 8 December 2005

Available online 14 February 2006

Abstract

Functional and structural traits of the Capparis spinosa flower were studied in order to understand

the mechanisms that allow this species to flower during the dry summer in the Mediterranean.

Stomata were found on the abaxial surface of sepals, and on both the adaxial and abaxial surfaces of

petals. Filaments and style were densely packed with small cells, exhibiting an increased density of

cell wall material that provides strength. Petals possessed vacuolated parenchyma cells with large

intercellular space. Turgor of petals was sustained mainly due to a decline in solute potential during

the night and to a rise in water potential and solute potential at sunrise, concomitantly with a

decrease in solute accumulation. Estimates of total sugars made in floral tissues of nine successive

flowers along newborn stems were 100-fold higher than those of proline accumulation. Unsaturated

fatty acids were identified as major components of lipids in petals, thereby influencing the fluidity of

membranes.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Caper; Nocturnal flowering; Solutes; Structure; Water relations

see front matter r 2006 Elsevier Ltd. All rights reserved.

.jaridenv.2005.12.009

nding author. Biology Department, Section of Botany, University of Athens, Panepistimioupolis,

1, Greece. Tel.: +30210 7274513; fax: +30210 7274702.

dresses: srhizop@biol.uoa.gr (S. Rhizopoulou), ioannidi@pharm.auth.gr (E. Ioannidi),

hotmail.com (N. Alexandredes), afargirop@hotmail.com (A. Argiropoulos).

ARTICLE IN PRESSS. Rhizopoulou et al. / Journal of Arid Environments 66 (2006) 635–647636

We are learning a great deal about inbred model organisms in the laboratory andmuch too little about real species in the wild

Fotis C. Kafatos and Thomas Eisner, 2004. Science 303 Editorial.

1. Introduction

Flowering time may be interpreted as an evolved response to predictably unfavorableenvironmental conditions prevailing during this period (Herrera, 1992; Patino and Grace,2002; van Doorn and van Meeteren, 2003). The few perennials that flower during summerin the Mediterranean region (Ish-Shalom-Gordon, 1993) exhibit adaptive mechanisms todrought. Among them, Capparis spinosa L. (Capparaceae) grows and blooms entirelyduring the dry and hot summer (Meikle, 1977; Eisikowitch et al., 1986; Rhizopoulou et al.,1997), contrary to the majority of co-occurring species. In the Mediterranean Basin, themain flowering period of the surrounding flora, which is primarily composed ofphanerophytes and chamaephytes, extends from February to June (Petanidou et al., 1995).During summer, drought is the factor essentially responsible for restricted growth in

Mediterranean-type regions, combined with high solar irradiation and high temperatures(Naveh and Lieberman, 1994; Larcher, 2000). The annual growing cycle of C. spinosa,which is native to the Mediterranean landscape, lasts from May to October, in Greece,while its flowering is exclusively nocturnal (Dafni et al., 1987; Petanidou et al., 1996). Thespecies (caper) is a winter-deciduous, hemicryptophytic plant that grows spontaneously infields and ruins. It is a spiny, perennial, wall-fissure plant that penetrates cracks andcrevices of rocks most probably by excreting acid compounds (Oppenheimer, 1960). Inearlier reports, the response of its leaves and roots to drought has been investigated(Rhizopoulou, 1990; Rhizopoulou et al., 1997; Rhizopoulou and Psaras, 2003). To the bestof our knowledge, environmental factors that control anthesis in C. spinosa have not yetbeen studied.The objective of this study was to identify functional and structural traits of the short-

lived flowers of C. spinosa in relation to the timing of anthesis during the dry summer. Wefocused in particular on water status.

2. Materials and methods

2.1. Study site and plant material

The study was conducted at the University campus of Athens, 5 km E of the center ofthe city (38114.30N, 23147.80E). Observations and experiments were performed from 5 Julyto 13 August, in a patch of several hundred individuals. Investigations of water potential(c) and osmotic potential (cs) of petals and sepals were made on elegant, solitary flowersselected at random at 2200 h, from new branches in the field. Additionally, water relationswere investigated in petals selected at periodic intervals, between 2400 and 1000 h, thosemade at 0600 h are called predawn measurements. Total sugars, lipids and proline contentwere measured at 2200 and at 0600 h. Additional estimates of total sugars and prolineaccumulation were made in successive flowers along stalks (bearing up to nine flower-buds), concomitantly with three leaves that surround a flower, namely upper, lower andmiddle leaf (i.e. the closest attached to a flower); these specimens were collected at 2200 h,

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throughout a period of 9 days. During the study period, the average daily maximumtemperatures ranged from 29 to 31 1C and precipitation was 0mm.

2.2. Measurements

Flower water status was estimated as petal and sepal water (c), solute (cs) andturgor (cp) potential. The parameters of water relations of fully expanded petals weremeasured with a dew point psychrometer on random nights during the blossoming period.Estimates of c were made on 6mm diameter petal and sepal discs, using C-52thermocouple psychrometers attached to a microvoltmeter (Wescor, HR-33T, Inc. Logan,Utah, USA). cs was measured on the same discs after freezing in liquid nitrogen andthawing, using the same technique. We estimated the equilibration time of floral tissues inthe chambers to be 2 h, for both c and cs (Schneider et al., 2000); cp was calculated bydifference (cs�c).

Free proline accumulation was measured in triplicate filtered extracts of dried, powderedsamples by the acidic ninhydrin method (Bates et al., 1973). Proline was calculated on adry weight basis using L-proline (Serva 33580), for the standard curve. Total sugars weredetermined in triplicate samples, too (Dubois et al., 1956). Total lipids of petals wereextracted from dried, powdered material using chloroform:methanol solution (2:1 v/v) intriplicate samples; qualitative and quantitative determination of fatty acids was performedby gas–liquid chromatography (Meletiou-Christou et al., 1990).

Data were subjected to analysis of variance among tissues. Values in the text, tables andfigures are followed by their standard error (7S.E.).

2.3. Microscopy

Cross-sections were obtained from petals, sepals, filaments and stigma of C. spinosa.Fixation of floral tissues was executed in phosphate buffered 3% glutaraldehyde for 2 h.The temperature was kept at 0 1C to reduce pigments leaching from cell vacuoles. Thetissue was post-fixed in 1% osmium tetroxide for 2 h at 4 1C, dehydrated in a gradedethanol series embedded in Durcupan ACM (Fluka) and incubated at 60 1C for 24 h. Semi-thin sections were cut on an LKB Ultratome III and observed with a ZEISS lightmicroscope (Christodoulakis, 1989). Intercellular space (ICS) was investigated on 51 petalcross-sections taken from 10 different petals (Psaras and Rhizopoulou, 1995); the area ofICS, as it appeared in the micrographs, was traced and blackened on a transparent overlayand measured with a Delta-T area meter. Scanning electron microscopic (SEM)observations of epidermis in sepals and petals were prepared according to a methoddescribed by Levizou et al. (2002). In brief, a dental impression material (Sandro-sil,Germany) was applied on adaxial and abaxial surfaces of petals and on the abaxial surfaceof sepals. After setting, the material was peeled off, washed, dried and filled with Spurrepoxy resin. After polymerization the negative moulds were peeled off. Micrographs wererecorded as video prints. In order to present the length of stomata in the case of sepals ofC. spinosa, the abaxial epidermis is illustrated as seen in the negative resin replicas,mounted on stubs, sputter coated with gold, and examined using a JEOL 6300 SEM.

The area of the corolla was measured using an area meter (Delta-T); measurements weremade in 80 corollas taken from 10 different shrubs.

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3. Results and discussion

3.1. Floral phenology and morphology

A flower of C. spinosa lasts only about one night. The flowers expand acropetally, i.e.when a lateral flower expands at the aerial edge of a stalk, eight to 10 successive flower-buds at all stages of development grow along the same newborn branch. We observed thatthe rapid extension of flower parts in C. spinosa takes place within an hour, i.e. from 1900to 2000 h. The ontogenetic sequence of flower appendages indicates that the differentiationof floral tissues of caper has been completed by the time of anthesis (Leins andMetzenauer, 1979).Rapid flower opening is associated with increased respiration (Galen et al., 1993).

Flowering at dusk is correlated with decreasing temperature and light intensity and withincreasing relative humidity (van Doorn and van Meeteren, 2003). Actually, sepals of amature flower-bud open slightly at 0700 h and remain in this sunlit position until 1900 h. Itseems likely that the waxy exposed surfaces of sepals in C. spinosa (Fig. 1A) reduce waterloss (Buchanan et al., 2000), while hypostomatous sepals (Fig. 1B) sustain evaporativecooling, which decreases the temperature of the gynoecium. Stomata on the abaxial, outersurface of sepals are larger (length 33.2070.04 mm� 23.6070.02 mm width) than stomataof petals (length 22.4070.03 mm� 20.2070.02 mm width), which concurs with results fromIpomoea sp. (Patino and Grace, 2002). Stomatal density of sepals is comparable to that ofthe abaxial epidermis of leaves of C. spinosa (Rhizopoulou et al., 1997). In the evening,sepals open concomitantly with four white petals (3–5 cm in length), numerous purplishstamens (2–4 cm in length) and a long stigma. Then, flowers emit a spicy fragrance, whiletheir nectar is secreted in the cavity of sepals. Although these flowers make poor contrastwith the background (Spaethe et al., 2001), they can be detected by bees (Vorobyev et al.,1999).The area of corollas of caper (2872 cm2) is larger than that of other plants flowering in

the Mediterranean region during summer and may be considered an improved opticalsignal for pollinators’ visits. The area of the corolla is central to plant advertisement andpollinator attraction (Herrera, 2005). Dafni et al. (1997) argued that large corollas inducehigher visitation per unit time than smaller ones and higher pollination intensity. Also, alarge corolla is positively correlated with nectar production (Herrera, 2005). Ishii andSakai (2000) proposed that flower size has a strong influence on flower maintenance, whilethe cost of producing large flowers is greater in dry than in wet environments. Also, largecorollas incur physiological costs of their greater water uptake (Herrera, 2005).Lobate epidermal cells on the adaxial surface of petals (Fig. 1C) are slightly larger than

those on the abaxial surface (Fig. 1D). Stomata have been observed on both epidermes ofpetals, though substantially reduced in frequency (1571 stomatamm�2), when comparedto that of sepals (9575 stomatamm�2) and leaves (12174 stomatamm�2); nonethelessthey appear fully functional (unpublished data). Petals possess large ICSs (approximately17% of the volume of the tissues) and few sieve elements (Fig. 1D).In the morning, when rigid petals of C. spinosa abscise, the stigma (pollen receptor) is

still held turgid, providing easy access to pollinators. Style (Fig. 1F) and filaments (Fig.1E), densely packed with small-sized cells, possess small ICSs and a tightly organizedvascular system, which may provide strength during the later stages of maturity. A smallcell volume allows turgor maintenance when water is scarce by increasing the density of

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Fig. 1. Images of epidermal cells (A) and stomata (B) on the waxy abaxial surface of sepals; (C) abaxial epidermal

cells of petals; (D) cross-section of petal, arrows indicate large intercellular space; (E) cross-section of filament

(diameter: 458.870.6mm); (F) cross-section of stigma (diameter 1.91278mm), from flowers of C. spinosa growing

under ambient conditions. Bars in A, B, C, and D represent 50 mm.

S. Rhizopoulou et al. / Journal of Arid Environments 66 (2006) 635–647 639

cell wall material per unit of vacuole volume that may also act as a mechanical barrier(Galen et al., 1999).

Flowers of C. spinosa expand at dusk, scent and remain open overnight, thus matchingfloral rewards (nectar, pollen) to the activity of some pollinators (Vogel, 1983; Cornes and

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Cornes, 1989; Petanidou and Ellis, 1993). Dafni et al. (1987) published that the solitary beeProxylocopa olivieri collects pollen from C. spinosa at 1900 and at 0400 h, while honeybeesfrom 0500 to 1000 h.

3.2. Water relations

Anthesis ends when anthers collapse and the corolla flops down (Li et al., 2001). c andcs of petals of C. spinosa were more negative than those in sepals. There were no significantdifferences in c of petals and sepals during the night, but a significant difference (po0.005)was detected in predawn values. The same holds true for cp. Predawn cpetal is higher thanboth cpetal during the night (Table 1) and predawn cleaf (Rhizopoulou et al., 1997;Rhizopoulou and Psaras, 2003). It is likely that cpetal returns during the night to a valueequal to soil water-content, which is in accordance with results from cotton flower petals(Trolinder et al., 1993). c and cs of petals of C. spinosa are comparable to c and cs givenfor floral organs of drought-stressed wheat (Westgate et al., 1996), as well as to those ofleaves of phreatophytes (Donovan et al., 2001). Turgor of petals was comparable to that ofsepals (Table 1). Cell enlargement requires the walls to yield to turgor, preventing turgorfrom reaching its maximum and in turn creating c values that favor water entry intoenlarging cells (Boyer, 1988). In a molecular analysis, we are still exploring the role of thecell wall in the expansion of floral tissues of C. spinosa (Ioannidi et al., 2001, 2003). Petalslose turgor at a c of �1.32MPa at 1000 h, enhancing the removal of pollen, and theyabscise until 1200 h. The life of a flower that was viable for 14 h has come to an end. In theevening, the next, mature flower-bud along the stem will open. If, as is sometimes the case,shrubs of C. spinosa grow close by wells, numerous corollas with rigid petals were

Table 1

Components of water relations, free proline accumulation and total sugar content in floral tissues of Capparis

spinosa L.

Floral tissues Time (h) Water

potential (c)(MPa)

Solute potential

(cs) (MPa)

Turgor

potential (cp)

(MPa)

Proline content

(mg g�1 d.w.)

Concentration of

total sugars

(mg g�1 d.w.)

Sepals 2200 �1.1570.02A �1.3570.03C 0.20 1.4270.07E 35570.6K

0600 �0.4270.01b* �0.9370.04d 0.51* 0.3770.02f* 8970.3l*

Petals 2200 �1.1670.03A �1.3770.02C 0.21 1.9670.04G 38470.6K

2400 �1.0470.05A �1.4870.05C 0.44

0600 �0.5970.06bd*�1.1270.04a 0.53* 1.5170.01e 10270.4m*

0800 �1.2270.06ac �1.3970.03c 0.17

1000 �1.3270.03c �1.3870.07c 0.06

Filaments 2200 0.9370.04H 27370.7N

0600 0.6470.03i 26870.9n

Pollen 2200 1.2270.02J 17570.5O

0600 1.7570.03eg* 14470.3mo*

The reported values for c and cs are the mean of five determinations. Values of total sugars and proline content

are means of three replicates7S.E. Means followed by the same letters, within tissues (a capital letter for

nocturnal measurements, a small letter for morning measurements) are not statistically different, while *

represents significant difference within a tissue between nocturnal and morning measurements, at po0.001.

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maintained on stalks until early afternoon; probably, adequate water flow might lengthenthe flower life-span for 2–5 h.

3.3. Metabolic aspects

Total lipid content in petals at 2200 h was similar to that recorded at 0600 h(32.570.8mg g�1 d.w.). However, fatty acid composition varied between newly expanded(night) and senescent (morning) petals (Table 2). Among the polyunsaturated fatty acids,elaidic, linolenic, linoleic and arachidonic acid were identified as major components innight petals, while in morning petals heptadecanoid, palmitoleic, elaidic, linoleic, behenicand arachidonic acid were identified as major components. More than 50% of the fattyacids of fresh (night) petals are unsaturated, thereby influencing the fluidity of membranes;also, they are important to insects, as they are fat-soluble vitamins (Kevan and Baker,1983).

Flowering involves translocation of material in response to the environment (Kemp,1983). It has been argued (van Doorn and van Meeteren, 2003) that flower opening may bedue to sugar uptake. The presence of sugars is beneficial since by lowering the solutepotential, it decreases the water requirement during periods of water deficiency. Freeproline is accumulated in the cytoplasm of plant tissues, which are subjected to droughtstress (Bohnert et al., 1995; Taylor, 1996). cs in sepals and petals is lower than that ofexpanding leaves of C. spinosa (Rhizopoulou and Psaras, 2003). Solutes and water can beexchanged between tissues, to meet the requirements of reproductive tissues. In petals, freeproline content was 20-fold lower than in leaves of caper, and ten-fold higher than in leaves

Table 2

Fatty acid levels in newly expanded (night) petals and slightly senescent (morning) petals of C. spinosa; fatty acids

given as a percentage of total fatty acids

Fatty acids Night petals (% of total fatty acids) Morning petals (% of total fatty

acids)

C15:0 1.802

C16:1 1.984 3.125

C17:0 5.862 8.765

C17:l 2.915

C18:0 2.933

C18:l 10.279 16.104

C18:2 16.518 20.143

C18:3 9.945 2.797

C20:2 3.093 7.844

C20:3 1.706

C20:4 24.697 16.115

C20:5 4.559 5.448

C22:0 2.189

C22:l 8.572 5.941

C22:2 4.259 2.538

C22:6 3.696 2.549

C23:0 3.570

Values given represent means obtained from five samples, with typically o3% S.E.

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of the majority of species growing in the same environment (Rhizopoulou et al., 1990;Diamantoglou and Rhizopoulou, 1992), probably contributing to water uptake andstabilizing subcellular structure (Hare et al., 1999). Also, C. spinosa grows in a nutrient-poor habitat and proline may be transferred from floral to reproductive tissues, as long asflower parts remain viable (Rhizopoulou et al., 1990; Taylor, 1996). Proline varied amongtissues (Table 1) and its predawn values in senescent flowers were lower than those detectedin newly expanded flowers; there were no significant differences in floral tissues (Table 3).The highest value was detected in petals (1.9mg g�1 d.w. in the evening and 1.5mg g�1 d.w.in the morning), followed by sepals (1.5mg g�1 d.w. in the evening and 1.4mg g�1 d.w. inthe morning), pollen (1.2mg g�1 d.w.) and filaments (0.9mg g�1 d.w.). Proline was notmeasured in nectar, but the concentration of its precursors was high (Petanidou et al.,1996).The highest sugar concentration was found in petals of C. spinosa at 2200 h

(384mg g�1 d.w.), which is five-fold higher than that of its expanded leaves (Rhizopoulouet al., 1997). Nectar, which contains sugars, is secreted by flowers of C. spinosa, in anecosystem that is water-limited and, as a consequence nectar-limited. In morning petals,sugar concentration declined by 26%, which coincides with the increase in Cspetals (by24.3%). In sepals, sugar content was 354mg g�1 d.w. at 2200 h, and it declined by 25% inthe morning (Table 1). In filaments, total sugar content was 273mg g�1 d.w. at 2200 h, and

Table 3

Summary of statistically significant differences in proline and sugar content in tissues of C. spinosa

Parameter p Post hoc comparisons

Free proline in individual tissues 0.0237 Petals–upper leaves

Sugar content in individual tissues 0.0014 Sepals–lower leaves

0.0023 Sepals–middle leaves

0.0015 Sepals–upper leaves

0.0005 Sepals–pollen

0.0137 Petals–lower leaves

0.0034 Petals–middle leaves

0.0013 Petals–upper leaves

0.0177 Petals–filaments

Free proline in all tissues of flowers

expanding along stalks

0.0212 1st–2nd flower

0.0309 1st–6th flower

0.0002 1st–7th flower

0.0004 1st–8th flower

0.0486 1st–9th flower

0.0001 3rd–7th flower

0.0009 3rd–8th flower

0.0016 4th–7th flower

0.0033 4th–8th flower

0.0044 5th–7th flower

0.0002 5th–8th flower

0.0334 7th–9th flower

Sugar content in all tissues of flowers

expanding along stalks

There were no significant differences

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it declined by 3% in the morning filaments. In pollen at 2200 h, total sugar content was174mg g�1 d.w., while in the morning samples a decrease of 17% was detected. In nectar ahigher total sugar and amino acid content was detected in senescent, morning flowers,when compared with the evening values (Petanidou et al., 1996); this may be due tochanges in volume and concentration of nectar, and to translocation of solutes from floraltissues into floral rewards.

In floral and foliar tissues of nine successive flowers growing along stalks, concentrationsof total sugars (Fig. 3) were 100-fold higher than those of proline (Fig. 2). Tests ofsignificance indicate that total sugar content did not vary with flower development alongstalks (F8;54 ¼ 0:04); in contrast, significant differences were detected in free prolineaccumulation (F8;54 ¼ 11:62) in successive flowers along stalks (Table 3). Thus, prolinecontent in the 1st flower differs from the 3rd, 4th and 5th flower, and all of them (i.e. 1st,3rd, 4th, 5th) differ from the 7th and 8th flower. The highest values of proline content infloral and foliar tissues were detected at the 7th flower along stalks; this finding issupported by Sozzi (2001), and it may be related to the fertility of caper buds. Concerningthe individual tissues, it is clear that the highest levels of proline were detected in the leavesthat surround a flower, namely upper leaf, followed by middle (the closest leaf to a flower)and lower leaf (Fig. 2); then came sepals, filaments and, lastly, petals. Tests of significancefor accumulated proline in tissues (F6;56 ¼ 2:83) indicate that petals differ from upperleaves (Fig. 2). Constitutively, sugar concentration was highest in petals, followed bysepals, filaments and, lastly, by leaves (Fig. 3); analysis of variance indicates significantdifferences in sugar content (F 6;56 ¼ 283:69) between sepals and leaves, petals and leaves,petals and pollen, sepals and pollen, filaments and leaves (Table 3).

The short life-span of flowers of C. spinosa indicates a low maintenance cost for water,nectar and respiration as well as a rapidly produced, sufficient amount of pollen grains(Ashman and Schoen, 1994). Short flower life is a characteristic of Selenicereus, Oenothera

0

4

8

12

16

1Flower along the axis

Prol

ine

cont

ent,

mg

g-1 d

.w.

lower leaf middle leaf sepals petals stament pollen upper leaf

acdeabcde abcde

abcde

gh gh

bcdefcbdef bfghi

98765432

Fig. 2. Distribution of free proline accumulation7S.E. in floral tissues (sepals, petals, filaments, pollen) and in

lower, middle and upper leaves surrounding flowers of C. spinosa growing along stalks. Each number along the X-

axis refers to a flower that opens for one night, i.e. number 1 represents the flower that was open during the first

night, 2 represents the flower that was open during the second night, and so on, 9 represents the flower that was

open during the last night of the experiment. Means followed by the same letters are not statistically different.

Mean bars indicated by an asterisk represent significant differences between petals and upper leaves. *po0.05.

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lower leaf middle leaf sepals petals stament pollen upper leaf

0

400

800

1200

1600

1Flower along the axis

`Sug

ar c

onte

nt, m

g g-1

d.w

.

FP

*

PS

LLLLLL

SP

FP

PS

SP

FP

FPFP

FP FP FP FP

PSPS PS PS PS PS PS

SP SP SP SPSP SP SP

LLL LLL LLL LLL LLL LLL LLL

98765432

Fig. 3. Distribution of total sugar concentrations7S.E. in floral tissues (sepals, petals, filaments, pollen) and in

lower, middle and upper leaves surrounding flowers of C. spinosa growing along stalks. Each number along the X-

axis refers to a flower that opens for one night, i.e. number 1 represents the flower that was open during the first

night, 2 represents the flower that was open during the second night, and so on, 9 represents the flower that was

open during the last night of the experiment. Means followed by the same letters within tissues (S for sepals, P for

petals, F for filaments) are not statistically different; the same holds true for lower, middle and upper leaves

(LLL).

S. Rhizopoulou et al. / Journal of Arid Environments 66 (2006) 635–647644

or Mirabilis, which do not grow in the Mediterranean region, the tropical Asian climberQuisqualis indica (Eisikowitch and Rotem, 1987), and Gynandritis, Haloxylon orRhanterium, which grow in desert environments (Omar, 2000), and Pangratium maritimum

(Eisikowitch and Galil, 1971). Fruit maturation of C. spinosa is completed by mid-August.We observed that although anthesis rarely occurs during this period, several prematureflower-buds, which coexist simultaneously, do not expand further, remaining asdehydrated, unopened flower-buds attached to stalks.

4. Conclusion

In conclusion, estimates of functional and structural characteristics of flowers of C.

spinosa provide some explanations for their nocturnal advertisement. The white petalsopen concomitantly with increasing relative humidity and declining temperature andexposure to sunlight: since corollas of C. spinosa grow close to the ground (20–30 cm highand often over 2m broad), where midday temperatures can exceed 40 1C. Floral tissues areexposed to visits of pollinators by remaining turgid overnight, when foliar stomata areclosed. Thus, c of the demanding floral tissues equilibrates with soil water potential,resulting in the maintenance of turgor. Also, turgor develops as a consequence of waterflux driven by gradients in water potential, created by the concentration of compatiblesolutes in petals (Bonner and Dickinson, 1990); the highest values of total sugars and freeproline were found in the evening (fresh) tissues. The decline in cs of petals of C. spinosa atdusk may result in withdrawal of water from other floral tissues. At sunrise, a rise in c andcs found for petals to values higher than those detected at midnight implies themaintenance of turgor (0.53MPa), despite a decline in sugar and proline content.

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The winter-deciduous C. spinosa grows and flowers entirely during the most stressfulperiod of the year, when the surrounding flora exhibits minimum growth rates. Thisperformance provides C. spinosa with a competitive advantage against other species; caper,thus, plays an important role in the dynamics of the Mediterranean ecosystem, during aperiod of limited resources.

Acknowledgments

This work was supported by the University of Athens special account for research grantsto S.R. (70/3/4232). We thank Dr. J.C. Davis, Ass. Prof. A. Legakis, Lecturer M.S.Meletiou-Christou and Ass. Prof. N. Christodoulakis for comments, B. Cotsopoulos forhelp with SEM, and the students K. Margazis, A. Sciandra and K. Spala for technicalassistance.

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