Journal of Phytology 2009, 1(4): ISSN: Journal of Phytology, 2009 Available Online:

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1 Journl of Phytology 9, 1(4): ISSN: Journl of Phytology, 9 Avilble Online: REGULAR ARTICLE INFLUENCE OF WATER STRESS ON SEED GERMINATION CHARACTERISTICS IN INVASIVE DIPLOTAXIS HARRA (FORSSK.) BOISS (BRASSICACEAE) IN ARID ZONE OF TUNISIA Mustph Gori*, Thr Tlig, Mohmed Neffti Lbortoire d Ecologie Pstorle, Institut des Régions Arides, 4119 Médenine, Tunisi SUMMARY Diplotxis hrr (Forssk.) Boiss (Brssiccee) hs wide ecologicl nd geogrphicl mplitudes in Tunisi nd grows under vriety of environmentl conditions. A lbortory experiment ws performed to evlute the effect of wter potentil on seed germintion nd recovery responses fter trnsfer to distilled wter. The germintion responses of seeds t 15 C in complete drkness were determined over wide rnge of PEG-6 solutions of different osmotic potentils: to 1 MP. Gretest germintion ws obtined in distilled wter (71%), nd increses in osmollity of solutions progressively inhibited seed germintion, less thn 1% of the seeds germinted t.8 MP. No germintion ws observed for the tretment of 1 MP. The rte of germintion decresed s osmotic potentil decresed. When seeds were trnsferred to distilled wter fter dys of wter stress, the recovery of germintion incresed with n increse in pretrnsfer PEG-6 tretments. Seeds subjected to high osmotic potentil hd higher recovery percentges. Keywords: Diplotxis hrr, Germintion, Recovery, Temperture, Wter stress M. Gori et l. Influence of Wter Stress on Seed Germintion Chrcteristics in Invsive Diplotxis hrr (Forssk.) Boiss (Brssiccee) in Arid Zone of Tunisi. J Phytol 1 (9) *Corresponding Author, Emil: gori_mustph@yhoo.fr 1. Introduction In rid ecosystems, the estblishment of species my differ in their life cycle (nnul/perennil), life form (shrubs/ herbceous), response to drought (tolernce/ voidnce), time of flowering, reproductive effort, seed dispersl, nd germintion behviour. Plnts in these ecosystems hve developed complementry dpttions nd survivl strtegies throughout the stges of their life cycles. Among these stges, the seed hs the highest resistnce to extreme environmentl fctors, wheres the seedling hs the lowest (Guttermn, 2). In extreme desert conditions, it is the pttern of germintion response to environmentl fctors tht determines whether prticulr species cn pper or not in specific

2 yer. Successful estblishment of plnts lrgely depends on successful germintion. Germintion is crucil stge in the life cycle of plnts nd tends to be highly unpredictble over spce nd time. Severl environmentl fctors such s temperture, slinity, light, nd soil moisture (Evns nd Etherington, 199; El-Keblwy nd Al-Rwi, 5; Zheng, et l 5; Gori nd Neffti, 7) interct in the soil interfce, which regulte seed germintion. Wter vilbility is one of the environmentl fctors tht influences most the germintive process (Bewley nd Blck, 1994; Lrson nd Kiemnec, 1997). A threshold mount of rinfll is necessry for desert nnuls to germinte in their growing seson, though the seeds of some species germinte in low soil wter potentil. Neffti (1994) reported tht the seeds of desert plnts germinte fster thn those from other hbitts. In these regions, germintion occurs during riny sesons when soil slinity levels re usully reduced (Gori nd Neffti, 7). Diplotxis hrr (Forssk.) Boiss of the fmily of Brssiccee, is n nnul herb tht grows in mny regions in rid Tunisi (Tlig et l 8). It is rmified stem herb which occurs in both sndy nd gypseous soils reching height of 5 to 6 cm (Pottier-Alpetite, 1979; Chieb nd Boukhris, 1998) nd produces mny smll seeds tht tend to persist for longer in the soil, which improved the existence of persistent soil seed bnks further (Hegzy, 1; Tlig et l 8). D. hrr hs ephemerl, modulr nd coppiced life-cycles nd reltively wet yers cuse the ephemerl plnts to live longer nd become perennils, with bility to shift from r- to K-selected strtegy. The intr-popultion vritions nd trde-off between r- nd K-life-cycles ensures plnt survivl under unpredictble desert environments (Hegzy, 1). In the field, estblishment of D. hrr vries ccording to the seson (cool/wrm), the geogrphicl nd ecologicl distribution s well s the mount of rinfll. Sesonl fluctutions of temperture nd rin in deserts of Tunisi explin the vrible estblishment pttern of this plnt during the course of yer, governed by sesonlly different of tempertures nd the rinfll events (Tlig et l 8). There is little vilble informtion in literture on the ecology of seed germintion of D. hrr. Thus, the objectives of this study were (1) to mesure the responses of D. hrr to wide rnge of osmotic potentil levels obtined with polyethyleneglycol, simulting drought conditions, in order to evlute the resistnce of seeds to wter stress; nd (2) to investigte the response of recovery of germintion under such conditions. 2. Mterils nd Methods Seed collection site Seeds of Diplotxis hrr were obtined from plnts which were collected from loction ner El Fjé, Médenine (1 39'N, 33 3'E; Southest Tunisi) in June 6. One thousnd seeds weighted, on verge, mg. See Tlig et l. (8) for detils on climte. Germintion experiments To determine the germintion response of this species to grdient of wter potentils we used different concentrtions of polyethylene glycol (PEG). PEG is nonpenetrting, inert osmoticum nd forms colloidl solution, the effect of which is similr to the mtric properties of soil prticles. The method gives good estimte of germintion behviour in reltion to soil moisture under field conditions (Hds, 1977). To simulte wter stress, PEG-6 solutions equivlent to the following osmotic potentils, prepred ccording to Michel nd Kufmnn (1973), were used:,.1,.2,.4,.6,.8, nd 1 MP. Seeds were surfce sterilized in.58 % sodium hypochlorite for one 25

3 minute, subsequently wshed with distilled wter nd ir-dried before being used in the germintion experiments to void fungus ttck. Ninety-millimetre Petri dishes contining two disks of Whtmn No. 1 filter ppers with 5 ml of test solution were prepred. Germintion experiments were conducted in incubtor (Luminincube II, nlys, Belgium; MLR-35, Snyo, Jpn) set t 15 C in complete drkness (Tlig et l 8). A completely rndomised design ws used in the germintion tests. For ech tretment, four replictes of 25 seeds ech were used. During dys the germinted seeds were counted nd removed every second dy. A seed ws considered to hve germinted when the emerging rdicle elongted to 2 mm. Solutions were renewed every 2 dys under sterile conditions to ensure reltively constnt in the tretments. Methods of germintion expression Three chrcteristics of germintion were determined: finl germintion percentge, rte of germintion, nd recovery of germintion. The rte of germintion ws estimted using modified Timson s index of germintion velocity = ΣG/t, where G is the percentge of seed germintion t 2-dy intervls nd t is the totl germintion period (Khn nd Ungr, 1984). The mximum vlue possible for our dt using this index ws 5 (i.e. 1/ ). The greter the vlue, the more rpid is the germintion. All seeds from the previous germintion tests which did not germinte fter dys t different PEG- 6 tretments, were plced in new Petri dishes with filter pper moistened with distilled wter, nd incubted under the sme conditions for dditionl dys to study the recovery of germintion. The recovery percentge ws determined by the following formul: ( b)/(c b)*1, where is the totl number of seeds germinted fter being trnsferred to distilled wter, b is the totl number of seeds germinted t different osmotic potentils, nd c is the totl number of seeds. Sttisticl nlysis Germintion dt were rcsine trnsformed before sttisticl nlysis to ensure homogeneity of vrince. Dt were nlysed using SPSS for Windows, version 11.5 (SPSS, 2). A one-wy nlysis of vrince (ANOVA) ws crried out to test effects of wter stress on the rte nd finl percentge of germintion. Tukey test (Honestly significnt differences, HSD) ws used to estimte lest significnt rnge between mens. 3. Results nd discussion Wter stress significntly (P <.1) ffected the finl germintion percentge of D. hrr (Tble 1). Tble 1. A one-wy ANOVA of the effect of PEG- 6 on germintion of Diplotxis hrr Mensqure df F-rtio Dependent vrible Percentge of germintion Rte of germintion Germintion recovery Note: Dt represent F-vlues significnt t P <.1. Mximum seed germintion ws obtined in distilled wter nd n increse in PEG-6 solutions of different osmotic potentils resulted in grdul decrese in percent germintion, nd only 5% of seeds germinted t.8 MP (Fig. 1,b). Seeds germinted rpidly in distilled wter during the initil 2 dys, while the dely of germintion incresed with incresing osmollity nd ws more obvious t 1 MP (Fig. 1). D. hrr germinte better t intermedite tempertures (1, 15 nd C), with therml optimum t 15 C (Tlig et l 8). This behviour is typicl strtegy of Mediterrnen plnts with 251

4 Germintion (%) Finl germintion (%) Germintion (%) Germintion rte optiml tempertures rnging between 15 nd C (Thnos et l 1995; Bskin nd Bskin, 1998). The temperture requirements of germintion cn be interpreted s n dpttion of this plnt species to sesonl fluctutions of temperture nd rin in deserts of Tunisi, becuse seed germintion strts following rins in erly winter, when tempertures were decresing (Tlig et l 8). Figure 1. Chnges in germintion percentge of Diplotxis hrr s influenced by PEG-6 solutions of different osmotic potentils: ( ) MP, ( ).1 MP, ( ).2 MP, (Δ).4 MP, ( ).6 MP, ( ).8 MP, nd ( ) 1 MP. (): germintion kinetics. The symbols re the observed numbers of germinted seeds, expressed s % of sown seeds. (b): Germintion percentge fter dys on the indicted PEG-6 solutions (open brs). Blck brs: recovery percentge of germintion fter being trnsferred to distilled wter for dditionl dys. Vlues of the germintion percentges (men ± 95% confidence limits, n = 4), hving different letter re significntly different t P <.5 (Tukey test). 1 8 Figure 2. Men rte of germintion of Diplotxis hrr seeds () nd reltionship between finl germintion percentge nd rte of germintion (b) s influenced by PEG-6 solutions of different osmotic potentils: ( ) MP, ( ).1 MP, ( ).2 MP, (Δ).4 MP, ( ).6 MP, ( ).8 MP, nd ( ) 1 MP. Vlues of rte of germintion (men ± 95% confidence limits, n = 4) t ech osmotic potentil hving different letter re significntly different t P <.5 (Tukey test). Lines describing the reltion between these prmeters were obtined by liner regression. Vlues (n = 28) re from the six PEG-6 tretments with four replictes ) ( b bc,1,2.1,4,6.2,8 1, PEG-6 ( MP) c d d b) ( y = 2.24x +.16 R 2 = b Dys Germintion in PEG-6 Recovery in distilled wter bc cd d b b b e e,1,2.1,4,6.2,8.4 1, PEG-6 ( MP) Rte of germintion In sline nd dry soils, wter potentil is not very different to tht of desiccted seeds. Therefore, t high osmotic potentils wter does not enter the seeds nd induces germintion. Our results show tht the highest germintion ws obtined in distilled wter (71%), nd n increse in osmollity of PEG solutions results in 252

5 decresing both the rte nd percentge of germintion. This ws in greement with the germintion behvior of most species (Evns nd Etherington, 199; Murillo-Amdor et l 2; Zheng et l 5; Tobe et l 6). Imbibition is n essentil prerequisite of germintion (Almnsouri et l 1), nd its rte nd extent is governed by the surrounding soil wter potentil nd the resistnce to movement of wter in the soil-seed system (Evns nd Etherington, 199). When seeds of D. hrr were trnsferred to distilled wter fter dys of wter stress tretment, the recovery of germintion percentges incresed significntly with n increse in pretrnsfer PEG-6 tretments (Tble 1, Fig. 1b). Seeds subjected to high osmotic potentil hd higher recovery percentges. Tlig et l. (8) reported tht D. hrr ws modertly slt tolernt t the germintion stge. Germintion percentge decreses with incresing slinity nd comes to the limit t mm. Slt stress decreses both rte nd percentge of germintion, but recovery is possible s long s slinity is low (Tlig et l 8). A frction of moistened seeds tht did not germinte with decresing osmotic potentil builds up seed bnk in the soil. Such seeds secure the long term existence of seed bnk helping the species spreding germintion over yers nd using opportunisticlly suitble germintion conditions (Cbllero et l 3). However, the lck of summer germintion in the field my be s well the result of dormncy tht reduces the risk of seedling mortlity, when moisture is limited nd slinity is incresed (Bskin nd Bskin, 1998). The rte of germintion, clculted by using modified Timson s index, decresed s osmotic potentil ws intensified (Fig. 2). A one-wy ANOVA of the rte of germintion indicted significnt effect (P <.1) of drought stress (Tble 1). Liner regression nlysis ws used to determine the reltionships between finl germintion percentge nd rte of germintion t different osmotic potentil levels. There ws strong positive reltionship identified between these prmeters, with coefficient of determintion R 2 =.99 (Fig. 2b). Seed imbibition rte, germintion percentge nd germintion rte generlly decrese s soil wter potentil decreses (Murillo-Amdor et l 2; Song et l 5; Sos et l 5), either by drought or by higher slinity. Seed-soil contct hs been ssumed to be the most importnt fctor for rpid trnsfer of wter from soil to seed. Moreover, in some species 85 % or more of the wter bsorbed by seeds cn be directly ttributed to vpor (Wuest, 7). It is well known tht wetting by solutions of incresed osmollity s rule high NCl concentrtions or PEG solutions my hve preconditioning effect. Preconditioning, or priming, increses the uniformity nd rte of germintion in comprison with non-primed seeds. In generl, nnul species need to control their germintion closely to minimize the risk of locl extinction cused by germinting under unfvorble conditions, but perennil species re free of such control since the lck of seedling cohort is fr less thretening for popultion survivl (Bskin nd Bskin, 1998; Fenner nd Thompson, 5). In ccordnce with this generlity, D. hrr, common nnul species in the desert of Tunisi, hs the bility to shift from r- to K-selected life-cycle nd to tke dvntge of yers with sufficient rinfll or yers of less environmentl stress (Hegzy, 1). Germintion of D. hrr occurs during lte winter nd erly spring, t time when seedlings cn survive, while no germintion occurred in summer, when tempertures were t their yerly mximum. In ddition, n inhibited germintion under the hrsh environmentl conditions would preserve the seeds enbling sufficient 253

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