sclerophylly and leaf anatomical traits of five field-grown olive cultivars growing under drought...

Summary Leaf-level morphological and structural adapta-tions to reduce water loss were examined in five olive (Oleaeuropaea L.) tree cultivars (Arbequina, Blanqueta, Cobran-çosa, Manzanilla and Negrinha) growing under field condi-tions with low water availability. Leaf measurements includedleaf tissue thickness, stomatal density, leaf area, leaf mass perunit area, density of leaf tissue, relative water content, succu-lence, water saturation deficit, water content at saturation andcuticular transpiration rate. We found considerable genotypicdifferences among the cultivars. Negrinha, Manzanilla andCobrançosa had more morphological and structural leaf adap-tations to protect against water loss than the other cultivars.Manzanilla and Negrinha enhanced their sclerophylly bybuilding parenchyma tissues and increasing protective struc-tures like the upper cuticle and both the upper and lower epider-mis. Cobrançosa exhibited good protection against water lossthrough high density of foliar tissue and by thick cuticle andtrichome layers. Compared with the Negrinha, Manzanilla andCobrançosa cultivars, Arbequina leaves had a thinner trichomelayer, implying that the leaves were less protected against wa-ter loss; however, the development of smaller leaves may re-duce water loss at the whole-plant level. Among cultivars,Blanqueta had the largest leaves and some anatomical traitsthat may lead to high water loss, especially from the adaxialsurface. The mechanisms employed by the cultivars to copewith summer stress are discussed at the morpho-structurallevel.

Keywords: cuticle thickness, leaf morphology, LMA, Oleaeuropaea, stem water potential, stomatal density, succulence,summer stress, trichomes.

Introduction

During the summer, olive (Olea europaea L.), like other Medi-terranean xerophytes, is usually subjected to high solar irra-diances, high air temperatures, high vapor pressure deficitsand limited water availability. The severity of these stresses is

predicted to increase in the future as a result of global environ-mental change (Fischer et al. 2001, Centritto 2002).

Plant responses to water scarcity are complex, involvingadaptive changes or deleterious effects (Chaves et al. 2002), orboth. Several studies (e.g., Castro-Díez et al. 1999, Bussoti etal. 2002) have shown that some species react to adverse envi-ronmental field conditions with an increase in sclerophylly.Because the leaf is the most flexible organ in its response to en-vironmental conditions (Nevo et al. 2000), its structure re-flects, more clearly than that of the stem and roots, the effectsof severe summer stress. Leaves of xeromorphic plants arecharacterized by a low surface/volume ratio (Karabourniotisand Bornman 1999, Richardson and Berlyn 2002) as a result ofchanges in cell number and cellular dimensions (Chartzou-lakis et al. 2000), and by a greater density of both the vascularsystem and stomata (Bolhar-Nordenkampf 1987). Other mor-pho-anatomical traits that help to minimize water loss duringdrought include leaf rolling (Schwabe and Lionakis 1996),dense leaf pubescence (Palliotti et al. 1994, Karabourniotisand Bornman 1999, Liakoura et al. 1999), a thick cuticle andepicuticular wax layer (Leon and Bukovac 1978, Liakoura etal. 1999, Richardson and Berlyn 2002), heavily lignified tissue(Richardson and Berlyn 2002), smaller mesophyll cells andless intercellular space (Bongi et al. 1987, Mediavilla et al.2001). Such traits are frequently accompanied by the accumu-lation of mucilage and other secondary metabolites (Margaris1981).

Olive leaves are well adapted to avoid excessive water lossduring the severe summer drought that characterizes the Medi-terranean region where this species grows (Fernández andMoreno 1999). These adaptations to drought imply a meta-bolic cost because reserves are diverted from growth (Bussotiet al. 2002). Mechanisms like the reduction in leaf area almostcertainly lead to a significant reduction in productivity (Turnerand Jones 1980). Because of this trade-off, differences havebeen observed among olive cultivars in their ability to adapt todrought conditions (Chartzoulakis et al. 1999).

Olive has traditionally been grown in Trás-os-Montes

Tree Physiology 24, 233–239© 2004 Heron Publishing—Victoria, Canada

Sclerophylly and leaf anatomical traits of five field-grown olivecultivars growing under drought conditions

EUNICE A. BACELAR,1,2 CARLOS M. CORREIA,1 JOSÉ M. MOUTINHO-PEREIRA,1 BERTAC. GONÇALVES,1 JOÃO I. LOPES3 and JOSÉ M. G. TORRES-PEREIRA1

1 Department of Biological and Environmental Engineering/CETAV, University of Trás-os-Montes e Alto Douro, Apartado 1013, 5001-911 Vila Real,Portugal

2 Author to whom correspondence should be addressed ([email protected])3 Direcção Regional de Agricultura de Trás-os-Montes, Quinta do Valongo, 5370 Mirandela, Portugal

Received April 30, 2003; accepted July 20, 2003; published online December 15, 2003

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(northeast Portugal) where it is of considerable economic andsocial importance. The region produces almost 25% of Portu-guese olive oil. Its excellent quality and unique characteristicsare responsible for the protected origin denomination of oliveoil from Trás-os-Montes. Although the cultivars most fre-quently grown in the region are considered to be well adaptedto drought, there are no studies documenting the drought adap-tations of these cultivars. We tested the hypothesis that adapta-tion to drought is effected by an increase in sclerophylly.Specifically, we compared, under field conditions, the olivecultivars most commonly grown in the region with foreigncultivars to identify morphological and structural adaptationsthat reduce water loss, and specific leaf-level mechanisms thatenable olive trees to cope with summer stress.

Materials and methods

Study site

The experiment was conducted in a shallow schistic soil atMirandela in northeast Portugal (41°31 N and 7°12 W) at250 m above sea level. The climate at this xeric study site istypically Mediterranean with high temperatures and severedrought during the summer (Figure 1) (INMG 1991). Meanannual rainfall is 520.1 mm, most of which falls in the winterwith negligible rainfall during the summer months, althoughperiods of drought can occur during winter. The warmestmonths are July/August and the coldest months are Decem-

ber/January, with mean daily temperatures of 23.6/22.9 and6.3/6.1 °C, respectively. During the study year (2001), temper-ature and rainfall differed from the mean values (Figure 1).Rainfall was extremely high during the winter months (reach-ing 284 mm in March) and rare during the summer months.Two months before the measurements, rainfall was almostzero, ensuring the development of leaves under drought condi-tions.

Plant material, leaf anatomy study and tissue measurements

We studied five cultivars of field-grown, unirrigated, 10-year-old, own-rooted olive trees. Two cultivars, Cobrançosa andNegrinha, originated in Trás-os-Montes (Portugal). Cobran-çosa is a regional cultivar with a wide distribution throughoutTrás-os-Montes, whereas Negrinha (denomination of origin)is a local cultivar from Freixo de Espada à Cinta. Bothcultivars have high economic significance in the region(Gomes et al. 1998). The other cultivars are native to differentregions of Spain. Arbequina is the major cultivar in Cataluña(Northeast), Blanqueta is of great importance in Valencia andAlicante (Southeast), and Manzanilla is important in Extre-madura (Centre). Arbequina and Blanqueta are from regionswith a Mediterranean climate, tempered by a maritime influ-ence, and Manzanilla comes from the central interior of theIberian Peninsula, with a climate similar to Trás-os-Montes.

Leaf material was collected on July 25, 2001, and all ana-tomical studies and tissue measurements were performed on10 healthy, current-year, sun-exposed, fully expanded matureleaves. Because sclerophylly does not change substantiallythrough time in mature leaves (Gratani 1996, Bussoti et al.2002), the study was restricted to the summer period.

The thickness of leaf blade, palisade and spongy paren-chyma, upper and lower epidermis, and trichome layer weremeasured in leaf cross sections of fresh material prepared formicroscopic examination. Sections were taken from the mid-dle of the leaves to avoid differential thickness along the leaf.

Upper cuticle thickness was assessed after staining freshmaterial with Sudan III, which stains the cuticle components adeep red (MacLean and Ivimey 1965). We were unable todistinguish between the lower epidermis and the lower cuticlelayer.

To make stomatal impressions, one or two coats of polish(colodium) were applied to the abaxial surface of each leaf af-ter peltate hairs were removed. The polish was then carefullypeeled off and placed on a microscope slide. The number ofstomata was determined for 10 peels per cultivar.

Morphology, sclerophylly and leaf water relationsparameters

At midday, three sun-exposed mature leaves were gatheredfrom each of five trees per cultivar. The following parameterswere examined: leaf area (LA (cm2), measured with anLI-3100 leaf area meter (Li-Cor, Lincoln, NE)), fresh mass(FM; g), fresh mass at full turgor (SM (g), measured after im-mersion of leaf petioles in demineralized water for 48 h in thedark at 4 °C), and dry mass (DM (g), measured after drying at70 °C to a constant weight).

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TREE PHYSIOLOGY VOLUME 24, 2004

Figure 1. Monthly air temperatures (T = mean for the period1951–1980 and T 2001 = mean for the year 2001) and rainfall (R =mean for the period 1951–1980 and R 2001 = mean for the year 2001)at Mirandela.

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The index of sclerophylly, leaf mass per unit area (LMA =DM/LA; g m–2) and density of foliar tissue (DM/FM; g kg–1)were calculated according to Groom and Lamont (1999). Sev-eral indices of leaf water status were also calculated: relativewater content (RWC = (FM – DM)/(SM – DM)100; %), succu-lence (S = (FM – DM)/LA; mg H2O cm–2), water saturationdeficit (WSD = (SM – FM)/(SM – DM)100; %) and watercontent at saturation (WCS = (SM – FM)/DM; g H2O gDM

–1 ).Cuticular transpiration rate (Ec; mg cm–2 h–1) was estimatedon the basis of leaf mass loss during dehydration.

Stem water potential

Predawn (Ψpd) and midday (Ψmd) stem water potentials weremeasured on six sun-exposed shoots with a pressure chamber(PMS, Corvallis, OR) according to Scholander et al. (1965).We measured stem water potentials rather than leaf waterpotentials because several field studies (Garnier and Berger1985, McCutchan and Schackel 1992, Naor and Wample1994, Naor 1998) have shown that Ψmd is better correlatedwith soil water availability than midday leaf water potential.Care was taken to minimize water loss during the transfer ofthe shoot to the chamber by enclosing it in a plastic bag imme-diately after excision (Turner and Long 1980).

Statistics

Data were subjected to analysis of variance with prior datatransformation when required. Proportional data expressed aspercentages and ratio were arcsine square-rooted and logtransformed, respectively. Mean separations were determinedby Duncan’s multiple range test (P < 0.05).

Results and discussion

Leaf anatomy study and tissue measurements

In leaf cross sections of all cultivars, the palisade parenchymaof the mesophyll comprised two parts, one being in contactwith the upper epidermis and the other with the lower epider-mis. The upper palisade parenchyma consisted of three com-pacted layers of elongated cells interspaced by tricosclereids.The lower palisade parenchyma consisted of one layer of rela-tively elongated cells. The presence of palisade parenchyma incontact with both the upper and lower epidermis is a common

feature of olive leaves under drought conditions (Bosabalidisand Kofidis 2002). Mesophyll compactness leads to low cellu-lar conductance thereby providing an efficient system to limitcellular water loss during drought (Bongi et al. 1987). Thepresence of this feature indicates that all cultivars are adaptedto the high-light regimes of the region and are protectedagainst water loss.

In leaves of all cultivars, a continuous rough wax layer and afew stellar trichomes covered the upper epidermis. The upperepidermis comprised one layer of irregularly shaped cells withheavy, thick outer walls and the lower epidermis comprisedone layer of isodiametric cells with a continuous layer of over-lapping, stellar trichomes hiding the small and abundantstomata.

Total lamina thickness differed significantly betweencultivars (Table 1); it was thick in Manzanilla and Negrinhabut thin in Arbequina and Cobrançosa. Differences in laminathickness were attributable to differing proportions of meso-phyll components in leaves.

Among cultivars, Negrinha had a thicker upper cuticle andthicker mesophyll tissue (palisade and spongy parenchyma).During the summer, cuticle thickening may enhance survivaland growth in this cultivar by improving water relations.

In Manzanilla, besides the palisade and spongy paren-chyma, the upper epidermis and lower epidermis were alsothicker than in the other cultivars. The upper cuticle layer wasless well developed, presumably because the epidermis al-ready provided protection for the inner tissues.

Leaves of Cobrançosa were thin compared with leaves ofother cultivars as a result of a thin spongy parenchyma (tissuewith numerous veins, polymorphic sclereids with relativelywide lacunae). The palisade/spongy parenchyma ratio washigh in this cultivar (1.06), suggesting a compact arrangementof cells and high mesophyll surface area per unit leaf area thatcould facilitate CO2 uptake and thus maintain photosynthesisunder drought conditions (Chartzoulakis et al. 2000). On theother hand, high-density leaf tissue leads to a decrease in thefractional volume of intercellular spaces and tends to decreasethe diffusion component of CO2 conductance (Chartzoulakiset al. 2000, Mediavilla et al. 2001). There is evidence, how-ever, that this component of the total internal conductance is ofminor importance in olive leaves (Syvertsen et al. 1995).

At the histological level, Cobrançosa was well protected

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SCLEROPHYLLY AND LEAF ANATOMICAL TRAITS OF OLEA EUROPAEA 235

Table 1. Mean values of leaf tissue thickness (µm) and stomatal density (stomata mm–2) of olive cultivars. The values represent the mean of 10 rep-lications. Means followed by the same letter are not significantly different at P < 0.05 (Duncan’s test).

Cultivar Thickness (µm) Stomatal

Total Upper Upper Upper Spongy Lower Palisade/ Upper/lower Lower Trichomedensity

lamina cuticle epidermis palisade mesophyll palisade spongy palisade epidermis1 layer

Arbequina 442.2 c 15.1 b 16.1 b 156.0 b 207.3 c 34.8 b 0.93 ab 4.48 a 12.9 b 30.3 c 451 bBlanqueta 491.1 b 13.3 b 16.4 b 164.1 b 234.9 c 46.8 a 0.90 b 3.51 b 15.6 b 43.2 ab 420 bCobrançosa 431.1 c 18.6 a 12.6 b 156.0 b 188.7 d 40.2 ab 1.06 a 3.88 ab 15.0 b 48.3 a 455 bManzanilla 554.4 a 14.9 b 21.7 a 198.0 a 252.9 a 46.2 a 0.97 ab 4.29 a 20.7 a 34.8 bc 442 bNegrinha 551.7 a 18.9 a 13.2 b 192.9 a 268.2 a 43.5 a 0.89 b 4.43 a 15.0 b 41.7 ab 537 a

1 Values also include the lower cuticle layer.

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against water loss as shown by the thick cuticle and thewell-developed trichome layer covering the abaxial surface.Leaf pubescence is a common feature of xeric genotypes(Abrams 1994, Karabourniotis and Bornman 1999, Liakouraet al. 1999). It is thought to increase water-use efficiency by in-creasing leaf boundary-layer resistance (Palliotti et al. 1994,Savé et al. 2000) and decreasing transpirational water losses(Fahn 1986, Baldini et al. 1997). Trichomes may also functionas an effective filter, protecting the underlying tissues againstultraviolet-B radiation damage (Karabourniotis and Bornman1999). Trichomes may allow olive leaves to take advantage oflight rain or condensation of water (Savé et al. 2000), therebyincreasing the probability of water uptake by leaves (Gramma-tikopoulos and Manetas 1994).

Arbequina is characterized by a thin lamina and a thintrichome layer that would not provide much protection againstwater loss. Furthermore, its leaf surface structure may facili-tate water removal through transpiration, and its thin lowerpalisade layer may enhance water loss.

The lamina of Blanqueta was intermediate in thicknessamong the cultivars examined, but it had a thin upper cuticle,upper epidermis and upper palisade layers. These features maylead to high water loss from the adaxial surface. However,Blanqueta had a high proportion of lower palisade paren-chyma and a dense trichome layer that may protect against wa-ter loss from the abaxial surface.

In olive leaves, the stomata are located on the abaxial side(hypostomatous), below the trichome layer, which preventsdehydration (Baldini et al. 1997). Leaves developed duringdrought usually have smaller and more numerous stomata thanleaves that develop under well-watered conditions (Larcher1995). Although Negrinha had the highest stomatal densityof the studied cultivars (Table 1), it probably avoids waterstress by possessing flexible regulation (Bolhar-Nordenkampf1987).

Morphology, sclerophylly and leaf water status

Leaf area was higher in Blanqueta and lower in Arbequina (Ta-ble 2), whereas values for Cobrançosa, Negrinha andManzanilla were intermediate. Several studies (Abrams 1994,Nevo et al. 2000, Pita and Pardos 2001) have shown that geno-types from xeric areas generally have smaller leaves than ge-notypes from mesic areas. Because small leaf areas transpireless water than large leaf areas, the production of small leaves

may help reduce water loss in Arbequina and balance the nega-tive anatomical features observed in this cultivar. In contrast,the large leaves produced by Blanqueta may be susceptible todesiccation, especially because this feature was associatedwith high vegetative growth (data not shown).

Leaves developed during drought generally have a higherLMA than leaves produced under well-watered conditions. Ahigh LMA is usually a consequence of an increase in densityor thickness of foliar tissue and normally occurs when thecosts of the assimilatory apparatus are increased (Centritto2002), such as during long periods of drought. In our study,LMA ranged from 183.9 g m–2 in Cobrançosa to 234.2 g m–2 inBlanqueta, indicating that leaves of all cultivars developed un-der drought conditions. Among the cultivars, Cobrançosa hadthe lowest LMA, a consequence of low leaf thickness. On theother hand, Cobrançosa had the highest density of foliar tissue(i.e., a greater fraction of the foliar volume was occupied bydry matter). These results corroborate the anatomical studyshowing a thinner mesophyll for this cultivar and a compactarrangement of cells. The high density of foliar tissue inCobrançosa is probably related to the thick cuticle layer and tothe small volume of intercellular spaces and may also be asso-ciated with an increase in supporting tissue or cell wall thick-ness. It is possible that leaves with high tissue density arebetter able to survive a severe drought because of a higher re-sistance to physical damage by desiccation (Mediavilla et al.2001).

The indices of leaf water status showed that, compared withthe other cultivars, Arbequina had lower RWC and higherWSD and WCS, indicating higher water loss. Arbequina hadthe highest Ec (Table 2) (reaching 45% more than inBlanqueta) as a result of the thin cuticle layer (Table 1) or dif-ferences in cuticle structure and composition, or both. Afterthe stomata, the main site for water loss by transpiration is thecuticle. High cuticular permeability affects not only thenon-stomatal transpiration pathway, but also water loss fromguard cells and, therefore, guard cell water status and stomatalaperture. During the hot, dry Mediterranean summer, whenstomata are tightly closed for most of the day (Lange 1988),the contribution of cuticular transpiration to water loss may beimportant in Arbequina.

The RWC of Arbequina was about 77%, which is consid-ered the turgor loss point for olive (Hinckley et al. 1980). Jorbaet al. (1985) found that a reduction in RWC from 96 to 65% in-

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Table 2. Leaf area (LA), leaf mass per unit area (LMA), density of the leaf tissue, relative water content (RWC), succulence, water content at satu-ration (WCS), water saturation deficit (WSD) and cuticular transpiration rate (Ec) of olive cultivars. Values represent the mean of 15 replications.Means followed by the same letter are not significantly different at P < 0.05 (Duncan’s test).

Cultivar LA LMA Density RWC Succulence WCS WSD Ec

(cm2) (g m–2) (g kg–1) (%) (mg H2O cm–2) (g H2O gDM–1 ) (%) (mg cm–2 h–1)

Arbequina 3.51 c 206.7 ab 459.8 b 77.1 b 24.27 b 1.53 a 13.87 a 4.27 aBlanqueta 7.69 a 234.2 a 459.7 b 83.4 a 27.49 a 1.42 b 9.82 b 2.36 bCobrançosa 4.54 b 183.9 b 482.0 a 84.6 a 19.73 c 1.27 c 8.67 b 2.68 bManzanilla 4.41 b 225.2 a 474.9 ab 85.7 a 24.77 ab 1.30 c 8.09 b 2.63 bNegrinha 5.14 b 227.5 a 463.2 ab 84.1 a 26.13 ab 1.38 bc 9.20 b 3.24 b

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duced an 85% reduction in photosynthesis of potted olivetrees. Thus, the low RWC of Arbequina likely inhibits photo-synthesis during the summer.

Abd-El-Rahman et al. (1966) reported that olive leaf WCSwas extremely low (1.59 g H2O gDM

–1 ) compared with fig andgrape growing in the same environment (5.77 and 5.85 g H2OgDM

–1 , respectively). We found that WCS values of Cobrançosaand Manzanilla were even lower (Table 2) than the value re-ported by Abd-El-Rahman et al. (1966), suggesting that thesecultivars have the capacity to withstand arid environments.Larcher (1960) suggested that a WSD of 8% is optimal forCO2 absorption and transpiration and 17% is the limit forstomatal closure. Blanqueta, Cobrançosa, Negrinha and Man-zanilla had WSD values of around 8%, whereas Arbequinahad a WSD near 12.5%, a value reported by Larcher (1960) asthe point of highest maximum water-use efficiency. Based onthese results, we conclude that Arbequina has adopted a desic-cation-tolerance mechanism that facilitates the maintenance ofhigh assimilatory rates during the onset of water deficit(Gulías et al. 2002). However, the leaves of this cultivar weresensitive to progressive desiccation during the summer, andsurvival may be threatened, especially during hot dry sum-mers.

Water storage mechanisms are important for desiccationavoidance, especially when they are coupled with surfacereduction and high transpiration resistance of the epidermis(Larcher 1995). A special form of water conservation is thebinding of water to mucilage in cells and ducts and inter-cellular cavities (Margaris 1981). Water reserves of this kindcan protect the plant from sudden wilting and severe leafshrinkage. Blanqueta, Negrinha and Manzanilla seemed to usethis kind of protection, showing a higher succulence index (Ta-ble 2). The close relationship between succulence and leafthickness, also reported by Bussoti et al. (2002), arises becausethicker leaves contain a greater volume of water per surfaceunit.

Stem water potential

Although data based on water content are informative in as-sessing the reserves available in the event of drought, the mostsuitable measures of specific desiccation tolerance of plantsare osmolarity and water potential of cells or tissue. The trendsof water potential are illustrated in Figure 2. Among cultivars,Arbequina and Blanqueta had the lowest Ψpd values, whichmay be indicative of incomplete overnight rehydration, proba-bly associated with a poor water transport system. The xero-morphic nature of olive roots has been observed in anatomicalstudies (Fernández et al. 1994) and by hydraulic functioning(Moreno et al. 1996).

Based on the daily pattern of stem water potential, differ-ences between Ψpd and Ψmd were significant in all cultivars.Arbequina presented the lowest Ψmd (–2.68 MPa), suggestingthat this cultivar is anatomically or physiologically less pro-tected against water loss. This result, combined with the lowRWC data, supports the hypothesis that Arbequina has a desic-cation-tolerance mechanism. In addition, Arbequina showedlarge changes in water potential for a given change in leaf

RWC, indicating that its leaves may have a high bulk elasticmodulus (Niinemets 2001). If so, Arbequina leaves may havelow cell wall elasticity, frequently cited as a mechanism en-abling drought-stressed plants to maintain cell volume andavoid deleterious reductions in RWC (Tyree and Jarvis 1982).In contrast, the other cultivars, especially Blanqueta, avoidedcritical water deficits by stomatal regulation, and by reducingleaf transpiration and the differences between Ψpd and Ψmd.

Conclusions

The five olive tree cultivars possessed different leaf-levelmechanisms to cope with summer stress. Among the Spanishcultivars, only Manzanilla seemed to be well protected at theanatomical level against water loss during the summer.Manzanilla and Negrinha enhanced their sclerophylly at theleaf level by building parenchymatous tissues, thereby in-creasing the protection provided by the upper cuticle(Negrinha) and upper and lower epidermis (Manzanilla). Theleaves of these two cultivars also had a higher succulence in-dex than the other cultivars, which affords protection againstsudden wilting and severe shrinkage. Cobrançosa is welladapted to drought conditions as a result of the high density(high palisade/spongy parenchyma ratio) of the foliar tissueand the presence of thick cuticle and trichome layers.

Arbequina leaves are anatomically less well protectedagainst water loss because of a thinner trichome layer thansome of the other cultivars. Because Arbequina leaves are sus-ceptible to progressive desiccation, survival may be threat-ened, especially during hot dry summers. However, thedevelopment of small leaves may reduce water loss at thewhole-plant level. Compared with some of the other cultivars,Blanqueta had larger leaves and some anatomical traits—in-cluding a thinner upper cuticle, upper epidermis and upper pal-isade layers—that may lead to high water loss, especially fromthe adaxial surface.

We identified several mechanisms at the morpho-structurallevel by which olive cultivars cope with summer stress. De-tailed studies are needed at the physiological and biochemicallevels to gain a mechanistic understanding of these processes.



Figure 2. Mean stem water potentials at predawn and midday of olivecultivars. Vertical bars represent the standard deviation of measure-ments on six shoots. Columns flanked by the same letter are not sig-nificantly different at P < 0.05 (Duncan’s test).

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sclerophylly and leaf anatomical traits of five field-grown olive cultivars growing under drought...

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