Constraints on floral water status of successively blossoming Mediterranean plants under natural conditions

Abstract

The water relations of showy laminar floral tissues (petals and tepals) were studied in 20 Mediterranean plants successively blossoming under ambient conditions. The water potential and osmotic potential of floral tissues decline according to the succession of the median day of flowering of the selected plant species. The highest (least negative) value of floral water potential (–0.32 MPa), among the examined species, was measured in petals of Anemone coronaria in March, and the lowest value (–1.25 MPa) in petals of Coridothymus capitatus in June. Low values of water potential of floral tissues coincided with constraints in declining values of osmotic potential at the onset of the dry period in the Mediterranean region; this apparently resulted in a reduction of turgor of floral tissues of Mediterranean plants subjected to water shortage. The reduced osmotic potential was correlated with enhanced soluble sugar content of floral tissues, presumably contributing to the expansion and water status of flowers under water scarcity, by decreasing water requirements.

Keywords:

• flowers
• Mediterranean
• osmotic potential
• seasonal
• sugars
• turgor
• water potential

Introduction

Flowers are among the most spectacular products of nature; their seasonal advertisement and life-span are essential for plant reproduction. Blossoming of plants is regulated by mechanisms that act to ensure that flower emergence occurs in suitable environmental conditions (Barrett 2010; Tooke and Battey 2010). Emerging flowers, which exhibit almost complete absence of sclerotic elements, have evolved mechanisms that require water supply in order to expand (Darlington and Dixon 1991; Lambrecht 2013). In the Mediterranean region, the main flowering period coincides with spring; induction of flowers before the dry summer may be interpreted as an evolved response of plants to predictably unfavourable environmental conditions when water is scarce (Bosch, Retana, and Cerda 1997; Tooke and Battey 2010; Argiropoulos and Rhizopoulou 2012; Scaven and Rafferty 2013; Farré-Armengol et al. 2014; Petanidou et al. 2014). Also, blossoming during spring appears to be an adaptation that diminishes the risk of floral senescence before fertilization. Although water availability has been identified as the main factor controlling anthesis of plant species (Skirycz and Inzé 2010), early summer blooming seems to be advantageous for some native Mediterranean plants, because they may benefit from the abundant pollinators and avoid competition for pollination that is prominent during spring (Voliotis 1984; Herrera 1992; Kigel et al. 2011). Furthermore, flowering during the drought period implies the ability of plant species to withstand water stress (Rhizopoulou et al. 2006; Chimona et al. 2012; Argiropoulos and Rhizopoulou 2013; Claeys and Inzé 2013). Summer water deficit is considered the main environmental constraint for plant growth in Mediterranean-type ecosystems; as water availability seasonally declines, it is reasonable to hypothesize that floral tissues of later-flowering species experience lower water potential and require lower osmotic potential to maintain turgor.

The objective of this study was to investigate components of the water relations of the visually striking floral tissues, i.e. petals (in dicotyledons) and tepals (in monocotyledons) of successively blossoming Mediterranean plants. Water flux from the soil to floral tissues is driven by water potential gradients; the water potential of floral tissues is related to the water status of the xylem that supplies water from the roots to the above-ground plant tissues (Forterre 2013). Earlier investigators studied the water relations of flowers of plant species mainly grown under controlled conditions (Trolinder, McMichael, and Upchurch 1993; Westgate, Passioura, and Munns 1996; Chapotin et al. 2003; field, Chatelet, and Brodribb 2009; van Doorn 2012). To the best of our knowledge, the components of water relations of petals and tepals of flowers and florets from naturally grown, native Mediterranean plants have not hitherto been investigated.

Material and methods

Twenty Mediterranean plants belonging to 16 families (Bremer et al. 2009; Chase and Reveal 2009) and growing in a nature reserve in the Botanic Garden of Diomedes (38°00’ N, 23°38’ E) were selected for the measurements of floral water status. The Botanic Garden of Diomedes is an asset of the University of Athens (Greece) that displays many elements of the Mediterranean ecosystems. The garden covers 460 acres, of which only 50 acres have been dedicated to the cultivation of ornamental plants. Landscape construction was based on the designs of Professor Herta Hammerbacher (1900–1985) of the University of Berlin (Hottentrager 1992), under the strict condition that the impact of landscape shaping on the natural ecosystem should be kept to a minimum, to conserve and protect native Mediterranean plants and habitats (Sarlis 1998; Potts et al. 2006; Rhizopoulou et al. 2012; Heywood 2014).

The study was conducted in the naturally occurring stands of wild shrubs and herbs in the Botanic garden of Diomedes (Rhizopoulou 2007; Rhizopoulou et al. 2012) and detailed field observations were made on a monthly basis between 2011 and 2012. The selected plants are growing under natural conditions in the Botanic Garden and are not artificially irrigated. Average monthly precipitation ranged from 44 to 3 mm and monthly temperature from 12 to 28°C between March and June, respectively, i.e. during the study period of two consecutive years (Figure 1). Time and duration of flowering of all species were monitored over the two years (2011–2013); for each species, a number of individual flowers (>35) was observed from the bud to the open-corolla stage and then to wilting. Median day of flowering, which is expressed as a Julian date, was estimated on flowers per individual plant (floral stems × number of flowers per stem), i.e. 20 flowers on each of the 31 different individual plants, for each species, selected at random.

Figure 1.  Climatic data of the research area during the study. The order of months is from March to July and the values represent the averages of the two years of the study; continuous line represents mean monthly temperature and bars represent monthly precipitation.

Predawn measurements of water relations were made on expanded floral tissues, i.e. during the first day of fully expanded floral corollas; 10 specimens were harvested from randomly selected plants of each species. The water potential (Ψ) was measured on discs of floral tissues (Richter 1997; Rhizopoulou et al. 2006) using C-52 sample chambers, 6 mm in diameter (Wescor Inc., Logan, UT, USA), attached to a Dew point psychrometer (HR-33T, Wescor Inc., Logan, UT, USA). The osmotic potential (Ψ_s) was measured on the same discs after freezing in liquid nitrogen and thawing, using the same technique (Grange 1983; Oosterhuis and Wullschleger 1989). Turgor was calculated as the algebraic difference between Ψ and Ψ_s. The reported values for Ψ and Ψ_s are means of five determinations ± standard error (SE).

Soluble sugars were extracted from dried, powdered floral material with acetone and were quantified colorimetrically in triplicate samples, according to the phenol-sulphuric acid method of Dubois et al. (1956); d-glucose (Serva (Heidelberg, Germany)) solutions were used for the standard curve.

Statistics were assessed with SPSS 12 software package (SPSS, Chicago, IL, USA). Correlations between parameters were tested through linear regressions fitted to the data.

Results

The highest (least negative) value of floral water potential (Ψ), among the 20 Mediterranean plants (Tables 1 and 2) was exhibited by Anemone coronaria (Ψ = –0.32 ± 0.05 MPa) in March and the lowest (most negative) value of floral Ψ was found in Coridothymus capitatus (Ψ = –1.25 ± 0.08 MPa) in June. In spring, Ψ of the short-lived petals of Cistus species and Eruca versicaria varied between –0.5 and –0.7 MPa, and the osmotic potential (Ψ_s) varied from –0.8 to –1.0 MPa; therefore, turgor of petals, representing a prerequisite for expansion of plant tissues, was sustained at approximately 0.30 MPa. Later flowering species exhibited lower perianth Ψ and Ψ_s in comparison with earlier blossoming species, and median Julian date of flowering was negatively correlated with Ψ_s (y = –0.006x – 0.14, R² = 0.52; p = 0.05) and Ψ (y = –0.008x + 0.47, R² = 0.34; p = 0.05) (Figure 2, Table 2). Ψ of floral tissues reached values that were more negative than –0.7 MPa in Medicago arborea, Satureja thymbra and Ruta chalepensis. The decline in Ψ was correlated with Ψ_s of floral tissues of the examined species (Figure 2). At the start of the dry period, more negative values of Ψ than –1.0 MPa were obtained in floral tissues, as, for example, these of Salvia fruticosa and Teucrium capitatum (Table 2). However, in the petals of Capparis spinosa flowers, relatively elevated values of Ψ and Ψ_s were obtained during the summer drought period (Table 1; Figure 3). Also, the decline in Ψ_s of floral tissues of the considered species was substantially linked to the accumulation of soluble sugars in petals and tepals (Figure 4). Soluble sugars of floral tissues of the considered species ranged from 7 to 22 mg g⁻¹ dry weight (Table 2); the lowest value was detected in the earliest flowering species, Asphodelus ramosus, and the highest value in the late-spring flowering Satureja thymbra (Table 2).

Table 1.  List of scientific names and families of the Mediterranean plants, presented according to the median day of flowering, expressed in Julian dates (1 for 1 January through 365 for 31 December).

Plant species	Family	Median day of flowering (mean Julian date)
Asphodelus ramosus L.	Xanthorrhoeaceae	75
Anemone coronaria L.	Ranunculaceae	83
Eruca vesicaria (L.) Cav.	Brassicaceae	97
Papaver rhœas L.	Papaveraceae	115
Chrysanthemum coronarium L.	Asteraceae	125
Cistus creticus Sm.	Cistaceae	126
Medicago arborea L.	Fabaceae	129
Ruta chalepensis L.	Rutaceae	130
Salvia fruticosa Mill.	Lamiaceae	131
Teucrium capitatum L.	Lamiaceae	132
Cistus salviifolius L.	Cistaceae	140
Nerium oleander L.	Apocynaceae	144
Acanthus spinosus L.	Acanthaceae	151
Convolvulus dorycnium L.	Convolvulaceae	154
Myrtus communis L.	Myrtaceae	155
Laurus nobilis L.	Lauraceae	169
Satureja thymbra L.	Lamiaceae	170
Alcea rosea L.	Malvaceae	183
Coridothymus capitatus Rchb.f.	Lamiaceae	182
Capparis spinosa L.	Capparaceae	184

Table 2.  Estimates of water (Ψ) and osmotic (Ψ_s) potential, and soluble sugar content of floral tissues during median day of flowering.

Plant species	Ψ (MPa)	Ψs (MPa)	Soluble sugars (mg g^−1 d.w.)
Asphodelus ramosus L.	–0.46 ± 0.03	–0.64 ± 0.07	6.98 ± 0.30
Anemone coronaria L.	–0.32 ± 0.05	–0.74 ± 0.03	10.02 ± 0.42
Eruca vesicaria (L.) Cav.	–0.50 ± 0.02	–0.80 ± 0.04	13.30 ± 0.61
Papaver rhœas L.	–0.65 ± 0.07	–0.88 ± 0.06	16.75 ± 0.35
Chrysanthemum coronarium L.	–0.72 ± 0.05	–1.01 ± 0.05	13.94 ± 0.22
Cistus creticus Sm.	–0.65 ± 0.04	–1.00 ± 0.08	14.68 ± 0.40
Medicago arborea L.	–0.73 ± 0.04	–0.96 ± 0.03	9.77 ± 0.32
Ruta chalepensis L.	–0.82 ± 0.02	–1.18 ± 0.07	18.26 ± 0.63
Salvia fruticosa Mill.	–1.03 ± 0.05	–1.32 ± 0.05	13.30 ± 0.64
Teucrium capitatum L.	–1.06 ± 0.08	–1.36 ± 0.06	21.04 ± 0.50
Cistus salviifolius L.	–0.68 ± 0.07	–1.02 ± 0.04	13.40 ± 0.25
Nerium oleander L.	–0.78 ± 0.06	–1.09 ± 0.08	9.78 ± 0.20
Acanthus spinosus L.	–0.79 ± 0.03	–1.06 ± 0.05	9.47 ± 0.38
Convolvulus dorycnium L.	–0.52 ± 0.02	–0.98 ± 0.02	14.91 ± 0.40
Myrtus communis L.	–0.61 ± 0.05	–1.02 ± 0.08	18.96 ± 0.12
Laurus nobilis L.	–0.88 ± 0.05	–1.18 ± 0.06	20.62 ± 0.80
Satureja thymbra L.	–0.94 ± 0.02	–1.28 ± 0.06	22.30 ± 0.67
Alcea rosea L.	–0.72 ± 0.08	–1.09 ± 0.09	13.30 ± 0.08
Coridothymus capitatus Rchb.f.	–1.25 ± 0.08	–1.41 ± 0.04	11.44 ± 0.10
Capparis spinosa L.	–0.52 ± 0.04	–0.94 ± 0.03	18.60 ± 0.09

Figure 2.  Relationship between water potential (Ψ) and osmotic potential (Ψ_s) of floral tissues of 20 Mediterranean plants (y = 0.86x + 0.24, R² = 0.74, p = 0.01); each spot represents mean values ± SE (n = 5).

Figure 3.  Values of water (Ψ) and osmotic (Ψ_s) potential of floral tissues of the winter-deciduous Capparis spinosa are indicated on a drawing (© S. Rhizopoulou) of a stem bearing a flower.

Figure 4.  Relationship between osmotic potential (Ψ_s) and soluble sugar content of floral tissues of 20 Mediterranean plants (y = – 18.45x – 5.57, R² = 0.80, p = 0.01); each spot represents mean values ± SE (n = 5).

Discussion

Ψ and Ψ_s of floral tissues decreased according to the succession of the median day of flowering of the Mediterranean plants. The lowered floral water status results in a gradient of water potential of plant tissues that provides the driving force for water movement along the soil–plant–atmosphere continuum during progressing soil water shortage in the Mediterranean ecosystem. Constraints in Ψ_s of floral tissues (Figure 2) led to an array of declining turgor (being the algebraic difference between Ψ and Ψ_s) of expanded flowers during a period of water deficit, which can be related to the imposition of constraints to corolla size in the Mediterranean region by the water maintenance costs for flowers (Teixido and Valladares 2014a, 2014b).

Relatively low values of components of water relations obtained in floral tissues of four species belonging to the family Lamiaceae (Table 2) may play a significant role in turgor maintenance and so in advertisement of flowers, represented by the spring blossoming Salvia fruticosa and Teucrium capitatum, and the late spring/early summer flowering Satureja thymbra and Coridothymus capitatus.

At the onset of the drought summer, Ψ of petals reflected the efficiency of plants at withholding water from drying soil. Although water deficiency affected floral water relations of the majority of species (Figure 2), the rapidly expanded petals of the large flowers of the deeply rooted, winter deciduous perennial Capparis spinosa exhibited relatively elevated values of Ψ and Ψ_s during the summer drought period (Table 1); the benefit of nocturnal flowering of C. spinosa reduces water loss during the dry period (Rhizopoulou 1990; Rhizopoulou, Heberlein, and Kassianou 1997; Rhizopoulou and Psaras 2003; Chimona et al. 2012; Yang et al. 2014).

The decline in Ψ_s of floral tissues, substantially linked to the accumulation of soluble sugars (Figure 4), may contribute to turgor maintenance in petals and tepals (Tarpley and Sassenrath 2006; Rosas et al. 2013; Beauzamy, Nakayama, and Boudaoud 2014). Indeed, petal and tepal expansion appears to be due to a pulsed increase in the concentration of soluble sugars (Evans and Reid 1986; Bieleski, Elgar, and Heyes 2000; van Doorn and van Meeteren 2003; van Doorn 2004; van Doorn and Kamdee 2014), which may be derived by both chlorophyllous parts of floral tissues and surrounding leaves, subsequently reducing the cost of the reproductive process and demand (Aschan and Phanz 2003, 2006; Rhizopoulou et al. 2006; Chimona et al. 2012). Also, there appears to be an influence on water uptake via osmotic adjustment; therefore, reduced values of floral Ψ_s may increase the capacity of Mediterranean plants to extract water from drying soil.

Authors’ contribution

The two authors contributed equally to this work.

Acknowledgements

The research was supported by the National and Kapodistrian University of Athens (ELKE 11238).