Histological characteristics of sugar beet leaves potentially linked to drought tolerance

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Histological characteristics of sugar beet leaves potentially linked to drought tolerance
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  Industrial Crops and Products 30 (2009) 281–286 Contents lists available at ScienceDirect Industrial Crops and Products  journal homepage: www.elsevier.com/locate/indcrop Histological characteristics of sugar beet leaves potentially linkedto drought tolerance  Jadranka Lukovi´c a , ∗ , Ivana Maksimovi´c b , Lana Zori´c a , Nevena Nagl c , Milorad Perˇ ci´c b , 1 ,Dubravka Poli´c a , Marina Putnik-Deli´c b a Faculty of Sciences, Department of Biology and Ecology, Trg D. Obradovi´ca 2, 21000 Novi Sad, Serbia b Faculty of Agriculture, Trg D. Obradovi´ca 8, 21000 Novi Sad, Serbia c Institute of Field and Vegetable Crops, M. Gorkog 30, 21000 Novi Sad, Serbia a r t i c l e i n f o  Article history: Received 11 February 2009Received in revised form 13 May 2009Accepted 15 May 2009 Keywords: Leaf anatomyDroughtSugar beetCuticleEpidermis a b s t r a c t Water is becoming more and more limiting factor of sugar beet production and the productivity of thecrop can be significantly improved by increased drought tolerance. It is therefore a great challenge toassess the degree of variability of anatomical and morphological traits of breeding material with respectto water use efficiency and drought, that can be used as potential markers for selection of sugar beetgenotypeswithbettertolerancetowatershortage.Toachievethis,thefirststepistoassessthedegreeof genetic variability with respect to anatomical and histological features linked to water management inplants,underoptimalwatersupply.Comparativehistiologicalanalysisoflaminaandpetiolewasdoneon12 sugar beet genotypes which previously showed divergent responses to lack of water in the field. Theplantsweregrowninsemi-controlledconditionsofaglasshouse,andwatereddaily.Mircromorphologicalanalysesweredonetoassessleafepidermalcharacteristics,bybothlightandSEM,andlaminaandpetiolehistological features. The measurements were used to calculate the percentage of individual tissues inrelation to the thickness of the lamina, main vein area and petiole area. The general structure of samplevariabilitywasestablishedbyprincipalcomponentanalysis(PCA),basedoncorrelationmatrix.Inmajor-ityofgenotypestheratioofthesizeofcellsofspongyparenchymaandpalisadecellsinaveragewas80%.Lowgenotypicvariabilityofthestudiedhistologicalparametersofthelaminaandpetiolemayreflectthenarrow genetic base of tested breeding material. The most significant genotypic difference, consideringleafepidermaltissue,wasin%ofadaxialandabaxialepidermis.Thehighestfoundnumberofstomatapermm 2 on both adaxial and abaxial epidermis was 40% higher than the lowest. During water stress, whenstomata are closed, plant survival depends on the amount of water lost through the cuticle. SEM analysisofadaxialepidermisofthelaminashowthatcuticlevariesintexture.Consideringtheobservedgenotypicvariability in cuticle ornamentation and the fact that plants develop various strategies of adaptation todrought,findinggenotypeswithincreaseddroughttolerancecouldbebasedonthecharacteristicsofthecuticle and epidermis.© 2009 Elsevier B.V. All rights reserved. 1. Introduction Plants respond to environmental variations, particularly to theamount of water and oxygen in the soil, through morphological,anatomicalandphysiologicaladjustmentsthathelpthemcopewithsuch variations.The yield potential of sugar beet ( Beta vulgaris  L.) depends pri-marilyonthesiteandyeareffects(Kenteretal.,2006).Theinfluence ∗ Corresponding author  . Tel.: +381 21 4852662; fax: +381 21 450620. E-mail address:  jlukovic@ib.ns.ac.yu (J. Lukovi´c). 1 Present address: Konzul d.o.o. Novi Sad, Stevana Musica 1, 21000 Novi Sad,Serbia. of the environment accounts for about 80% of the total variance(Hoffmann et al., 2009). The effect of a year reflects the weather conditions during the vegetation period, which directly influenceplant growth, and also affects sowing and harvest dates and thusthe length of the growing season. There are a number of studiesconcerning the impact of weather variables on the growth of sugarbeet, conducted under controlled conditions or from single fieldexperiments ( Jaggard et al., 1998; Kenter and Hoffmann, 2003; Qiet al., 2005; Kenter et al., 2006).Development of varieties with increased drought tolerancewould result in more stable yield under unfavourable condi-tions, but breeding, specifically for drought tolerance, is stilltime-consuming and expensive (Pidgeon et al., 2006). Therefore, searching for simple and fast methods for the screening of the 0926-6690/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.indcrop.2009.05.004  282  J. Lukovi´c et al. / Industrial Crops and Products 30 (2009) 281–286 breedingmaterialwithrespecttodroughttoleranceplaysanimpor-tant role in breeding programs (Boyer, 1996; Jaggard et al., 1998;Pidgeon et al., 2001). One of the best ways to improve sugar beetproduction in regions prone to drought is to search for varietieswhich have greater resistance to drought (Ober and Luterbacher,2002). Biennial root crops are equally susceptible to water stressthroughout their development (Salter and Goode, 1967). Sugar beet is among those crop species that can economize water verysuccessfully(ˇ Caˇ ci´cetal.,1997),butwaterpotentialmayaffectaccu- mulationofessentialelementsinsugarbeetleaves(Maksimovi´cet al., 2003). The task of plant breeders is to define genotypes withsuch structural characteristics, that make them more adaptive todrought and which are at the same time able to produce a highyield of good quality. This is particularly important for sugar beet,characterized by synthesis of large amounts of organic matter andtherefore demanding considerable water amounts (Petrovi´c andStiki´c, 1992).Anatomical characteristics can also be efficiently used as anindicator of drought tolerance (Martins and Zieri, 2003). In the majority of cases, there is a correlation between some of xeromor-phic features and dry conditions of the habitat (Ristic and Cass,1991;Belhadjetal.,2007).First,droughtcausesastomatalclosure,reduces CO 2  uptake for photosynthesis, and reduces plant growthandyield.Plantsvaryinthetypesandspeedofresponsestodroughtconditions, depending on their genetic and ecotypic backgrounds,but a number of drought responsive genes are conserved acrossplant taxa. This is especially true for genes involved in the osmoticadjustment,detoxification,andcellcommunicationandsignalling.Someoftheanatomicalcharacteristicsrepresentingadaptationsto water deficit are: a greater stomata number per unit area, theirsmaller dimensions, smaller epidermal cells, thicker cuticle anda greater number of layers of smaller mesophyll cells (Merkulovet al., 1997). Since most water in plants is lost via stomata, themost obvious and probably the most efficient way to improvedrought tolerance is by modifying guard cell behaviour ( Jong KukNa, 2005). Cuticular waxes play an important role in drought tol-erance. This, as well as the easiness of wax extraction and analysis,suggests that characteristics of the cuticular wax may prove tobe useful selectable traits in a breeding program (Cameron et al.,2002).Water deficit inhibits both cell division and cell expansion inall axis tissues of radish plants, but there is no evidence that therelationship between the various tissues is disturbed. The walls of hypocotylparenchymacellssignificantlythickenduringaperiodof water deficit. Stress significantly inhibits expansion of the cells of the cortex, pericycle, cambium, and ray parenchyma. It also causesa reduction of the number of cells in all tissues but the cortex, inwhich cell division presumably ceases prior to the stress ( Joyce etal., 1983). The walls of parenchyma cells significantly thicken dur-ing a period of water deficit. Staining with toluidine blue suggeststhat this is due to lignification ( Joyce et al., 1983). In  Triticum aes-tivum , an increase of the hemi-cellulose content of the cell wallhas been reported under water stress (Wakabayashi et al., 1997).Water moving from the leaf xylem to the mesophyll cell walls,which are adjacent to the leaf intercellular spaces, occurs throughboth the cell walls and the protoplasts. Therefore, the character-istics of both the walls and the protoplasts of the tissues, whichare not specialized for water conduction, can have an importantinfluenceontheleafhydraulicconductance(Aasamaaetal.,2005).Because the hydraulic conductance of unlignified walls is signifi-cantlyhigherthanthatofprotoplasts(SteudleandPeterson,1998),thickwallsmayfavourwaterflowthroughthemesophyll(Aasamaaet al., 2005).Generally speaking, it is known that shoots are not the mainresistance to water flow, about half of the total plant resistancelies in roots, and most of the other half may be in leaves (Meinzer,2002). With sugar beet, leaf growth was much more susceptibleto long-term water shortage than taproot growth (Kretschmer andHoffmann, 1985). Also, it is worth noting that in sugar beet there isadistinctsectorialzonationofdonor–acceptorlinkbetweenleavesand taproot (Merkulov et al., 1996).There is a limited amount of information on structural partic-ularities of sugar beet genotypes that are linked to water regimeand potential for efficient water use. Genotypes show divergentresponses to drought under field conditions. Therefore, the aim of this paper is to use comparative histological analysis of the laminaand petiole to estimate the degree of structural variability in suchsugar beet genotypes under optimal water supply. 2. Material and methods Twelve sugar beet genotypes were analyzed ( Beta vulgaris  ssp. vulgaris  L. from a population made by successive hybridization.The selected genotypes previously showed divergent responses towater shortage in the observation tests in the field. In this exper-iment the plants were grown in semi-controlled conditions of aglasshouse, in a mixture of soil (2/3) and sand (1/3), with dailywatering to maintain 100% field capacity and 24-h lighting for75 days. After that, 8-h periods of darkness were introduced. Tenplants per genotype were sampled on the 90th day. For anatomi-calanalysis,pairsofyoungestcompletelyformedleaveswereused.Cross-sections of the middle part of petiole and lamina (betweenthethirdandfourthlateralnerveandthepartwiththemainnerve)were made using a cryostat (Leica CM 1850), at 18 to − 20 ◦ C, with20–30  m intervals. Leaf epidermal characteristics were observedunder light and scanning electron microscope (SEM). For lightmicroscopy observations leaf epidermal prints were made follow-ingtheproceduredescribedbyWolf(1954).ForSEManalysissmall piecesofdryleavesoffiveplantspergenotypeweresputter-coatedwith gold for 180s, 30mA (BAL-TEC SCD 005) and viewed with JEOL JSM-6460LV electron microscope at an acceleration voltageof 20kV. The measurements were used to calculate the percentageofindividualtissuesinrelationtothethicknessofthelamina,mainvein cross-section area and petiole cross-section area. The datawere statistically processed with Statistica for Windows, version8.0(StatosftInc.,Tulsa,OK,USA).ThecomparisonofgenotypeswasdoneusingDuncan’stest.Thevaluesmarkedwiththesameletterdonot differ significantly for  p <0.05. The general structure of samplevariability was established by principal component analysis (PCA),using a standardized basic matrix. This analysis was performed inorder to investigate patterns of variation of analysed parameters,aswellastopointoutanatomicalvariablesthatmoststronglycon-tributetothetotalvariationofthesampleanddifferencesbetweengenotypes. 3. Results SEManalysisshowedthattheleafsurfaceultrastructurewasdif-ferent among different genotypes. Smooth to granular cuticle wasfound (Fig. 1a–c) as well as genotype differences in the width and texture of epidermal cell walls (Fig. 1d–f). The thickness of lamina cross-section did not show any sig-nificant variation across genotypes. A somewhat more significantdifferenceamonggenotypeswasfoundin%ofthethicknessofadax-ial and abaxial epidermis and in % of the palisade tissue thickness(Tables 1 and 2). The amphistomatic lamina has a larger number of slightly smaller stomata on the abaxial side. The average num-ber of stomata per mm 2 ranges from 101 to 223 in the adaxial andfrom 138 to 229 in the abaxial epidermis. All genotypes exhibit alargercellcross-sectionareaintheadaxialepidermis.Duncan’stestindicatesthatthereisasimilargenotypicvariabilityofallobserved   J. Lukovi´c et al. / Industrial Crops and Products 30 (2009) 281–286  283 parametersofadaxialandabaxialepidermis.Thehighestnumberof stomata per mm 2 on both adaxial and abaxial epidermis has geno-type G7 and this is 40% more than the genotype G12 which has thelowest stomatal number (Table 2).Photosynthetictissueismadeupof2–3layersofpalisadetissuecellsand4–5layersofspongytissuecells.Theratioofpalisadeandspongytissueandareaofspongytissueonthecross-sectiondidnotshow significant genotypic variability. More significant genotypicdifferences exist in the area of palisade tissue cells in the cross-sectionandtheratioofwidthandheightofpalisadecells(Table2).The number of vascular bundles per mm 2 of cross-section areaofthemainveinrangesfrom1.4to2.3.Asignificantlyhighernum-ber of vascular bundles per mm 2 , with a larger number of smallervessels is found in genotypes G1, G3 and G4 (Table 2, Fig. 2). Per- cent of vascular tissue (% of phloem and % of xylem) in the mainveinrangesfrom2.4%to4.8%(respectively).Highergenotypicvari-abilitywasobservedin%ofxylem.Sclerenchymaticelementswithproportionof3.9–10.6%aremainlypresentadjacenttothephloem.Subepidermally there are 2–3 layers of collenchyma cells. The pro-portion of collenchyma tissue showed low genotypic variability(Table 1).The petiole of all genotypes contains three large bundles anda few smaller ones. When compared to the main vein, the petiolehas quite a larger number of bundles per mm 2 of cross-section,with a significantly lower % of vascular tissue (phloem 1.8%; xylem3.4%). The proportion of mechanical tissue in the petiole is 3.0%for sclerenchyma and 15.6% for collenchyma. A similar genotypicvariability was recorded for both types of mechanical tissue. Highvariationcoefficientswererecordedforalmostallcharacteristicsof the main vein and petiole (Table 1).The results of principal component analysis (PCA) showed thatexamined characters had a generally low variability, since the firstfouraxesexplainedonly53.7%oftotalvariation.Charactersthatareprominent on the first axis and contribute most to total variability(16.2%) are: lamina thickness, % adaxial and % abaxial epidermis,and cross-section area of palisade and spongy tissue cells. The sec-ond axis is responsible for 12.8% of variability and is defined by thesizeofstomataontheabaxialepidermis.Thethirdaxis,with12.9%ofvariability,isdefinedbyproportionofpalisadeandspongytissueand palisade/spongy tissue ratio. On the fourth axis there are char-actersofthepetiole(%ofphloem,%ofxylemand%ofcollenchyma)which contribute with 11.8% to the total variability (Table 3). Fig. 1.  SEM micrographs of adaxial leaf epidermis – surface view – a (G3), b (G9), c (G5) and margin of lamina—d, e, f.  284  J. Lukovi´c et al. / Industrial Crops and Products 30 (2009) 281–286 Fig. 2.  Cross-section of the mean vein – (a) large number of smaller vessels (G3); (b) small number of larger vessels (G5).  Table 1 Averagesofleafanatomicalcharacteristicsofsugarbeetgenotypes(mean ± standarderror and coefficient of variation—% given in brackets).CharactersLaminaThickness (  m) 267  ±  5.4 (21.8)Adaxial epidermisNo. of stomata/mm 2 140  ±  4.0 (31.2)Stomata length (  m) 24.9  ±  0.3 (10.7)Stomata width (  m) 16.6  ±  0.2 (12.7)Percent of epidermal thickness 7.5  ±  0.1 (16.8)Cell cross-section area 541  ±  13.2 (26.8)Abaxial epidermisNo. of stomata/mm 2 178  ±  5.0 (30.9)Stomata length (  m) 23.8  ±  0.3 (11.8)Stomata width (  m) 16.5  ±  0.2 (10.3)Percent of epidermal thickness 7.3  ±  0.2 (17.6)Cell cross-section area 451  ±  11.5 (28.0)Photosynthetic tissuePercent of palisade tissue thickness 29.2  ±  0.3 (12.2)Percent of spongy tissue thickness 55.9  ±  0.4 (7.5)High/width palisade cell 1.6  ±  0.05 (18.7)Palisade/spongy ratio 0.53  ±  0.01 (18.3)Cell cross-sec. area palisade tissue 809  ±  21 (28.4)Cell cross-sec. area spongy tissue 652  ±  15.6 (26.1)Main veinNo. of vascular bundles/mm 2 1.8  ±  0.1 (45.7)Percent of phloem 2.4  ±  0.1 (31.3)Percent of xylem 4.8  ±  0.2 (42.8)Percent of sclerenchyma 3.9  ±  0.1 (30.3)Percent of colenchyma 10.6  ±  0.3 (30.0)PetioleNo. of vascular bundles/mm 2 12.6  ±  0.4 (35.0)Percent of phloem 1.8  ±  0.04 (35.0)Percent of xylem 3.4  ±  0.08 (26.6)Percent of sclerenchyma 3.0  ±  0.08 (31.7)Percent of colenchyma 15.6  ±  0.6 (42.0) Discussion Generally, the structural characteristics of the plant that areassociated with its ability to survive under dry conditions arereferred to as xeromorphic characteristics.Sugarbeetgenotypescandifferconsiderablyinyieldandqualitycharacteristics which are additionally modified by environmentalconditions. In the last decades drought stress became the mostimportant of them because of its restricting effect on growth andalteration of the chemical composition of the beet (Bloch andHoffmann, 2005).During water stress, when stomata are closed, plant survivaldepends on the amount of water lost through the cuticle. Froma whole-plant point of view, the interplay between stomatal reg-ulation and cuticular water permeability is therefore essential(Kerstiens, 1996). The waxy layer varies in continuity and in thick- nesswiththeageoftheleafandwithgrowingconditions(Bystromet al., 1968). Genetic variation in waxiness has been associatedwith drought resistance in many species (wheat, barley, tomato).Inmanyofthesespecies,therewasphenotypicplasticityshownbyan increase in glaucousness during drought (Thomas, 1997). Adax- ial cuticles of plants grown in humid air were more permeableto water than those from dry air (Karbulkova et al., 2008). Cuti- cles in sugar beet leaves, according to Holloway’s classification of structural types of cuticle, belong to the reticulate-lamellate type( Jeffree, 2006). Our results of the SEM analysis of adaxial epider- mis of the lamina show that cuticle varies in texture. Consideringtheobservedgenotypicvariabilityincuticleornamentationandthefactthatplantsdevelopvariousstrategiesofadaptationtodrought,with sugar beet, as elsewhere, finding genotypes with increaseddroughttolerancecouldbebasedonthecharacteristicsofthecuti-cle. We suggested that variation in cuticular wax in sugar beetmay be related to drought resistance. The increase in wax con-tent might be used to improve drought resistance and water useefficiency.Our results showed that in relation to the characteristics of epi-dermal tissue, the most significant genotypic difference was in %of adaxial and abaxial epidermis. Stomata were numerous abaxi-allyforallgenotypes,exceptinG7genotype,theirsizeandnumberpermm 2 oftheleafsurfaceandtheareaofcellsinthecross-sectionareaofadaxialandabaxialepidermiswerenotveryvariable.Higherstomata density and smaller stomata size is a form of adaptationto drought because it enables plants to regulate water transportandtranspirationmoreeffectively(FahnandCutler,1992;Dickison,2000). Low stomatal density in sugar beet seemed to correlatewith water stress resistance. Thomas and Clarke (1995), confirm- ing results from the studies of  Visser (1951), found that stomatal densities ranged from 70 to 150 stomata per square millimeterand that varieties more capable of tolerating water deficiency hadlowerstomataldensities.Ontheotherhand,sincestomataldensitycorrelates positively with sugar yield (Thomas and Clarke, 1995),attempting to improve resistance to drought by selecting for fewerstomata might lead to a reduction in sugar yield. In dry conditions,however, when turgor pressure is negative, small epidermal cellswith thickened walls exhibit considerable resistance to collapse(Fahn and Cutler, 1992). According to Joyce et al. (1983), in sugar beet leaves a small reduction in turgor had a greater effect on celldivision than on cell expansion.The degree of differentiation of the mesophyll and the propor-tion of palisade and spongy parenchyma vary in relation to plantspeciesandhabitat(Esau,1965).Asforthecharacteristicsofphoto- synthetictissue,allgenotypesshowedlowgenotypicvariability.Of totalvariability,whichforthefirstfouraxeswas53.5%,characteris-tics of photosynthetic tissue such as % thickness of palisade tissue,   J. Lukovi´c et al. / Industrial Crops and Products 30 (2009) 281–286  285  Table 2 Some leaf anatomical characteristics (mean ± standard error). The difference between the values with the same letter was not statistically significant at  p ≤ 0.05.Genotypes Adaxial epidermis Abaxial epidermis Cell cross-section area of palisade tissueNo. of main vein vascularbundles /mm 2 Thickness (%) Stomata number Thickness (%) Stomata numberG1 7.6  ±  0.3 abc 110  ±  4 d 7.4  ±  0.2 abc 138  ±  7 d 741  ±  45 bc 2.3  ±  0.4 a G2 7.7  ±  0.3 abc 131  ±  10 cd 7.6  ±  0.3 abc 157  ±  9 bcd 651  ±  42 c 1.8  ±  0.2 ab G3 7.0  ±  0.2 bc 154  ±  6.0 bc 7.0  ±  0.2 bcd 203  ±  12 abc 737  ±  56 bc 2.2  ±  0.2 a G4 6.5  ±  0.2 c 133  ±  11 cd 6.0  ±  0.2 d 175  ±  11 abcd 860  ±  95 abc 2.3  ±  0.2 a G5 8.0  ±  0.4 ab 128  ±  12 cd 8.0  ±  0.5 ab 182  ±  17 abcd 712  ±  72 bc 1.7  ±  0.3 ab G6 6.9  ±  0.4 bc 140  ±  10 bcd 6.6  ±  0.4 cd 177  ±  13 abcd 846  ±  81 abc 1.9  ±  0.3 ab G7 7.3  ±  0.6 bc 223  ±  25 a 6.6  ±  0.5 cd 185  ±  38 abcd 877  ±  56 ab 1.4  ±  0.3 ab G8 7.5  ±  0.3 abc 140  ±  7b cd 7.1  ±  0.3 bcd 180  ±  8 abcd 919  ±  46 ab 1.4  ±  0.1 b G9 7.6  ±  0.3 abc 155  ±  12 bc 7.6  ±  0.4 abc 229  ±  22 a 881  ±  95 ab 1.7  ±  0.2 ab G10 8.0  ±  0.7 ab 124  ±  16 cd 7.8  ±  0.7 abc 145  ±  17 cd 1043  ±  53 a 1.4  ±  0.2 b G11 8.6  ±  0.6 a 181  ±  16 b 8.5  ±  0.5 a 210  ±  22 ab 748  ±  56 bc 1.4  ±  0.1 b G12 8.1  ±  0.3 ab 101  ±  11 d 7.6  ±  0.3 abc 142  ±  16 d 858  ±  67 abc 1.5  ±  0.2 ab  Table 3 Principal component analysis (PCA) of measured parameters. Factor coordinates of the variables, based on correlations and cumulative percentages of the vectors (markedloadings are >0.700).Characters Factor 1 Factor 2 Factor 3 Factor 4Lamina thickness (  m)  0.7696*  − 0.1802 0.3782  − 0.1939Percent of adaxial epidermis thickness  − 0.8787*  0.0808  − 0.1681  − 0.0716Stomata length abaxial epidermis (  m) 0.0290  0.8555*  − 0.3000 0.3149Stomata width abaxial epidermis (  m)  − 0.0494  0.7934*  − 0.2943 0.1024Percent of abaxial epidermal thickness  − 0.8379*  0.1147  − 0.2379  − 0.1215Percent of palisade tissue thickness 0.0283 0.0144  − 0.9702*  0.0713Percent of spongy tissue thickness 0.4034  − 0.0602  0.8806*  − 0.0093Palisade/spongy ratio  − 0.0912 0.0163  − 0.9744*  0.0350Cell cross-sec. area palisade tissue  0.7489*  − 0.0162  − 0.1982  − 0.1626Cell cross-sec. area spongy tissue  0.7227*  0.1643  − 0.0239  − 0.2772Percent of phloem petiole 0.1106  − 0.1865 0.1190  − 0.8845* Percent of sclerenchyma petiole  − 0.0106  − 0.1027 0.0592  − 0.7977* Percent of colenchyma petiole  − 0.1537 0.0342 0.0258  − 0.7293* Percentage of total variance explained 0.1616 0.1279 0.1288 0.1185Cumulative percentage of total variance 16.16 28.95 41.83 53.68 palisade tissue area at the cross-section and height/width of pal-isadecellsstandoutinthethirdaxiswithonly12.9%variability.Onaverage,cellsofthepalisadeparenchymawerebiggerthanthoseof thespongyparenchyma.AccordingtoMartinezetal.(2007),under controlled conditions cells of spongy parenchyma were 70% of thesize of the palisade cells of cultivars of common beans ( Phaseolusvulgaris L.)Thisdifferencewaslargerunderwaterstresstreatment,wherethesizeofthespongycellparenchymarepresentedonly40%of the size of the palisade cell. Under our experimental conditionsin majority of genotypes, this ratio is in average 80%.Xylem conductivity is determined by the structure and the sizeofthevessels(SchultzandMatthews,1993).Accordingtoourdata, the genotypes with a larger number of vascular bundles per mm 2 of the cross-section area of the main vein have a larger numberof smaller vessels as well as a lower proportion of sclerenchyma.Sclerenchymatic elements along the vascular bundle help to avoidcollapse of vascular bundle elements under turgor loss conditions(Grilletal.,2004).Generally,plantsortissueswithasmallercellsize will be more tolerant of low water potential (Cutler et al., 1977).Since significant correlations were recorded between the size of the vascular bundle of the main vein, number and length of stom-ata on abaxial side, palisade cells height and physiological traits,cited structural characteristics may point to the adaptation rangesto drought (Merkulov et al., 1997). In their analysis of xylem traits associated with water transport efficiency and water stress toler-ance, Jacobsenetal.(2007)thinkthatxylemdensitymaybeauseful tool in estimating the xylem characteristics and drought toleranceof large number of species. Our observations indicate that a largernumber of vascular bundles per mm 2 of cross-section area of themain vein, with a larger number of smaller vessels, could be usedin the selection of drought tolerant genotypes.The petioles of all genotypes contain three large bundles andseveral smaller bundles. When compared to the main vein, thepetiole has a larger number of bundles, with a smaller proportionof phloem and xylem in relation to the area of the cross-section.Higher genotypic variability in the number of petiole bundles isrelated to genotypic characteristics of the petiole, affected primar-ily by the variability of the form of leaf rosette. According to theresults of PCA analysis, all examined petiole characteristics hadlow variability, and therefore were considered as less significantin selection for drought tolerance, compared to lamina characters.Low genotypic variability of the studied histological parameters of thelaminaandpetiolemayreflectthenarrowgeneticbaseoftestedbreeding material.Breeding for the agronomic characters had a marked influenceon reducing diversity (McGrath et al., 1999). When estimating the degree of genotypic variation in drought tolerance within a widerange of sugar beet germplasm and gene bank accessions within Beta , Ober and Luterbacher (2002) and Pidgeon et al. (2006) point out the existence of genotypic variation and individual genotypesthat exhibit greater drought tolerance than selected commercialvarieties. In this case, introgression of germplasm from wild pop-ulations would be the only way to increase the genetic variationfor characters that could contribute to drought tolerance in sugarbeet. Therefore, when searching for valid parameters in assessingdrought tolerance of individual genotypes, it is necessary to takeinto consideration the interrelation of a large number of morpho-anatomical parameters together with physiological–biochemicalcharacteristics of individual organs at different growth and devel-opment stages and at different degrees and durations of watershortage. As shown here, there is a variability of some leaf andpetiole features (e.g. larger number of vascular bundles per mm 2
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