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Effects of seed maturation time and dry storage on light and temperature requirements during germination in invasive Prosopis juliflora

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Effects of seed maturation time and dry storage on light and temperature requirements during germination in invasive Prosopis juliflora
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  Flora 201 (2006) 135–143 Effects of seed maturation time and dry storage on light and temperaturerequirements during germination in invasive  Prosopis juliflora Ali El-Keblawy  , Awatif Al-Rawai Department of Biology, Faculty of Science, UAE University, P.O. Box 17551, Al-Ain, UAE  Received 2 March 2005; accepted 27 April 2005 Abstract The effects of time of seed maturation and dry seed storage and of light and temperature requirements during seedincubation on final germination percentage and germination rate were assessed for the invasive shrub  Prosopis juliflora (Sw.) D.C., grown under desert environmental conditions of the United Arab Emirates (UAE). Seeds were collectedfrom Fujira on the northern coast of the UAE at different times during the growing seasons (autumn, winter andspring) and were germinated immediately and after 8 months of dry storage under room temperature (20 7 3 1 C). Seedswere germinated at three temperatures (15, 25 and 40 1 C) in both continuous light and darkness. The results showedsignificant effects for time of seed collection, seed storage, light and temperature of seed incubation and many of theirinteractions on both germination percentage and rate. Fresh seeds matured during autumn and winter germinatedsignificantly greater at 40 1 C and in light than at lower temperatures and in dark. Storage significantly increasedgermination percentage and rate; the increase was greater for seeds matured during winter than for seeds maturedduring spring. This indicates that dormancy breakage was greater in seeds of winter than seeds of spring. The need forhigh temperature to achieve greater germination was significantly reduced after seed storage, especially for seedsmatured in autumn and winter. r 2005 Elsevier GmbH. All rights reserved. Keywords:  Desert habitat; Germination; Harvest time; Maternal effect; Seed dormancy; Seed maturation; Storage Introduction The conditions under which seeds mature on themother plant can determine the level of seed germin-ability and dormancy and consequently affect time of seed germination and fate of next generation (Baskinand Baskin, 1998; Meyer and Allen, 1999; Roach and Wulff, 1987). It has been documented that seeddormancy and germination responses vary greatlydepending on maternal habitat and time of seeddevelopment and maturation on mother plants. Severalstudies have demonstrated that seed germination variesbetween populations of different species (El-Keblawyand Al-Ansari, 2000; El-Keblawy et al., 1996; Gutter- man, 2000; Meyer and Allen, 1999). Environmental conditions experienced by parental plants during thegrowing season have also shown to play a significantrole in determining subsequent germination responses inseeds of many species including  Artemisia tridentata (Meyer and Monsen, 1991),  Spergularia marina  (Ungar,1988),  Portulaca oleracea  (El-Keblawy and Al-Ansari,2000),  Eruca vesicaria  (Pita Villamil et al., 2002), ARTICLE IN PRESS www.elsevier.de/flora0367-2530/$-see front matter r 2005 Elsevier GmbH. All rights reserved.doi:10.1016/j.flora.2005.04.009  Corresponding author. Permanent address: Department of Biol-ogy, Faculty of Education in Al-Arish, Suez Canal University, Egypt. E-mail address:  a.keblawy@uaeu.ac.ae (A. El-Keblawy).  Campanula americana  (Galloway, 2002) and in threeperennial shrubs (Gutterman, 1991). Maternal repro-ductive phenology has also been shown to influencegermination response as a result of temporal changes inboth external and intrinsic (e.g., maternal resourcestatus) factors ((El-Keblawy and Lovett-Doust, 1998,1999; Galloway, 2002; Grzesik et al., 1998). The most important factors that affect maternalplants during the growing season have been suggestedto be temperature, rainfall and day length (Baskin andBaskin, 1998; Fenner, 1992). Several studies have reported the importance of temperature during seeddevelopment and maturation as an important factoraffecting seed germination (Alexander and Wulff, 1985;Allen and Meyer, 2002; El-Keblawy and Al-Ansari,2000; Harrington and Thompson, 1952; Jensen and Eriksen, 2001; Nosova, 1981; Probert et al., 1985; Qaderi and Cavers, 2000; Qaderi et al., 2003; Sharif- Zadeh and Murdoch, 2000; Wurzburger and Koller,1976). Germination response has been also shown to beaffected by day length experienced by maternal plants(El-Keblawy and Al-Ansari, 2000; Evenari et al., 1966; Heide et al., 1976; Munir et al., 2001; Richardson, 1979). In addition, moisture condition at which maternalplants are growing has been shown to affect seedgermination in some species (Allen and Meyer, 2002;Meyer and Allen, 1999). In most of these studies, seedsproduced by plants at higher temperatures, water stressand shorter days have higher germination percentageand/or rates (i.e., lower dormancy) than those producedat lower temperatures, long days and favourablemoisture conditions. It has been proposed that theenvironmental conditions under which plants are growncan affect seed germination by affecting their chemicalcomposition and seed provisioning (e.g., mineral,photosynthetic and phytohormone resources) through-out the growing season (Baskin and Baskin, 1998;Galloway, 2002; Grzesik et al., 1998; Roach and Wulff, 1987).Despite most studies have evaluated the impact of thetemporal changes in prevailing environmental factorsduring seed maturation on germination level and speedat certain temperature and light condition, fewer studiesassessed the impacts of these factors on light andtemperature requirements during seed germination (e.g.,El-Keblawy and Al-Ansari, 2000; Gutterman, 1991,2000; Meyer et al., 1989). Similarly, several studies have reported the increase in germination after dry seedstorage (e.g., Allen and Meyer, 2002; El-Keblawy and Al-Ansari, 2000; Gutterman, 2000; Hidayati et al., 2002), but few studies examined the effect of storageon light and temperature requirements during germina-tion (Gutterman et al., 1998; Orozco-Segovia et al., 2000; Qaderi et al., 2003). Prosopis juliflora  (Sw.) D.C. (mesquite), native toCentral and South America, was introduced to severaldeserts in tropical and subtropical regions, including theArab Gulf, for greening of landscapes and for sand anddesertification control (Ghazanfar, 1996; Western, 1989). In the United Arab Emirate (UAE), it hasescaped forested areas and invaded both natural andmanaged habitats, including farms, and has beenassociated with habitat degradation and land abandon-ment. Mesquite is highly aggressive and coppices well,so that it often crowds out native vegetation (El-Keblawy and Al-Rawai, 2005; Shiferaw et al., 2004). P. juliflora  is an evergreen shrub. In the UAE, itproduces flowers and fruits in March–May in inlands(about 100–150km from the coasts), but from Octoberto May near the coastal areas (Al-Rawai, 2004).Shiferaw et al. (2004) have shown that animals, bothdomestic and wild, are very important dispersal agentsfor seeds of   P. juliflora . Seeds are adapted forendozoochory, i.e., dispersal by animals internally. Theyare embedded in an attractive succulent nutritious fruit,technically known as ‘reward’ (for the disperser), andsurvive the passage through the animal’s gut by thepossession of tough seed coats, which protect them fromthe chemical and abrasive action they encounter in thegut (Shiferaw et al., 2004; Razanamandranto et al., 2004). The hard seed coat creates a physical dormancyin  P. juliflora , which can be broken by seed pre-treatment with sulphuric acid, by passing through thedigestive track of animals, and by the treatment withboiling water (Shiferaw et al., 2004; Pasiecznik et al., 1998).The invasion of an introduced species to its newhabitats is depending mainly on its adaptation in thosehabitats. Several ecological and life history traits dousually determine the invasive ability of an introducedspecies to new habitats. Understanding some of the lifehistory traits, such as growth, reproduction andgermination ecology, would help in understanding thebiology of an invasive species and consequently help todevelop guidelines for its control (Fowler and Larson,2004; Sakai et al., 2001; Woitke and Dietz, 2002). The aim of the present study was to evaluate the effects of the time of seed maturation and dry seed storage on seeddormancy and light and temperature requirementsduring germination of the invasive  P. juliflora  underthe arid environment of the UAE. Material and methods Seeds of   P. juliflora  were collected during November2001 (autumn), February 2002 (winter) and May 2002(spring) from a large population near Fujira, on thecoast of the Gulf of Oman in the eastern region of UAE.Ripe pods were collected from the trees to ensure thatthe seeds are ripened on the targeted season. Seeds were ARTICLE IN PRESS A. El-Keblawy, A. Al-Rawai / Flora 201 (2006) 135–143136  randomly collected from the whole population torepresent the genetic diversity of the population. Seedswere separated from the pods using sharp knives anddivided into two groups. The first was germinated within7 days of their harvest; hereinafter, these seeds will bereferred to as fresh seeds. The second was dry-stored inpaper bags at room temperature (20 7 3 1 C) for 8months. Stored seeds were surface sterilized with thefungicide Phygon.The germination was conducted in 9-cm petri-dishescontaining one disk of Whatman No. 1 filter paper, with10ml distilled water. Fresh and stored seeds of the threecollections were germinated in three incubators, set at15, 25 and 40 1 C in both continuous light and darkness.Three replicates of 25 seeds each were used for eachtreatment. The dishes were wrapped in aluminium foil toprevent any exposure to light (during dark treatment).Seeds were considered to be germinated with theemergence of the radicles. Germinated seedlings werecounted and removed every alternate day, for 20 daysfollowing seed sowing.The rate of germination was estimated using amodified Timson index of germination velocity ¼ P G  / t ,where  G   is the percentage of seed germination at 2dintervals and t is the total germination period (Khan andUngar, 1984). The maximum value possible was 1000/20 ¼ 50. The greater the value, the more rapid is thegermination.A four-way analysis of variances (ANOVA) wasperformed to test the effects of the main factors (time of seed collection, seed storage and temperature and lightof incubation) on final germination percentage of  P. juliflora . A three-way ANOVA was carried out toevaluate the effects of time of seed collection, seedstorage and temperature of incubation on the germina-tion rate. Tukey test (Honestly significant differences,HSD) was used to estimate least significant rangebetween means. The germination percentages werearcsine-transformed to meet the assumptions of ANO-VA. This transformation improved normality of thedistribution of the data. All statistical methods wereperformed using SYSTAT, version 10.0. Results Four-way ANOVA showed significant effects for timeof seed maturation, seed storage, light and temperatureof incubation on final germination percentage of  P. juliflora  ( P  o 0 : 01, Table 1). In addition, three-wayanalysis of variances showed significant effects for timeof seed maturation, seed storage and temperature of incubation on germination rate of   P. juliflora  ( P  o 0 : 05,Table 2). Generally, seeds collected in autumn attainedsignificantly greater and faster germination than thosecollected in winter and spring. Storage significantlyincreased final germination percentage and germinationrate. The overall germination was greater in light than indark.The interactions between seed storage and both timeof seed maturation and temperature of incubation aswell as the interaction between the three factors on finalgermination and germination rate were significant( P  o 0 : 001, Tables 1 and 2 and Fig. 1). The germination at 40 1 C was significantly greater and faster than at 15and 25 1 C for fresh seeds of autumn and winter, but notfor fresh seeds of spring. Fresh seeds matured in autumngerminated significantly greater than seeds matured inwinter. The germination was significantly increased afterseed storage. Germination of stored seeds was signifi-cantly greater and faster for seeds of winter than seeds of spring, but was lower than seeds of autumn. This ARTICLE IN PRESS Table 1.  Four-way ANOVA tests the effect of seed storage,time of seed maturation and light and temperature of incubation on final germination percentage of   Prosopis juliflora Source of variation df MS  F  -ratio  P  Maturation time (MT) 2 0.579 118.1  o 0.001Storage (S) 1 2.894 590.7  o 0.001Temperature (T) 2 0.166 33.8  o 0.001Light (L) 1 0.387 78.9  o 0.001MT  S 2 0.346 70.6  o 0.001MT  T 4 0.002 0.34 nsMT  L 2 0.074 15.12  o 0.001S  T 2 0.124 25.25  o 0.001S  L 1 0.013 2.55 nsT  L 2 0.127 25.99  o 0.001MT  S  T 4 0.059 12.02  o 0.001MT  S  L 2 0.002 0.39 nsMT  T  L 4 0.009 1.88 nsS  T  L 2 0.040 8.18  o 0.01S  MT  T  L 4 0.010 2.14 nsError 72 0.005 ns ¼ insignificant at  P  ¼ 0 : 05 : Table 2.  Three-way ANOVA tests the effect of seed storage,time of seed maturation and temperature of incubation ongermination rate of   Prosopis juliflora Source of variation df MS  F  -ratio  P  Maturation time (MT) 2 345.406 64.18  o 0.001Storage (S) 1 2480.66 460.92  o 0.001Temperature (T) 2 343.878 63.89  o 0.001MT  S 2 291.378 54.14  o 0.001MT  T 4 7.170 1.33 nsS  T 2 18.774 3.49  o 0.05S  MT  T 4 38.340 7.12  o 0.001Error 36 5.382 ns ¼ insignificant at  P  ¼ 0 : 05.A. El-Keblawy, A. Al-Rawai / Flora 201 (2006) 135–143 137  indicates that dormancy breakage was greater for winterthan for spring seeds. Germination rate was slow forfresh seeds and there was no great variation betweenseeds of the three seasons at the three tempera-tures. Storage significantly increased germination rate,especially for autumn seeds. Final germination and ARTICLE IN PRESS Germination (%) Germination rate 0020406080100 Final germination (%) Fresh seeds 1020304050 Germination rate Fresh seeds 0020406080100 Final germination (%) Stored seeds 1020304050 Germination rate Stored seeds 152540Temperature ( ° C)Temperature ( ° C)020406080100 Final germination (%) Overall fresh and stored seeds 15254001020304050 Germination rate Overall fresh and stored seeds 152540Temperature ( ° C)Temperature ( ° C)152540152540Temperature ( ° C)Temperature ( ° C)152540 Fig. 1.  Effect of time of seed maturation, seed storage and temperature during incubation on final germination percentages andgermination rate of   Prosopis juliflora  seeds. Dark columns ¼ autumn-matured, white columns ¼ winter-matured and hatchedcolumns ¼ spring-matured seeds. Vertical bars represent SE. A. El-Keblawy, A. Al-Rawai / Flora 201 (2006) 135–143138  germination rate were greater, respectively, in storedseeds of autumn and winter than in fresh seeds of thesame seasons by 280% and 392% at 15 1 C and by 96%and 174% at 25 1 C, but by 28% and 82% at 40 1 C. Thedifference between stored and fresh seeds of spring,however, was 22% at both 15 1 C and 25 1 C (Fig. 1). Thisresult indicates that the need for high temperature toachieve greater germination in fresh seeds was signifi-cantly reduced after seed storage, especially for seedsmatured in autumn and winter.The interaction between time of seed maturation andlight of incubation on final germination was significant( P  o 0 : 001, Table 1). Germination in light was signifi-cantly greater than in dark for seeds matured in autumn,but not for seeds matured in winter and spring. Theinteraction between seed storage, and temperature andlight of incubation on final germination was alsosignificant ( P  o 0 : 001, Table 1). Germination of freshseeds at 40 1 C was significantly greater than at both15 1 C and 25 1 C in light, but was significantly greaterthan it at 15 1 C only in dark. However, there were nosignificant differences between germination of storedseeds at the three temperatures in both light and dark.At 15 and 40 1 C, the germination in light was greaterthan in dark by 59% and 106% for fresh seeds, but by12% and 20%, respectively, for stored seeds. At 25 1 C,there was no significant difference between light anddark germination for both fresh and stored seeds(Fig. 2). The results indicate that seed storage reducedlight requirement for attaining high germination,especially at 40 1 C.The ANOVA showed also that the interactionbetween light and temperature is significant ( P  o 0 : 001,Table 1). There was no significant difference between thegermination in light and dark at 15 and 25 1 C, but lightgermination was significantly greater than that of darkwhen seeds germinated at 40 1 C. The germination at 15,25 and 40 1 C were 35.3%, 47.2% and 41.7%, respec-tively, in dark, but were 43.3%, 47.5% and 62.2%,respectively, in light. Discussion Temporal and spatial heterogeneity in natural envir-onments tend to enhance the selective advantage of multiple strategies (Venable et al., 1995). Seed hetero- morphism, which is the production by individual plantsof seeds that differ in morphology, dormancy andgermination requirements is a strategy described fairlycommonly in species that inhabit unpredictable andharsh environments, such as frequently disturbedhabitats (Bra ¨ndel, 2004; Harper, 1977; Manda  ´k andPys ˇek, 2005) and arid and semiarid environments (El-Keblawy, 2003; Venable and Lawlor, 1980). The result of the present study showed that the germinationpercentage, germination speed and germination require-ments of   P. juliflora  differ significantly between differenttimes of seed collection. This heteroblasty ensures thateven under optimal conditions only a portion of theseeds will germinate at one time, which is particularly ARTICLE IN PRESS A: Germination in light B: Germination in dark FreshStoredSeed storage020406080100 Final germination (%) FreshStoredSeed storage020406080100 Final germination (%) Fig. 2.  Effect of seed storage and temperature during incubation on final germination of   Prosopis juliflora  seeds. Blackcolumns ¼ 15 1 C, white columns ¼ 25 1 C and hatched columns ¼ 40 1 C. Vertical bars represent SE. A. El-Keblawy, A. Al-Rawai / Flora 201 (2006) 135–143 139
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