plant life-form and germination in a mexican inter-tropical desert: effects of soil water potential...

Journal of Arid Environments (2001) 47: 485–497doi:10.1006/jare.2000.0728, available online at http://www.idealibrary.com on

Plant life-form and germination in a Mexicaninter-tropical desert: effects of soil water potential

and temperature

Joel Flores & Oscar Briones*

Instituto de Ecologn&a, A.C., Apartado Postal 63, C.P. 91000, Xalapa,Veracruz, Me&xico

(Received 6 April 2000, accepted 22 September 2000, published electronically 15 February 2001)

We investigated the effects of soil water potential (SWP) and temperatureon seed germination of six coexisting species of an inter-tropical desert. Thesespecies include three life-forms; the shrubs Cercidium praecox and Prosopislaevigata; the columnar succulents Neobuxbaumia tetetzo and Pachycereushollianus; and the arborescent semi-succulents Beaucarnea gracilis and Yuccapericulosa. In the six species germinability increased and germination time andspeed of germination (t50) decreased as SWP decreased. The SWP treatmentswere 0 MPa, !0)12 MPa, !0)2 MPa, !0)41 MPa and !0)66 MPa. Therewas, however, a SWP threshold below which germination time and t50 increasedand germinability decreased. The shrubs had the highest germinability whereasthe columnar succulents had the lowest. The shrubs also had shorter germinationtime and t50 than arborescent semi-succulents whereas seeds of the columnarsucculents were indeterminate. In all species except P. laevigata germinabilityincreased and the germination time and t50 decreased as temperature increased.The temperature treatments were 123C, 203C and 263C. The shrubs had theshortest t50 and germination time and the highest germinability at all temper-atures. Arborescent semi-succulents had the lowest germinability and longestgermination time and t50 at the three temperatures treatments. Our resultssupport the hypothesis that in desert environments different plant life-formsutilise different germination strategies to persist.

( 2001 Academic Press

Keywords: arid environments; germination; plant life-forms; soil waterpotential; temperature

Introduction

In desert environments rainfall is extremely variable in space and time; and droughts areextended (Noy-Meir, 1973; McGinnies, 1979). These characteristics are likely toaffect plant physiology and ecology (Westoby, 1980). Two critical steps in thelife-cycle of most plant species are seed germination and seedling establishment(Kozlowski & Gunn, 1972; Harper, 1977; Gutterman, 1993; 1994; Esler & Phillips,1994). It has been suggested that, in desert environments, soil water potential (SWP)and temperature are the key factors affecting seed germination (Potter et al., 1986;Cluff & Roundy, 1988; El-Sharkawi & Farghali, 1988).

*Corresponding author. Fax: #52 (28) 187809. E-mail:[email protected]

0140}1963/01/040485#13 $35.00/0 ( 2001 Academic Press

486 J. FLORES & O. BRIONES

Germination is the physiological process through which growth is re-established in theembryo (Bewley & Black, 1978). The process starts with seed hydration and ends withthe emergence of the radicle (Bewley & Black, 1994; GonzaH lez-Zertuche & Orozco-Segovia, 1996; Bewley, 1997; Welbaum et al., 1998). It is believed that the seed criticalhydration level (i.e. the amount of water absorbed by a seed before embryo emergence)is species-specific (Hadas & Russo, 1974a, b). Therefore, after a seed is embedded insoil, the rate of water intake directly affects the germination. Seed water intake maybe regulated by the water potential and the resistance of water movement at the seed-soilinterface (Evans & Etherington, 1990).

It has been reported that seeds of some plant species are adapted to germinate understressing water conditions. In arid zones some species germinate at water potentialslower than !0)8 MPa (Potter et al., 1986; BriedeH & McKell, 1992; Qi & Redmann,1993; Facelli & Ladd, 1996; IbaH n8 ez & Passera, 1997; Larson & Kiemnec, 1997; Adams,1999). This is not surprising since high SWP are rarely achieved in the soil surface ofarid environments. Furthermore, it has been reported that some plant species exhibit aninverse correlation between germination and substrate water potential until a lowerthreshold below which germination decreases (Potter et al., 1986; IbaH n8 ez & Passera,1997).

Desert plants are reported to have faster germination than other type of plants (Meyer& Monsen 1992; Shaw et al., 1994; Flores & Jurado, 1998; Teketay, 1998; Jurado et al.,2000). Jurado & Westoby (1992) found that germination is in general faster in aridenvironments than in those with high water availability. Although it has been argued thata high speed of germination is vital in arid and semi-arid ecosystems in which wateravailability is low (Evans & Etherington, 1990), we are aware of no study addressinghow different SWP affects the speed of germination of desert plants.

Another factor that may be important in seed germination in arid environments is soiltemperature (Baskin & Baskin, 1998a). There are groups of species in which optimumgermination temperatures coincide with the average summer or winter temperatures oftheir local habitats (Went, 1949; Juhren et al., 1956; Tevis, 1958; Beatley, 1974; Friedelet al., 1988; Jurado & Westoby, 1992; Gutterman, 1993). Other investigations havedocumented that when seed germination temperature is close to the optimum, waterpotential become less limiting (Scifres & Brock, 1969; Knipe, 1973; Bokhari et al., 1975;Hegarty, 1975; Potter et al., 1986; Romo & Haferkamp, 1987; Cluff & Roundy,1988; Eddleman & Romo, 1988; El-Sarkawi & Farghali, 1988; Romo et al., 1991;Qi & Redmann, 1993; Dahal et al., 1996).

We conducted two experiments to investigate the effects of SWP and temper-ature on germinability, time of germination and speed of germination (t50) of sixco-existing species representing three plant life-forms. If the desert life-forms werefunctional groups (KoK rner, 1994) with different persistence strategies (Shreve,1951; Crosswhite & Crosswhite, 1984; Cody, 1986; 1989; 1991), we expected to finda correlation between life-form and germination.

Materials and methods

Study site

The study site is located in the ZapotitlaH n Valley (18320@N;97328@W). This valley is partof the more extensive inter-tropical desert of TehuacaH n in west central Mexico(Rzedowski, 1978). The dominant vegetation in the site is xerophytic heath-land withdominance of shrubs, columnar succulents and arborescent semi-succulents (Montan8 a& Valiente-Banuet, 1998). The TehuacaH n Valley is considered as a high biodiversitycore world wide in which the research on, and the conservation of natural resources arepriorities (IUCN, 1990).

PLANT LIFE-FORM AND GERMINATION IN DESERT 487

The weather is dry for most of the year with sporadic rain falls from May to August inmost years but rain may fall in September in some years. The total average annual rainfall in the area is 400 mm and the annual temperature ranges from 183C to 223C(Zavala-Hurtado, 1982).

Plant species and life-forms

We selected two species of each of the three dominant life-forms in the study site; (1) theshrubs Cercidium praecox (Ruiz & PavoH n), Caesalpiniaceae and Prosopis laevigata(Humb. & Bonpl. ex Willd.) M.C. Johnston, Mimosaceae; (2) the columnar succulentsNeobuxbaumia tetetzo (F.A.C. Weber) Backeb. var. tetetzo, Cactaceae and Pachycereushollianus (F.A.C. Weber) F. Buxb., Cactaceae; (3) the arborescent semi-succulentsBeaucarnea gracilis Lem., Nolinaceae and Yucca periculosa F. Backer, Agavaceae. Theseeds used in the experiments were collected from at least 10 individuals of each species.

In order to avoid life-history phylogenetic constraints (Harvey & Pagel, 1991) weattempted to select species in different families for a life-form. In the case ofcolumnar succulents this was not possible since all species are in the family Cactaceae,but the two species studied belong to different sub-tribes, Pachycereinae(P. hollianus) and Cephalocereinae (N. tetezo).

Germination under different soil water potentials (SWP)

We calculated a characteristic curve of the relationship between water content in a givenamount of soil and SWP (Rundel & Jarrel, 1989). To do this, we used five metal potseach containing 25 g sieved (2 mm) and dried (48 h at 803C) soil from the study site. Weadded 25 ml of distilled water to each pot; this was enough to take the soil to its fieldwater capacity. Then, all pots were weighed individually and air-dried for several days.Every day the pots were weighed and SWP determined in C-52 sample chambers witha Wescor HR-33T dew-point microvoltmeter (Wescor Inc., Logan, Utah, U.S.A.)calibrated with standard NaCl solutions (Rundel & Jarrel, 1989).

We investigated seed germination performance under five different SWP:0 MPa, !0)12 MPa, !0)2 MPa, !0)41 MPa and !0)66 MPa. These SWP wereachieved by adding 23)5 ml, 23 ml, 21 ml, 15 ml and 10 ml of distilled water to Petridishes containing 25 g of soil, respectively. There were five replicates per treatment andeach one included 20 seeds. All dishes were placed in a germination chamber (Lab-LineInstruments, Inc., Illinois, U.S.A.) with 12 h light at 263C and 12 h dark at 203C. Inorder to avoid temperature fluctuations we utilised flourescent lamps and the chamberwas air ventilated.

In order to avoid significant fluctuations of SWP in the Petri dishes we followed themethod of Etherington & Evans (1986). We placed a 3 mm thick transparent polyethy-lene disk and a polyethylene mesh disk on the top of the seeds. Etherington & Evans(1986) showed that the mesh disk did not affect O2 availability for the seeds. ThePetri dish lid was firmly pressed and sealed with parafilm. With this design, seeds wereexposed to light but water condensation on the lid, the seeds, or the soil surface wasprevented. At the beginning of the experiment, all dishes were weighed and thenreweighed daily. Seed germination was recorded every day. From these observations weobtained; (1) germinability, calculated as the percentage of germinated seeds over a timeperiod of 15 days (Flores & Jurado, 1998). Baskin & Baskin (1998b) have mentionedthat germination tests should be terminated after about 2 weeks, because most of theseeds germinate within 10 days or less. We also used this time span to simulate theexpected number of germinated seeds after a single rain fall event (Jurado & Westoby,1992; Flores & Jurado, 1998); (2) Germination time, calculated as the day at which the


first germination ocurred (Martin et al., 1995); (3) Speed of germination or half-time ofgermination (t50), calculated as the time (in days) at which 50% of the seeds germinated(Grime et al., 1981; Thompson & Grime, 1983; Jurado & Westoby, 1992; Flores& Jurado, 1998; Teketay, 1998; Adams, 1999).

Germination under different temperature regimes

Seed germination at 123C, 203C and 263C (corresponding to average temperatures ofwinter, autumn and summer at the study site respectively) was investigated. Sets of seedsfor each species were placed on moist filter paper in Petri dishes; five replicates with 20seeds for each temperature and plant species. Seeds were exposed to 12-h light and 12-hdark. Seed species with hard ectoderm (C. praecox and P. laevigata) were scarified withsand paper since germinability is too low otherwise (Everitt, 1983; Jurado & Westoby,1992; VaH zquez-Yanes & Orozco-Segovia, 1993; Flores & Jurado, 1998; Teketay, 1998;Jurado et al., 2000). Seeds were watered with distilled water and germination waschecked daily. From these we estimated the germination time, germinability and t50 foreach replicate. We did not investigate the interactive effects between temperatureand SWP due to the difficulty to manage a large number of replicates in thegermination chamber.

Statistics

We used GLM-ANOVA to explore the effects of SWP and temperature ongermination time, germinability and t50 among plant species and life-forms. The ana-lyses were performed with the GLIM 4 software (Francis et al., 1993). The six plantspecies and the five SWP or three temperatures were used as main factors. Potentialdifferences within each plant species were explored with the t-test weighed byBonferroni (Sokal & Rohlf, 1995). We also pulled the data for each life-form (1) shrubs;(2) columnar succulents; (3) arborescent semi-succulents; and the differencesamong them were tested by contrasts. Because germinability was measured as percent-age we used the binomial error distribution to fit the GLM model; we also re-scaled thedeviance since our data were over-dispersed and we used the statistic F instead of s2

(Crawley, 1993). In the models fitted to germination time and t50 we used the gammaerror distribution since the variance was not homogeneously distributed and the errordistributions was bias (Aitkin et al., 1989; Crawley, 1993).

Results

Germination under different soil water potentials (SWP)

None of the SWP treatments inhibited seed germination except in P. hollianus at 0 MPa.As SWP decreased, germinability increased and t50 and germination time decreased.There was, however, a threshold (!0)41 MPa) below which the three germination-related parameters were negatively affected (Tables 1 and 2). Must species hadtheir highest germinability at !0)41 MPa except for P. laevigata, in which germinabilitywas not significantly different between the treatments !0)41 MPa and !0)66MPa, and N. tetetzo between !0)66 MPa and !0)2 MPa. Only B. gracilis andY. periculosa had a statistical significant reduction of germination below the !0)41 MPathreshold.

The germination time of C. praecox and P. hollianus was shortest (p(0)05) at!0)41 MPa. The germination time of N. tetetzo and B. gracilis was shortest (p(0)05)

Table 1. Effects of soil water potential (SWP) on germinability of six species of the desert in the Tehuaca&n Valley, Me&xico

Germinability*

Species Life-form (LF) SWP (MPa)

0 !0)12 !0)2 !0)41 !0)66 LF Average

Cercidium praecox Shrub 77a 77a 90a 100b 74a 82)51

Prosopis laevigata Shrub 64a 71a 80a 100b 92b

Neobuxbaumia tetetzo Columnar succulent 7a 15a 47b 70b 78b 42)62

Pachycereus hollianus Columnar succulent 0a 3a 50b 94c 62b

Beaucarnea gracilis Arborescent semi-succulent 31a 40a 74a 69a 3b 52.03

Yucca periculosa Arborescent semi-succulent 47a 62a 82a 83a 29b

*Average from five replicates.Different letters and numbers indicate significant differences within each species (p(0)05) and among life-forms (p(0)01), respectively.

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Table 2. Effects of soil water potential (SWP) on germination time and speed of germination (t50) of six species of the desert in theTehuaca&n Valley, Me&xico

Germination time* t50*

Species Life-form (LF) SWP (MPa) SWP (MPa)

0 !0)12!0)2 !0)41!0)66 LF 0 !0)12!0)2 !0)41!0)66 LF

Cercidium praecox Shrub 1)6b 2)0c 1)6b 1)0a 4)6d 1)61 2)2b 2)2b 2)0b 1)4a 5)8c 2)41

Prosopis laevigata Shrub 1)0a 1)0a 1)0a 1)0a 1)4b 2)2b 2)2b 2)0b 1)0a 3)0c

Neobuxbaumia tetetzo Columnar succulent 3)3c 1)2a 1)8b 1)8b 3)2c 4)71,2 3)0b 1)8a 3)4c 3)0b 5)4d 5)41,2

Pachycereus hollianus Columnar succulent — 12)0c 6)4b 6)0a 6)2b — 13)0c 7)4b 7)0a 7)8b

Beaucarnea gracilis Arb. semi-succulent 9)4c 7)2a 8)2b 7)4b 11d 7)72 10)2c 9)0a 10)8c 9)8b 11c 9)82

Yucca periculosa Arb. semi-succulent 6)4a 6)4a 6)0a 7)6b 8)8c 8)8b 8)4a 8)6a 11)8c 10b

*Average value from five replicates.Different letters and numbers indicate significant differences within each species (p(0)05) and among life-forms (p(0)05), respectively.

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Table 3. Effects of temperature on germinability, germination time and speed of germination (t50) of six species of the desert in theTehuaca&n Valley, Me&xico

Species Life-form (LF) Germinability* Germination time* t50*

123C 203C 263C LF 123C 203C 263C LF 123C 203C 263C LF

Cercidium praecox Shrub 83b 98a 98a 95)71 7c 2b 1a 3)71 7)6c 2)4b 1)4a 3)81

Prosopis laevigata Shrub 95a 100a 100a 8b 2a 2a 8)2b 1)6a 1)4a

Neobuxbaumia tetetzo Columnar succulent 5b 11b 100a 48)82 9b 6b 2a 5)8 2 9)5b 6)5b 1)6a 6)02

Pachycereus hollianus Columnar succulent 15b 76a 86a 8b 6b 4a 8)0c 6)5b 3)8a

Beaucarnea gracilis Arb. semi-succulent 0b 2b 95a 43)83 — 12b 6a 8)33 — 11)0b 6)8a 9)93

Yucca periculosa Arb. semi-succulent 0c 72b 94a — 9b 6a — 10)7a 11)2a

*Average values from five replicates.Different letters and numbers indicate significant differences within each species (p(0)05) and among life-forms (p(0)01), respectively.

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at !0)12 MPa; Y. periculosa had its shortest germination times (p(0)05) at 0, !0)12and !0)2 MPa, and P. laevigata had its longest germination time (p(0)05) at!0)66 MPa.

The t50 was shortest in C. praecox, P. laevigata and P. hollianus (p(0)05) at!0)41 MPa. In N. tetetzo and B. gracilis the shortest t50 (p(0)05) was at !0)12 MPa,whereas in Y. periculosa (p(0)05) was at !0)12 and !0)2 MPa.

We also found that germinability and plant life-form were correlated (Table 1).The shrubs had a higher percentage of germination than arborescent semi-succulentswhich had higher germinability than the columnar succulents (p(0)05). We alsofound similar relationships between plant life-forms and t50 and germination time(Table 2). The shrubs had the shortest germination time and t50; the arborescentsemi-succulents had the longest times (p(0)01); columnar succulents wereintermediate.

Germination under different temperature regimes

Temperature had significant effects on germinability, t50 and germination time(p(0)01; Table 3). In general, germinability increased as temperature increased butP. laevigata. C. praecox, and P. hollianus had their highest germinability (p(0)05) at203C and 263C whereas N. tetetzo, B. gracilis and Y. periculosa had their highestgerminability (p(0)05) at 263C.

The germination time and t50 decreased as temperature increased. Cercidium praecox,N. tetetzo, P. hollianus and B. gracilis had their shortest germination time andt50 (p(0)05) at 263C whereas germination time and t50 of P. laevigata was shortest(p(0)05) at 203C and at 263C; Y. periculosa had a shorter germination time (p(0)05)at 263C than at 203C, but the t50 was similar in these temperature treatments.

We found that germination correlated with plant life-form (Table 3). The shrubs hadthe highest germinability, and shortest germination time and t50 whereas the arborescentsemi-succulents had the lowest germinability and longest t50 and germination time(p(0)01). Columnar succulents were intermediate from the other two life-formgroups. In addition, arborescent semi-succulents did not germinate at low (123C)temperature.

Discussion

The relationships among patterns of germinability, t50 and germination time with SWPand temperature document important adaptations of the studied species to arid environ-ments. Rainfall at the study site is highly variable and the soil rarely is maintained at itsfield water capacity. Thus, seeds adapted to germinate at high soil humidity, but notnecessarily at the soil field water capacity, would have an advantage germinating in aridenvironments (Evans & Etherington, 1990). However, this may have a trade-offbetween germination at relatively low SWP and seedling establishment since seedlingsusually need higher SWP to compensate for transpiration (Mott, 1974; Evans& Etherington, 1991; Adams, 1999).

The observed effects of SWP on shrubs are consistent with the effectsreported in other arid environments. Potter et al. (1986) found that some populations ofthe North American shrub, Atriplex canescens had higher germinability at low waterpotential, than other populations of this species. Also, Choinski & Tuohy (1991) foundthat the South African shrubs Combretum apiculatum and Colophospermum mopanehad high germinability over a wide range of soil water potentials. A similar result wasfound by IbaH n8 ez & Passera (1997) for the Spanish shrub Anthyllis cytisoides. Thepublished evidence and our observations, however, oppose the germination behaviour of


most plants in tropical and temperate habitats, and crops and annual plants in aridenvironments. For these the optimum water potential for germination coincides with thesoil field water capacity and seed hydration, germination rate and germinability alldecrease as water potential drops (Williams & Shaykewich, 1971; Mott, 1972; Romo& Haferkamp, 1987; Cluff & Roundy, 1988; Meidan, 1989; Evans & Etherington,1990; Choinski & Tuohy, 1991; Khatri et al., 1991; Kiemnec & Larson, 1991; Qi& Redmann, 1993; Facelli & Ladd, 1996; Swaegel et al., 1997; Al-Karaki, 1998; Rowseet al., 1999).

It is known that oxygen becomes limiting for seed germination in close to anaerobicconditions. This could be the reason why we observed a low germination at 0 MPa, sincethe excessive water content of soil could diminish the availability of oxygen to the seeds(Bradbeer, 1988).

We are aware that naturally occurring seeds are exposed to temperature fluctuationswhile germinating. Therefore our findings under constant temperatures may differfrom field germination (Thompson & Grime, 1983; Probert, 1992). However, webelieve that even our oversimplified simulation of seasonal temperatures in the control-led experiments uncovered naturally occurring differences in the germinationbehaviour among different plant life-forms. Our data suggest that columnar succu-lents and arborescent semi-succulents are adapted to germinate at high temperaturessince germination of these species was low at 123C. This is in accordance with previousstudies in TehuacaH n with the columnar succulents N. tetetzo, P. hollianus andCephalocereus chrysacanthus (Rojas-AreH chiga et al., 1998). These species showed60–80% germination at 203C and 253C but germination dropped to 10–40% and 5–25%at the extreme temperatures of 103C and 403C respectively. In the Sonoran Desert,McDonough (1964) found that more than 90% germination of the columnar succulentsCarnegiea gigantea and Lemaireocereus thurberi occurred at 253C but neither of themgerminated at 153C. The arborescent semi-succulents Yucca elata and Y. brevifoliagerminated 5–20% and 0% respectively at 103C (McClearly & Wagner, 1973), but theformer species germinated 94% and the latter germinated 61% at 233C (Keeley& Meyers, 1985). By contrast, the temperatures used in this study did not affect thegermination of shrub species. A similar finding has been reported from xerophyticshrubs in different arid environments (Jurado & Westoby, 1992; Flores & Jurado,1998; Teketay, 1998; Jurado et al., 2000).

Some species presented higher germination in the temperature-controlled experimentthan in the SWP-controlled experiment. Emmerich & Hardegree (1991) suggest thatrepeated changes in the substrate water potentials due to water evaporation and the sub-sequent addition of more water to substrate has a significant effect of increasinggermination. Although we did not quantify the water potential in the temperature-controlled experiment, it varied repeatedly since the substrate used in the experiment(filter paper) loses water by evaporation and we constantly added water to keep thesubstrate wet. Dubrovsky (1996; 1998) suggests that such responses to discontinuoushydration indicate that seeds are tolerant to drought even after they are hydrated.

Overall our results showed that different life-forms have differential re-sponses to a range of temperatures and SWP. This is in accordance with the idea that inarid and semiarid environments different life-forms use distinct strategies to persist(Shreve, 1951; Crosswhite & Crosswhite, 1984; Cody, 1986; 1989; 1991). This ishowever not restricted to arid environments since Grime et al. (1981) found that grassesgerminated faster than annual weeds and woody species in plant communities nearSheffield, U.K.

We thank Carlos Montan8 a and Arturo Flores for help discussions. We also thank Irma G. de losSalmones and Alejandro Flores for technical assistance. Enrique Jurado made useful comments onan earlier version of the manuscript. Support was provided by CONACYT.


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plant life-form and germination in a mexican inter-tropical desert: effects of soil water potential...

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