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Scientia Horticulturae 178 (2014) 145–153

Contents lists available at ScienceDirect

Scientia Horticulturae

journa l h om epage: www.elsev ier .com/ locate /sc ihor t i

esponses of Mediterranean ornamental shrubs to droughttress and recovery

tefania Toscano, Domenica Scuderi ∗, Francesco Giuffrida, Daniela Romanoepartment of Agricultural and Food Science (DISPA), University of Catania, Via Valdisavoia 5, 95123 Catania, Italy

r t i c l e i n f o

rticle history:eceived 29 May 2014eceived in revised form 5 August 2014ccepted 14 August 2014vailable online 22 September 2014

eywords:ater stress

iomassas exchangehotosynthesisater relations

a b s t r a c t

The aim of this study was to evaluate the differences in the mechanisms that are involved in the resis-tance of ornamental species to drought stress resulting from a regular suspension and recovery of thewater supply. Plants of five ornamental shrubs [Callistemon citrinus (Curtis) Skeels (Callistemon), Lau-rus nobilis L. (Laurus), Pittosporum tobira (Thunb.) W.T. Aiton (Pittosporum), Thunbergia erecta (Benth.)Anderson (Thunbergia) and Viburnum tinus L. ‘Lucidum’ (Viburnum)] were subjected to two consecutivecycles of suspension/rewatering (S-R) and compared with plants that were watered daily (C). The relativewater content (RWC), leaf water potential (� ), net photosynthetic rate (A), transpiration rate (E) and sto-matal conductance (Gs) parameters were monitored during the experiment. The five species that wereinvestigated exhibited different responses to drought stress. At the end of the experimental period, S-Rtreatment had no effect on dry weight in all species, except Pittosporum. In Pittosporum, drought stressreduced total plant biomass by 19%. Drought stress induced alterations in shrubs, including decreases in

the shoot dry matter and increases in the root to shoot ratio, strongly affecting Callistemon and Pittospo-rum. All species adapted to water shortages using physiological mechanisms (RWC and water potentialadjustment, stomatal closure and reductions in photosynthesis). Following rewatering, the species fullyrecovered and thus can be considered appropriate for green spaces in the Mediterranean environment.However, Laurus and Thunbergia seem to be less sensitive to drought stress than the other species.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Water deficits are considered the main limiting factor for plantrowth in the Mediterranean region during the summer because ofhe high levels of solar radiation and high temperatures (Di Castri,981). However, the global climate changes that this region hasxperienced during the last several decades have led to signifi-antly increased water shortages throughout the year (IPCC, 2007;

ang et al., 2007). Thus, the issues that are linked to water short-ge are of interest to landscape management, and the area of green

ustainability is becoming increasingly relevant. The possibilityf isolating specific plants that are resistant to particular abiotictresses is currently being studied (Franco et al., 2006) because this

Abbreviations: A, net photosynthetic rate; ANOVA, analysis of variance; DW,ry weight; E, transpiration rate; Gs, stomatal conductance; RWC, relative waterontent; S-R, cycle suspension/rewatering; SLA, specific leaf area; R/S, root/shootatio.∗ Corresponding author at: Via Valdisavoia 5, 95123 Catania, Italy.el.: +39 95 234322.

E-mail addresses: domenica.scuderi@gmail.com, dscuderi@unict.it (D. Scuderi).

ttp://dx.doi.org/10.1016/j.scienta.2014.08.014304-4238/© 2014 Elsevier B.V. All rights reserved.

knowledge is useful for the establishment of tools to improve sus-tainable green areas. The high number of ornamental plants that areused in Mediterranean areas (Romano, 2004) allows for the isola-tion of suitable genotypes that are able to cope with environmentalstresses. All this has led to increased interest in the study of orna-mental plant responses to water deficits in urban and suburbanlandscape environments.

Plant responses to drought are multiple and interconnected(Efeoglu et al., 2009) and their capacities to adapt to this stress mayvary considerably within genera and species (Sánchez-Blanco et al.,2002; Torrecillas et al., 2003). Mediterranean species have devel-oped physiological and morphological adaptations to water stress(Dickson and Tomlinson, 1996), including the regulation of gasexchange (Moriana et al., 2002), osmotic adjustment (Chartzoulakiset al., 1999), the development of leaf protective structures (i.e.,hairs, thick cuticles and schlerenchymatic cells), leaf modifica-tions (i.e., inclination variations, increased thicknesses and reducedsurface areas) (Castro-Díez et al., 1998; Gratani and Bombelli,

2000; Karabourniotis, 1998) and more extensive root systems(Malinowski and Belesky, 2000). Numerous morphological adap-tations to water stress involve the aerial portions of plants. Leafgrowth is the most sensitive plant process to water deficits

1 orticulturae 178 (2014) 145–153

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Fig. 1. Timeline for suspension/rewatering (S-R) treatments used in experiment.

46 S. Toscano et al. / Scientia H

Bradford and Hsiao, 1982; Hsiao, 1973; Jones, 1985). The verti-al orientations of the leaves allow the plants to reduce the radiantnergy that is intercepted (Pereira and Chaves, 1993). The specificeaf area, which is often used as an indirect indicator of leaf thick-ess, is reported to be reduced under drought conditions (Liu andtützel, 2004; Marcelis et al., 1998). The reduction of the specificeaf area is assumed to be a way to improve water use efficiencyWUE) (Craufurd et al., 1999; Wright et al., 1994) because thickereaves usually have higher densities of chlorophyll and proteinser unit leaf area and thus have greater photosynthetic capacitieser unit leaf area than thinner leaves (Liu and Stützel, 2004). Thepecific leaf area was shown to be reduced in Asteriscus maritimusollowing water stress as a direct consequence of reduced leaf areasRodríguez et al., 2005). Similar results were found for Eragrostisurvula, Oryza sativa, Abelmoschus esculentus and Asteriscus mar-timus following water stress, and significantly decreased total leafreas were observed (Rucker et al., 1995; Shubhra et al., 2003).

Several authors have found frequent increases in the root-hoot ratios in plants under water stress (Blum, 1996; Zwack andraves, 1998), which has been considered to be an adaptive strategy

Bargali and Tewari, 2004; Guo et al., 2007; Li et al., 2008) because larger investment in roots improves the absorption of water.

Reduced photosynthesis is one of the main consequences ofater stress (Hsiao and Acevedo, 1974; Huang, 2004) and is related

o stomatal closure, which is implemented by the plant to reduceater loss through transpiration (Nayyar and Gupta, 2006; Yang

t al., 2006). However, the duration and speed of the stomatal clo-ure vary depending upon the species (Schulze and Hall, 1982).

Evergreen trees have adopted mechanisms to cope with the typ-cal conditions of the Mediterranean, including the ability to endure

ater limitation and to recover after rainfall (Galmés et al., 2007).urther, lemon plants respond to water stress and rewatering byeveloping drought avoidance mechanisms, such as stomatal clo-ure, leaf rolling and partial defoliation (Ruiz-Sànchez et al., 1997).feoglu et al. (2009) demonstrated that the relative water contentn maize was significantly reduced under drought stress conditionsut significantly increased during the recovery period, reaching the

evels of the control plants. Other authors (Sánchez-Blanco et al.,002) have shown that plants of Cistus albidus and C. monspeliensishat experienced water stress and recovery have developed differ-nt avoidance mechanisms, for example, C. albidus limits growthnd cell expansion, while C. monspeliensis reduces photosyntheticrocesses.

Water stress could limit plant vegetative growth, performancend also the survival of shrubs and trees (Fernández et al., 2006),nd consequently, the selection of drought-tolerant plants maye considered a strategy for the improvement of landscape man-gement (Niu et al., 2008). However, information regarding theesponses of some ornamental species in Mediterranean environ-ents to short-term water stress is still lacking. Thus, the aim of

his study was to evaluate differences in the mechanisms that arenvolved in the resistance of ornamental species to water stresss a result of a regular suspension and recovery of the water sup-ly. These different mechanisms were studied in five ornamentalhrubs that are commonly used in Mediterranean landscapes.

. Material and methods

.1. Plant materials, growing conditions and experimentalreatments

The experimental trial was carried out in an unheated green-ouse that was located in Catania, Italy (37◦30′N 15◦06′E 20 m a.s.l.).he five ornamental shrubs [Callistemon citrinus (Curtis) SkeelsCallistemon), Laurus nobilis L. (Laurus), Pittosporum tobira (Thunb.)

Plants were irrigated daily to container capacity or subjected to suspension/rewatering treatment [Start trial (T0), 7 days no water (S1), 7 days daily irrigation(R1), 7 days no water (S2), 14 days daily irrigation (R2)].

W.T. Aiton (Pittosporum), Thunbergia erecta (Benth.) Anderson(Thunbergia) and Viburnum tinus L. ‘Lucidum’ (Viburnum)], fromcommercial nursery, were transplanted into 3.3-L pots (one plantper pot) that were filled with a mixture of sand (75%), silt (18%) andclay (7%). Six-month-old plants were watered daily to pot capacity(determinated by gravimetric method) at regular intervals prior tothe initiation of the treatments using a drip irrigation. The initialbiomasses of the plants were 69.3 g plant−1 (Callistemon), 180.6 gplant−1 (Laurus), 172.6 g plant−1 (Pittosporum), 145.6 g plant−1

(Thunbergia) and 38.4 g plant−1 (Viburnum). After 30 days, 30plants from each species were subjected to two consecutive cyclesof suspension/rewatering (S-R), while another 30 plants from eachspecies were watered daily (C). For every cycle in the S-R treatment,the water suspension lasted for 7 days, after which the plants wererewatered to pot capacity for another 7 days. After the second watersuspension cycle, S-R treatment plants were maintained under thesame conditions as the control plants for 14 days (Fig. 1).

The mean air temperatures, relative humidity levels and globalradiation levels during the experimental periods were recorded ona data logger (CR 1000; Campbell Scientific Ltd., Loughborough, UK).The maximum and minimum temperatures were 22.6 and 18.2 ◦C,respectively, and the mean relative humidity levels ranged from 60to 68%. The total radiation levels ranged from 8.2 to 11.6 MJ m−2.

2.2. Data collection

On days 37 (S1), 44 (R1), 51 (S2) and 64 (R2) (Fig. 1) of the exper-imental period, the midday relative water content (RWC) and themidday leaf water potential (� ) were measured between 12:00 and14:00 (solar time). The RWC were measured on fully opened leaves.Five leaf discs that were 10 mm in diameter were excised from theinterveinal areas of each plant. For each replicate, 30 discs werepooled, and their fresh weights (FW) were determined. They werefloated on distilled water in Petri dishes for 4 h to regain turgid-ity and then the turgid tissue was quickly blotted to remove excesswater and reweighed [turgid weight (TW)]. The samples were driedat 80 ◦C for 24 h to determine the dry weights (DW) (Rouphael et al.,2008). The RWC were calculated according to Jones and Turner(1978):

RWC% = (FW − DW/TW − DW) ∗ 100

The leaf water potential were estimated according to Scholanderet al. (1965) using a pressure chamber (PMS Model 1000, Instru-ment Company, Albany, Oregon, USA). The leaves were removedwith a sharp blade near the petiole bases and immediately tested.

On days 30 (T0), 37 (S1), 44 (R1), 51 (S2) and 64 (R2) (Fig. 1) of theexperimental period, the net photosynthetic rate (A), transpirationrate (E) and stomatal conductance (Gs) were measured on mature,fully expanded leaves using a CO2/H2O IRGA (LCi, ADC BioscientificLtd., Hoddesdon, UK). The measurements were carried out in clearconditions from 10:00 to 13:00 (solar time). All of the photosynthe-sis measurements were performed on outer, fully expanded leaves

that were sampled from branches that were located in the middleof the canopy.

At the end of the experiment, nine pots per treatment (threeper replication) were randomly chosen for the measurement of

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he following parameters: dry weight production and partition-ng, leaf number, leaf characteristics (unit area, specific leaf area,hlorophyll content) and root length. Plants that were located at theorders of blocks were not harvested. The dry weights (DW) werebtained by drying the weighed samples in a thermoventilatedven at 70 ◦C to constant weights. Root lengths were determinedy Newman’s modified line intersect method (Newman, 1966).he chlorophyll content was determined using a SPAD-502 chloro-hyll meter (Minolta Camera Co., Osaka, Japan). As proposed byang et al. (2005), a calibration curve was plotted considering

he relationship between the SPAD values and the chlorophyllontent as extracted according to Moran and Porath (1980).he following equation was used to obtain the chlorophyll con-ent (�g cm−2): chlorophyll (�g cm−2) (Callistemon) = 2.567 SPADndex–123.8 (R2 = 0.765***); chlorophyll (�g cm−2) (Laurus) = 0.958PAD index–13.74 (R2 = 0.761***); chlorophyll (�g cm−2) (Pit-osporum) = 0.929 SPAD index–20.58 (R2 = 0.759***); chlorophyll�g cm−2) (Thunbergia) = 0.787 SPAD index–4.960 (R2 = 0.744***);hlorophyll (�g cm−2) (Viburnum) = 0.932 SPAD index–13.66R2 = 0.844***) (n = 30). The leaf areas were determined using a leafrea meter (Delta-T Devices Ltd, Cambridge, UK). The specific leafrea (SLA) were calculated as the ratio of the leaf area to the leafry weight.

.3. Statistics

The experiment was conducted as a randomised complete blockesign with three replicates (the species were randomized withinlocks); each experimental unit consisted of ten plants. The statis-ical analyses were performed using CoStat version 6.311 (CoHortoftware, Monterey, CA, USA). Data were subjected to a two-waynalysis of variance (ANOVA), to determine the effects of droughttress and species as main effect and their interaction. The meansere compared using the Student–Newman–Keuls test (P ≤ 0.05).

he interactions, when significant, are presented separately in thegures.

Leaf RWC, leaf water potential and leaf gas exchange data wereubjected to one-way variance (ANOVA) at each data (T0, S1, R1,2 and R2) to determine the effects of stressed and control plantor each species. Pair-wise comparisons were done using Student’s-test for means of samples with unequal variances.

. Results

At the end of the experiment, the S-R treatment only alteredotal plant dry weight of Pittosporum (Table 1; Fig. 2). The S-R treat-

ent reduced total dry weight of Pittosporum by ∼66 g (19%). Theseodifications were a consequence of a 27% reduction in the shoot

ry weights (Fig. 2).In all species, S-R treatments increased the root-to-shoot ratio

Table 1). Root/shoot ratio (R/S) differed among treatment from.55 (control plants) to 0.71 (stressed plants). Among the species,

n the mean of the treatments, R/S varied from 0.18 of Viburnumo 0.96 of Laurus. In all species, root length increased among treat-

ent from 71.2 cm g−1 (control plants) to 94.2 cm g−1 (stressedlant). Among the species, in the mean of the treatments, root

ength varied from 52.5 cm g−1 in Pittosporum to 128.2 cm g−1 inhunbergia.

The SLA and leaf chlorophyll content in all species were unaf-ected at the end of the second rewatering cycle (Table 2).

The effect of S-R treatment on leaf numbers and leaf areas dif-

ered among species (Table 2). The S-R treatment had no influencen the number of leaves, total leaf area or leaf size (unit leaf area)n Laurus or Thunbergia (Fig. 3). The S-R treatment decreased theumber of leaves in Callistemon by 32% (Fig. 3) and resulted in a

lturae 178 (2014) 145–153 147

reduction in the leaf area of 49% (Fig. 3). In Pittosporum, S-R treat-ment reduced leaf area by 40%; however, this reduction was notrelated to reduced leaf numbers but rather related to reduced leafsize (unit leaf area) (Fig. 3). Similar to Pittosporum, S-R treatmentalso reduced leaf area (∼22%) and leaf size (∼19%) in Viburnum butnot leaf number (Fig. 3).

The effect of S-R treatment on leaf RWC at different times duringthe experiment varied among species. In Laurus and Thunbergia, S-R treatment decreased leaf RWC by 30 and 27%, respectively, butonly during the first drought event (Fig. 4B and D). In contrast, S-R treatment decreased leaf RWC in all other species, during bothdrought events (Fig. 4A, C and E). During the first drought event(S1), leaf RWC in Callistemon, Pittosporum and Viburnum plantsdecreased by 21, 55 and 9%, respectively, compared with the con-trol. However, during the second drought event, S-R treatment hadless effect on leaf RWC, although they were significant, by 3% inCallistemon, by 16% in Pittosporum and by 4% in Viburnum. At theend of the second rewatering period (R2), S-R treatment had noinfluence on the leaf RWC in any species (Fig. 4).

For all species, the S-R treatment decreased leaf water poten-tial during each drought event compared to controls (Fig. 4). Thelowest values were measured during first drought event (S1). Dur-ing the first drought event, the S-R treatment reduced leaf waterpotential by 44% in Callistemon, 52% in Laurus, 38% in Pittospo-rum, 50% in Thunbergia and 100% in Viburnum. During the seconddrought event, the S-R treatment reduced leaf water potential by8% in Callistemon, 28% in Laurus, 39% in Pittosporum, 14% in Thun-bergia and 13% in Viburnum compared to controls (Fig. 4F–L). Atthe end of the second recovery period (R2), the S-R treatment hadno influence on leaf water potential in any species similar to thetrend observed with the RWC.

The net photosynthesis (A) rates in the control plants were atbeginning of the trial (T0), on average, 6.7 �mol CO2 m−2s−1 (Cal-listemon), 6.0 �mol CO2 m−2s−1 (Laurus), 5.4 �mol CO2 m−2s−1

(Pittosporum), 5.7 �mol CO2 m−2s−1 (Thunbergia) and 6.0 �molCO2 m−2s−1 (Viburnum).

During the first suspension (S1), the S-R treatment reduced A by64% in Callistemon, by 67% in Laurus, by 87% in Pittosporum, by 54%in Thunbergia and by 81% in Viburnum. During the first recovery(R1), Thunbergia and Viburnum maintained the values below ofthe control plants (by 28 and 32%, respectively). During the seconddrought event (S2), the S-R treatment reduced photosynthesis by57% in Callistemon, by 67% in Laurus, by 80% in Pittosporum and by77% in Viburnum (Fig. 5).

Transpiration (E) was lower in S-R-treated plants of all speciesduring drought events (S1 and S2); in Thunbergia instead thedecrease was not significant throughout the experiment. In allspecies, S-R treatment had no influence on E during recoveryperiods (R1 and R2) (Fig. 5).

The first S-R treatment reduced Gs by 54% in Callistemon, 70% inLaurus, 100% in Pittosporum and 83% in Viburnum (Fig. 5). Duringthe second period of S-R, the reduction were similar in Pittosporumand Viburnum (by 100 and 83%), while in Laurus the reduction levelreached 100%. S-R treatment had no influence on Gs in Thunbergiaplants at all times of the experiment.

4. Discussion

The study of stress/recovery responses is instrumental forachieving a better understanding of the mechanisms underlyingthe abilities of plants to adapt to different environments and cli-

matic conditions (Sapeta et al., 2013). Our results indicate thatthe shrubs that were used in this experiment employ variousmechanisms, such as the differential partitioning of dry matterbetween roots and shoots parts, the reduction of the number and

148 S. Toscano et al. / Scientia Horticulturae 178 (2014) 145–153

Table 1Total and shoot dry weights, root/shoot ratio and root length in five species of ornamental shrubs (Callistemon citrinus, Callistemon; Laurus nobilis, Laurus; Pittosporum tobira,Pittosporum; Thunbergia erecta, Thunbergia; Viburnum tinus ‘Lucidum’, Viburnum) grown in containers.

Species Treatment Total dry weight (g plant−1) Shoot dry weight (g plant−1) Root/shoot ratio (g g−1) Root length (cm g−1)

Callistemon 118.6c 62.7d 0.88a 68.3c

Laurus 282.5a 144.3b 0.96a 79.7b

Pittosporum 298.7a 212.7a 0.40c 52.5d

Thunbergia 196.1b 116.3c 0.66b 128.2a

Viburnum 76.4d 64.9d 0.18d 84.8b

C 202.9a 131.6a 0.55b 71.2b

S-R 185.8b 108.7b 0.71a 94.2a

Significancea

Treatment (T) * *** ** ***Species (S) *** *** *** ***T × Sb * *** n.s. n.s.

Plants were irrigated daily to container capacity (C) or subjected to suspension/rewatering treatment (S-R treatment: 7 days no water, 7 days irrigation, 7 days no water, 14days irrigation). Values are means for main effects of species (S) and irrigation treatment (T).

a n.s. = not significant; *,**,*** represent significance of main effects and interactions at 0.01 < P < 0.05, 0.001 < P < 0.01 and P < 0.001, respectively, from ANOVA. The valuesin the same column followed by the same letter are not significantly different at P≤0.05 (Student–Newman–Keuls).

b The data concerning significant interactions are presented separately.

Fig. 2. Effects of water treatments on the total dry weights (g plant−1) and shoot dry weights (g plant−1) in five species of ornamental shrubs (Callistemon citrinus, Callistemon;Laurus nobilis, Laurus; Pittosporum tobira, Pittosporum; Thunbergia erecta, Thunbergia; Viburnum tinus ‘Lucidum’, Viburnum) grown in containers. Plant were irrigated dailyto container capacity (C) or subjected to suspension/rewatering treatment (7 days no water, 7 days irrigation, 7 days no water, 14 days irrigation). Columns denoted with thesame letters are not significantly different, as determined by Student–Newman–Keuls (P≤0.05 test).

Table 2Leaf number, total plant leaf area, unit leaf area, specific leaf area (SLA) and chlorophyll content in five species of ornamental shrubs (Callistemon citrinus, Callistemon; Laurusnobilis, Laurus; Pittosporum tobira, Pittosporum; Thunbergia erecta, Thunbergia; Viburnum tinus ‘Lucidum’, Viburnum) grown in containers.

Species Treatment Leaf(n.plant−1)

Totalleaf area(cm2 plant−1)

Unitleaf area(cm2plant−1)

SLA(cm2 g−1)

Chlorophyllcontent(�g cm−2)

Callistemon 1370.7a 3626.7d 2.7e 87.1c 49.4a

Laurus 541.9c 6218.1b 11.9b 81.6c 31.9c

Pittosporum 1155.8b 10827.9a 9.2c 87.0c 21.0d

Thunbergia 1311.6a 6919.0b 5.4d 168.5a 42.9b

Viburnum 127.2d 4751.5c 39.4a 117.7b 37.5c

C 999.1a 7416.9a 14.7a 106.8 36.6S-R 803.8b 5520.3b 12.7b 110.0 37.5

Significancea

Treatment (T) *** *** *** n.s n.s.Species (S) *** *** *** *** ***T × Sb ** *** *** n.s n.s

Plants were irrigated daily to container capacity (C) or subjected to suspension/rewatering treatment (S-R treatment: 7 days no water, 7 days irrigation, 7 days no water, 14days irrigation). Values are means for main effects of species (S) and irrigation treatment (T).

a ons ati 0.05 (

stad�

n.s. = not significant; *,**,*** represent significance of main effects and interactin the same column followed by the same letter are not significantly different at P≤

b The data concerning significant interactions are presented separately.

ize of leaves and leaf area, stomatal closure, declines in Gs and A,

hat allow them to tolerate repeated cycles of drought. Our resultsre similar to those by Zollinger et al. (2006), who reported thatifferent mechanism of drought avoidance (declines in Gs, Pn ands) were found in six ornamental herbaceous perennials. Despite

0.01 < P < 0.05, 0.001 < P < 0.01 and P < 0.001, respectively, from ANOVA. The valuesStudent–Newman–Keuls).

the short duration of our experiment, the effects of the alterations in

irrigation appear to involve both morphological and physiologicalparameters.

Growth and photosynthesis are the primary processes that areaffected by drought stress (Chaves and Oliveira, 2004). The effects of

S. Toscano et al. / Scientia Horticulturae 178 (2014) 145–153 149

Fig. 3. Interactive effects of water treatments and species on the leaf numbers (n◦ plant−1), the total leaf areas (cm2 plant−1) and unit leaf areas (cm2 plant−1) in five speciesof ornamental shrubs (Callistemon citrinus, Callistemon; Laurus nobilis, Laurus; Pittosporum tobira, Pittosporum; Thunbergia erecta, Thunbergia; Viburnum tinus ‘Lucidum’,V ) or sud ignific

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iburnum) grown in containers. Plant were irrigated daily to container capacity (Cays no water, 14 days irrigation). Columns denoted with the same letters are not s

he suspension/rewatering treatment led to differences in shrubs,ncluding decreased shoot dry matter in S-R treatment plants. Theegative impact of suspension in irrigation strongly affected Callis-emon and Pittosporum but not Laurus, Viburnum or Thunbergia.hese results support previous studies that indicated that reducediomass production is a result of lower water availability (Shaot al., 2008; Mugnai et al., 2009); in our study, Callistemon and Pit-osporum were less tolerant to water stress conditions comparedo the other ornamental shrubs studied.

Root-to-shoot ratio increased in plants suffering from waterhortages. This response is attributable to the development ofigger roots systems in relation to shoot biomass in accordanceith previous reports, which suggested that the shoot growth is

nhibited relatively more than root growth as a consequence ofeducing water loss (Monneveux and Belhassen, 1996). Increasedoot-to-shoot ratios have been frequently observed in plants underrought conditions (Blum, 1996; Zwack and Graves, 1998) toeduce water consumption (Wu et al., 2008) and increase waterbsorption (Nicholas, 1998). This result was confirmed by increasedoot lengths allowed the plants to improve their absorbent sur-aces. Through this mechanism of avoidance, the plants adapt tohe stress without changing their shoot biomasses. In other species,uch as Callistemon and Pittosporum, this ratio also increased as aesult of reductions in the shoot dry weight. Additionally, Nayyarnd Gupta (2006) showed that water deficits affect the shoot parts,articularly the photosynthetic apparatus.

Water deficit not only decreased shoot dry weights but alsoecreased leaf area. These reductions may be due either to reduced

eaf numbers or leaf size. According to Mugnai et al. (2009), deficitrrigation reduces the leaf area in Callistemon citrinus. This was alsoonfirmed by Álvarez and Sánchez-Blanco (2013) in Callistemon cit-inus, in which the reduced leaf areas were attributed to decreasedeaf numbers.

Drought stress reduced leaf area in Pittosporum and Viburnumimilar to the effects of drought in Cistus monspeliensis and C. albidusSanchez-Blanco et al., 2002). In our study, Pittosporum and Vibur-um, which were subjected to drought-recovery cycles, reduced

eaf area at the end of the trial. This response is attributed to anvoidance mechanism that allowed the minimisation of water losshrough stomatal closure in addition to the reduction of carbonssimilation in whole plants and consequently plant growth (Banont al., 2006).

Chlorophyll is one of the main chloroplastic components thatas employed in sustaining photosynthesis, and the relative

hlorophyll concentration is positively correlated with the pho-osynthesis rate (Guo and Li, 1996). Flexas and Medrano (2002)eported that water stress reduces green leaf colour in C3 plantsue to chlorophyll degradation. However, our study indicated that

bjected to suspension/rewatering treatment (7 days no water, 7 days irrigation, 7antly different, as determined by Student–Newman–Keuls (P≤0.05 test).

water deficits did not significantly affect the relative chlorophyllconcentrations in the leaves. Lack of detectable change in chloro-phyll concentrations may have been due to the relatively shortduration of the experiment. Other authors have demonstrated thatthe leaf chlorophyll concentrations of Carrizo citrange plants werenot affected by relatively short-term salinity or drought-stresstreatments (Pérez-Pérez et al., 2007).

In our study, plants were able to survive the water shortagesmainly due to altered physiological mechanisms. In fact, the adap-tation of plants to water stress most commonly involves stomatalclosure, reduced photosynthesis rates and the adjustment of thewater potential (Ludlow, 1980).

Many studies have shown that when plants are subjected todrought, leaves exhibit large reductions in their relative watercontent and water potential (Kyparissis et al., 1995; Scarascia-Mugnozza et al., 1996). This dehydration is often reversible (Efeogluet al., 2009). In fact, in our study, the RWC values were significantlyreduced under stress conditions but noticeably increased duringrecovery, reaching values that were similar to those of the con-trol plants. It has been suggested that increases in stomatal andosmotic sensitivities following an initial drought episode may helpplants better tolerate repeated episodes of drought (Williams et al.,2000). This was confirmed by our results, in which we revealed use-ful knowledge regarding the mechanisms by which select speciesare able to better tolerate repeated episodes of drought. In fact,although the RWC was significantly reduced in the first suspension,in correspondence of the second suspension, the differences amongstressed plants and control were small and sometimes not signifi-cant. The reductions measured in the second suspension were onlymore pronounced for Pittosporum and Viburnum. However, at theend of the stress period, all of the species fully recovered the leafwater status with RWC values similar to control plants.

The midday leaf water potential were markedly differentbetween the control and stressed plants, although the latter recov-ered leaf water potential values similar to control plants at the endof the recovery period. Water stress has been shown to influencethe sensitivity of photosynthetic apparatus photoinhibition (Ferrarand Osmond, 1986; Osmond, 1994; Osmond and Chow, 1988). Inaddition to affecting stomatal closure, drought stress reduced gasexchange in the plants by limiting the transpiration and photosyn-thetic rates. A previous study showed that the reduction of waterstress from −1.0 to −2.0 MPa results in smaller cells and less devel-oped leaves; thus, the photosynthetic areas are reduced (Medranoet al., 2002). This occurred in Callistemon, which exhibited

reduced leaf numbers, and Viburnum, which showed decreased leafsize.

Lenzi et al. (2009) demonstrated that the net photosynthesis ofdrought stressed oleanders recovered levels equivalent to control

150 S. Toscano et al. / Scientia Horticulturae 178 (2014) 145–153

Fig. 4. Relative water content (RWC, %) and water potential (�, MPa) of leaves in five species of ornamental shrubs (Callistemon citrinus, Callistemon; Laurus nobilis, Laurus;Pittosporum tobira, Pittosporum; Thunbergia erecta, Thunbergia; and Viburnum tinus ‘Lucidum’, Viburnum) grown in containers. Plant were irrigated daily to container capacityfor the species in the control (�) or subjected to suspension/rewatering treatment (�) (7 days no water, 7 days irrigation, 7 days no water, 14 days irrigation). Their absenceindicates that the size was less than the symbol. The vertical bars indicate the S.E. of the means. The asterisks depict statistically significant differences between control andstressed plants at each date (S1, R2, S2 and R2) for each species. Significance values were obtained from a t-test for means of samples with unequal variances.

S. Toscano et al. / Scientia Horticulturae 178 (2014) 145–153 151

Fig. 5. Net photosynthesis (A), transpiration (E) rates and leaf conductance (Gs) in five species of ornamental shrubs (Callistemon citrinus, Callistemon; Laurus nobilis, Laurus;Pittosporum tobira, Pittosporum; Thunbergia erecta, Thunbergia; Viburnum tinus ‘Lucidum’, Viburnum) grown in containers. Plants were irrigated daily to container capacityfor the species in the control (�) or subjected to suspension/rewatering treatment (�) [7 days no water (S1), 7 days irrigation (R1), 7 days no water (S2), 14 days irrigation(R2). The vertical bars represent the S.E. of the means. Their absence indicates that the size was less than the symbol. The asterisks depict statistically significant differencesbetween control and stressed plants at each date (T0, S1, R2, S2 and R2) for each species. Significance values were obtained from a t-test for means of samples with unequalvariances.

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lants at the end of drought stress. Further, the recovery of the pho-osynthetic rates following water stress depends on the intensityf the stress (Sapeta et al., 2013). The recovery of photosynthe-is may be fast and complete following moderate stress, but theecovery after severe stress is progressive and slow and sometimesncomplete (De Souza et al., 2004; Flexas et al., 2006; Miyashitat al., 2005). Following rehydration, when the stomata reopened,he photosynthesis rates in the stressed plants increased to thosef the control plants for all of the species in our study. This sug-ests that both the dark and light reactions of photosynthesis wereot damaged during the stress period (Lenzi et al., 2009). It shoulde noted that the transpiration rates in all of the species follow-

ng the recovery period were similar to those of the control plants. rapid recovery after stress condition can be correlated with areater physiological tolerance to drought and this proves that thelants had activated improved adaptive mechanisms of avoidancehich allows the plant to regain full turgor more efficiently. This is

n line with results that were obtained by Momen et al. (1992) inelation to Quercus wislizenii plants.

These results have allowed us to obtain useful informationegarding the responses of some shrubs to suspension/rewateringycles. All of the species that were examined were able to employoth morphological and physiological mechanisms to adapt toater stress and recovery when the stress was over. Drought stress

educed allocation to aboveground structures (dry weight, leafrea and leaf number), and increased root growth (dry weightsnd root lengths), allowed the plants to survive the suspensioneriod. The effects of drought on Callistemon, Pittosporum andiburnum almost exclusively involved aboveground growth. Theaurus and Thunbergia plants appear to be more tolerant of droughtompared to the other species, increased root development as andaptive mechanism. Measurement of physiological parametersllow further separation amongst species in their drought. Dur-ng the suspension period, E, A, Gs and � values decreased inll species. However, Laurus and Thunbergia showed similar RWCalues between the stressed plants and control plants during theecond suspension period. Both of these species also had a moreighly developed root systems. The ability of Thunbergia plants toolerate periods of drought stress was also demonstrated by therend in transpiration, which increased during the first droughtvent without being influenced by the second drought event.

cknowledgments

The research Responses of Mediterranean ornamental shrubs torought stress and recovery was supported by the research projectRIN 2009. Molecular, physiological and morphological aspects ofrnamentals response to sub-optimal water resources and ionictress founded by the Italian Ministry of University and Researchnd marked by grant number 2009BW3KL4 002.

eferences

lvarez, S., Sánchez-Blanco, M.J., 2013. Changes in growth rate, root morphology andwater use efficiency of potted Callistemon citrinus plants in response to differentlevels of water deficit. Sci. Hort. 156, 54–62.

anon, S., Ochoa, J., Franco, J.A., Alarcón, J.J., Sánchez-Blanco, M.J., 2006. Hardeningof oleander seedlings by deficit irrigation and low air humidity. Environ. Exp.Bot. 56 (1), 36–43.

argali, K., Tewari, A., 2004. Growth and water relation parameters in drought-stressed Coriaria nepalensis seedlings. J. Arid Environ. 58, 505–512.

lum, A., 1996. Crop responses to drought and the interpretation of adaptation. PlantGrowth Regul. 20, 135–148.

radford, K.J., Hsiao, T.C., 1982. Physiological responses to moderate water stress,

in: Lange, O.L., Nobel, P.S., Osmond, C.B., Ziegler, H. (Eds.), Physiological PlantEcology. II. Water Relations and Carbon Assimilation. Encyclopedia of PlantPhysiology. New Series Vol. 12B, Springer-Verlag, Berlin, pp. 264–324.

astro-Díez, P., Villar-Salvador, P., Pérez-Rontomé, C., Maestro-Martínez, M.,Montserrat-Martí, G., 1998. Leaf morphology, leaf chemical composition and

lturae 178 (2014) 145–153

stem xylem characteristics in two Pistacia (Anacardiaceae) species along a cli-matic gradient. Flora 193, 195–202.

Chartzoulakis, K., Patakas, A., Bosabalidis, A.M., 1999. Changes in water relations,photosynthesis and leaf anatomy induced by intermittent drought in two olivecultivars. Environ. Exp. Bot. 42, 113–120.

Chaves, M.M., Oliveira, M.M., 2004. Mechanisms underlying plant resilienceto water deficits: prospects for water-saving agriculture. J. Exp. Bot. 55,2365–2384.

Craufurd, P.Q., Wheeler, T.R., Ellis, R.H., Summerfiled, R.J., Williams, J.H., 1999. Effectof temperature and water deficit on water-use efficiency, carbon isotope dis-crimination, and specific leaf area in peanut. Crop Sci. 39, 136–142.

De Souza, R.P., Machado, E.C., Silva, J.A.B., Lagôa, A.M.M.A., Silveira, J.A.G., 2004.Photosynthetic gas exchange, chlorophyll fluorescence and some associatedmetabolic changes in cowpea (Vigna unguiculata) during water stress and recov-ery. Environ. Exp. Bot. 51, 45–56.

Di Castri, F., 1981. Mediterranean-type shrublands of the world, in: Di Castri, F.,Goodall, D.W., Specht, R.L. (Eds.), Mediterranean-Type Shrublands. Elsevier,Amsterdam, pp. 1–52.

Dickson, R.E., Tomlinson, P.T., 1996. Oak growth, development and carbonmetabolism in response to water stress. Ann. Sci. Forest. 53, 181–196.

Efeoglu, B., Ekmekc i, Y., C ic ek, N., 2009. Physiological responses of three maize cul-tivars to drought stress and recovery. South Afr. J. Bot. 75, 34–42.

Fernández, J.A., Balenzategui, L., Banón, S., Franco, J.A., 2006. Induction of droughttolerance by paclobutrazol and irrigation deficit in Phyllirea angustifolia duringthe nursery period. Sci. Hort. 107, 277–283.

Ferrar, P.J., Osmond, C.B., 1986. Nitrogen supply as a factor influencing photoinhi-bition and photosynthetic acclimation after transfer of shade-grown Solanumdulcamara to bright light. Planta 168, 563–570.

Flexas, J., Medrano, H., 2002. Drought-inhibition of photosynthesis in C3 plants:stomatal and non-stomatal limitations revisited. Ann. Bot. 89, 183–189.

Flexas, J., Bota, J., Galmés, J., Medrano, H., Ribas-Carbó, M., 2006. Keeping a posi-tive carbon balance under adverse conditions: responses of photosynthesis andrespiration to water stress. Physiol. Plant. 127, 343–352.

Franco, J.A., Martínez-Sánchez, J.J., Fernández, J.A., Banón, S., 2006. Selection andnursery production of ornamental plants for landscaping and xerogardening insemi-arid environments. J. Hort. Sci. Biotechnol. 81 (1), 3–17.

Galmés, J., Flexas, J., Savé, R., Medrano, H., 2007. Water relations and stomatal char-acteristics of Mediterranean plants with different growth form and leaf habits:responses to water stress and recovery. Plant Soil 290, 139–155.

Gratani, L., Bombelli, A., 2000. Correlation between leaf age and other leaf traits inthree Mediterranean maquis shrub species: Quercus ilex, Phillyrea latifolia andCistus incanus. Environ. Exp. Bot. 43, 141–153.

Guo, P., Li, M., 1996. Studies on photosynthetic characteristics in rice hybrid proge-nies and their parents. I. Chlorophyll content, chlorophyll-protein complex andchlorophyll fluorescence kinetics. J. Trop. Subtrop. Bot. 4, 60–65.

Guo, S.W., Chen, G., Zhou, Y., Shen, Q.R., 2007. Ammonium nutrition increases pho-tosynthesis rate under water stress at early development stage of rice (Oryzasativa L.). Plant Soil 296, 115–124.

Hsiao, T.C., 1973. Plant responses to water stress. Ann. Rev. Plant Physiol. 24,519–570.

Hsiao, T.C., Acevedo, E., 1974. Plant responses to water deficits, water use efficiency,and drought resistance. Agr. For. Metereol. 14, 59–84.

Huang, B., 2004. Recent advances in drought and heat stress physiology of turfgrass:a review. Acta Hort. 661, 185–192.

IPCC, 2007. Summary for policymakers of climate change 2007: the physical sciencebasis. Contribution of Working Group I to the Fourth Assessment Report of theIntergovernmental Panel on Climate Change. Cambridge, Cambridge UniversityPress.

Jones, H.G., 1985. Partitioning stomatal and non-stomatal limitations to photosyn-thesis. Plant Cell Environ. 8, 95–104.

Jones, H.G., Turner, N.C., 1978. Osmotic adjustment in leaves of sorghum in responseto water deficits. Plant Physiol. 61, 122–126.

Karabourniotis, G., 1998. Light-guiding function of foliar sclereids in the evergreensclerophyll Phillyrea latifolia: a quantitative approach. J. Exp. Bot. 49, 739–746.

Kyparissis, A., Petropoulou, Y., Manetas, Y., 1995. Summer survival of leaves in asoft-leaved shrub (Phlomis fruticosa L., Labiates) under Mediterranean field con-ditions: avoidance of photoinhibitory damage through decreased chlorophyllcontents. J. Exp. Bot. 46, 1825–1831.

Lenzi, A., Pittas, L., Martinelli, T., Lombardi, P., Tesi, R., 2009. Response to waterstress of some oleander cultivars suitable for pot plant production. Sci. Hortic.122, 426–431.

Li, W.X., Oono, Y., Zhu, J., He, X.J., Wu, J.M., Iida, K., Lu, X.Y., Cui, X., Jin, H., Zhu,J.K., 2008. The Arabidopsis NFYA5 transcription factor is regulated transcrip-tionally and posttranscriptionally to promote drought resistance. Plant Cell 20,2238–2251.

Liu, F., Stützel, H., 2004. Biomass partitioning, specific leaf area, and water use effi-ciency of vegetable amaranth (Amaranthus spp.) in response to drought stress.Sci. Hort. 102, 15–27.

Ludlow, M.M., 1980. Adaptive significance of stomatal responses to water stress,in: Turner, N.C., Kramer, P.-J. (Eds.) Adaptation of Plants to Water and HighTemperature Stress. Wiley, New York, pp. 123–128.

Malinowski, D.P., Belesky, D., 2000. Adaptations of endophyte-infected cool-seasongrasses to environmental stresses: mechanisms of drought and mineral stresstolerance. Crop Sci. 40, 923–940.

Marcelis, L.F.M., Heuvelink, E., Goudriaan, J., 1998. Modelling biomass productionand yield of horticultural crops: a review. Sci. Hort. 74, 83–111.

orticu

M

M

M

M

M

M

M

N

N

N

N

O

O

P

P

R

R

R

R

S. Toscano et al. / Scientia H

edrano, H., Escalona, J.M., Bota, J., Gulías, J., Flexas, J., 2002. Regulation of photo-synthesis of C3 plants in response to progressive drought: stomatal conductanceas a reference parameter. Ann. Bot. 89 (SPEC. ISS.), 895–905.

iyashita, K., Tanakamaru, S., Maitani, T., Kimura, K., 2005. Recovery responses ofphotosynthesis, transpiration, and stomatal conductance in kidney bean follow-ing drought stress. Environ. Exp. Bot. 53, 205–214.

omen, B., Menke, J.W., Welker, J.M., 1992. Tissue water relations Quercus wis-lizenii seedlings: drought resistance in a California evergreen oak. Acta Oecol.13, 127–136.

onneveux, P., Belhassen, E., 1996. The diversity of drought adaptation in wide.Plant Growth Regul. 20, 85–92.

oran, R., Porath, D., 1980. Chlorophyll determination in intact tissues using N,N-dimethylformamide. Plant Physiol. 65, 478–479.

oriana, A., Villalobos, F.J., Fereres, E., 2002. Stomatal and photosynthetic responsesof olive (Olea europaea L.) leaves to water deficits. Plant Cell Environ. 25,395–405.

ugnai, S., Ferrante, A., Petrognani, L., Serra, G., Vernieri, P., 2009. Stress-inducedvariation in leaf gas exchange and chlorophyll a fluorescence in Callistemonplants. Res. J. Biol. Sci. 4 (8), 913–921.

ayyar, H., Gupta, D., 2006. Differential sensitivity of C3 and C4 plants to water deficitstress: association with oxidative stress and antioxidants. Environ. Exp. Bot. 58,106–113.

ewman, E.I., 1966. A method of estimating the total length of roots in a sample. J.Appl. Ecol. 3, 139–145.

icholas, S., 1998. Plant resistance to environmental stress. Curr. Opin. Biotechnol.9, 214–219.

iu, G., Rodrigues, D.S., Mackay, W., 2008. Growth and physiological response todrought stress in four oleander clones. J. Am. Soc. Hort. Sci. 133, 188–196.

smond, C.B, 1994. What is photoinhibition: some insights from comparisons ofshade and sun plants, in: Baker, N.R., Bowyer, J.R. (Eds.), Photoinhihition of Pho-tosynthesis: From Molecular Mechanisms to the Field. BIOS Scientific PublishersLtd., Oxford, pp. 1–24.

smond, C.B., Chow, W.S., 1988. Ecology of photosynthesis in the sun and shade:summary and prognostications. Aust. J. Plant Physiol. 15, 1–9.

ereira, J.S., Chaves, M.M., 1993. Plant water deficits in Mediterranean ecosys-tems, in: Smith, J.A.C., Griffiths, H. (Eds.), Plant Responses to WaterDeficits—From Cell to Community. BIOS Scientific Publishers Ltd., Oxford,pp. 237–251.

érez-Pérez, J.G., Syvertsen, J.P., Botía, P., García-Sánchez, F., 2007. Leaf water rela-tions and net gas exchange responses of salinized Carrizo citrange seedlingsduring drought stress and recovery. Ann. Bot. 100 (2), 335–345.

odríguez, P., Torrecillas, A., Morales, M.A., Ortuno, M.F., Sánchez-Blanco, M.J., 2005.Effects of NaCl salinity and water stress on growth and leaf water relations ofAsteriscus maritimus plants. Environ. Exp. Bot. 53, 113–123.

omano, D., 2004. Strategie per migliorare la compatibilità del verde orna-mentale con l’ambiente mediterraneo, in: Pirani, A. (Ed.), Il verde incittà. La progettazione del verde negli spazi urbani. Edagricole, Bologna,pp. 363–404.

ouphael, Y., Cardarelli, M., Colla, G., 2008. Yield mineral composition, water rela-tions, and water use efficiency of grafted mini-watermelon plants under deficitirrigation. Hort. Sci. 43 (3), 730–736.

ucker, K.S., Kvien, C.K., Holbrook, C.C., Hook, J.E., 1995. Identification of peanutgenotypes with improved drought avoidance traits. Peanut Sci. 22, 14–18.

lturae 178 (2014) 145–153 153

Ruiz-Sànchez, M.C., Domingo, R., Savè, R., Biel, C., Torrecillas, A., 1997. Effects ofwater stress and rewatering on leaf water relations of lemon plants. Biol. Plant.39 (4), 623–631.

Sánchez-Blanco, M.J., Rodríguez, P., Morales, M.A., Ortuno, M.F., Torrecillas, A.,2002. Comparative growth and water relation of Cistus albidus and Cistusmonspeliensis plants during water deficit conditions and recovery. Plant Sci. 162,107–113.

Sapeta, H., Costa, J.M., Louren, T., Maroco, J., van der Lindee, P., Oliveira, M.M., 2013.Drought stress response in Jatropha curcas: growth and physiology. Environ. Exp.Bot. 85, 76–84.

Scarascia-Mugnozza, G., de Angelis, P., Matteucci, G., Valentini, R., 1996. Long-termexposure to elevated (CO2) in a natural Quercus ilex L. community: net photo-synthesis and photochemical efficiency of PSII at different levels of water stress.Plant Cell Environ. 19, 643–654.

Scholander, P.F., Bradstreet, E.D., Hemmingsen, E.A., Hammel, H.T., 1965. Sap pres-sure in vascular plants. Science 148, 339–346.

Schulze, E.D., Hall, A.E., 1982. Stomatal responses, water loss and CO2 assimilationrates of plants in contrasting environments, in: Lange, O.L., Nobel, P.S., Osmond,C.B., Ziegler, H. (Eds.), Physiological Plant Ecology. II. Water Relations and CarbonAssimilation. Encyclopedia of Plant Physiology New Series 12B. Springer, Berlin,pp. 181–230.

Shao, H.B., Chu, L.Y., Jaleel, C.A., Zhao, C.X., 2008. Water-deficit-stress-inducedanatomical changes in higher plants. Compt. Rend. Biol. 331, 215–225.

Shubhra, V., Dayal, J., Goswami, C.L., 2003. Effect of phosphorus application ongrowth, chlorophyll and proline under water deficit in clusterbean (Cyamopsistetragonoloba L. Taub). Indian J. Plant Phys. 8, 150–154.

Torrecillas, A., Rodriguez, P., Sanchez-Blanco, M.J., 2003. Comparison of growth,leaf water relations and gas exchange of Cistus albidus and C. monspelien-sis plants irrigated with water of different NaCl salinity levels. Sci. Hort. 97,353–368.

Yang, X., Chen, X., Ge, Q., Li, B., Tong, Y., Zhang, A., Li, Z., Kuang, T., Lu, C., 2006.Tolerance of photosynthesis to photoinhibition, high temperature and droughtstress in flag leaves of wheat a comparison between a hybridization line and itsparents grown under field conditions. Plant Sci. 171, 389–397.

Wang, Q., Chen, J., Stamps, R.H., Li, Y., 2005. Correlation of visual quality grading andSPAD reading of green-leaved foliage plants. J. Plant Nutr. 28, 1215–1225.

Wang, Y., Zhou, G., Wang, Y., 2007. Modelling responses of the meadow steppedominated by Leymus chinensis to climate change. Clim. Change 82, 437–452.

Williams, M., Rosenqvist, E., Buchhave, M., 2000. The effect of reducing productionwater availability on the post-production quality of potted miniature roses (Rosa× hybrida). Postharvest Biol. Technol. 18, 143–150.

Wright, G.C., Nageswara, R.C., Farquhar, G.D., 1994. Water-use efficiency and carbonisotope discrimination in peanut under water deficit conditions. Crop Sci. 34,92–97.

Wu, F., Bao, W., Li, F., Wu, N., 2008. Effects of drought stress and N supply onthe growth, biomass partitioning and water-use efficiency of Sophora davidiiseedlings. Environ. Exp. Bot. 63, 248–255.

Zollinger, N., Kjelgren, R., Cerny-Koenig, T., Kopp, K., Koenig, R., 2006.

Drought responses of six ornamental herbaceous perennials. Sci. Hort. 109,267–274.

Zwack, J.A., Graves, W.R., 1998. Leaf water relations and plant development ofthree freeman maple cultivars subjected to drought. J. Am. Soc. Hort. Sci. 123,371–375.