Journal of Arid Environments (1999) 43: 437–448Article No. jare.1999.0567Available online at http://www.idealibrary.com on

Germination of Callitris seeds in relation totemperature, water stress, priming, and

hydration}dehydration cycles

Robyn Adams*

School of Ecology and Environment, Deakin University (Rusden),662 Blackburn Road Clayton, Victoria 3168, Australia

( Received 12 March 1998, accepted 1 July 1999)

The effects of temperature, water stress, hydration}dehydration cyclesand seed priming on the germination of Callitris verrucosa and Callitris preissii,two Australian semi-arid coniferous tree species, were investigated. Optimumgermination occurred at 183C, with a minimum germination time of 8}9 daysfor both species. At this temperature, germination was inhibited at osmoticpotentials lower than !1 ) 0 MPa, but the capacity to germinate at low osmoticstress increased as the temperature decreased. Seed priming and hydration}dehydration cycles did not reduce seed viability, and Callitris seeds appear toretain the physiological changes induced by short-term hydration, as the timeto the onset of germination was decreased to about 3 days. The capacity ofCallitris seeds for incremental germination is likely to increase overall germina-tion success in a low rainfall environment.

( 1999 Academic Press

Keywords: Callitris; germination; water stress; hydration}dehydration; semi-arid; Australia

Introduction

Water availability is a major factor limiting the regeneration of arid and semi-arid species(Noy}Meir, 1973). Moisture conditions immediately prior to, and during germination,rather than temperature, play a dominant role in regulating germination (Koller, 1955;Karssen, 1982). In arid and semi-arid zones, rainfall events are highly unpredictable,droughts common, and germination followed by establishment events for tree speciesare rare (Chesterfield & Parsons, 1985; Kennenni & van der Maarel, 1990; Woodell,1990).

Callitris verrucosa (Cunn. ex Endl.) F. Muell. and Callitris preissii Miq. are twosemi-arid zone conifers endemic to Australia. They occupy a typically Mediterraneanenvironment of summer drought and winter rainfall, although unpredictable summerrainfall is not uncommon (Hobbs et al., 1995). Their distributions have been severelyreduced as a result of clearing (Bowman & Harris, 1995), and even in protectedconservation reserves, their regeneration is highly sporadic. There is no appreciable soilseed bank (Adams, 1982), and as C. preissii is a non-serotinous species, and C. verrucosa

* E-mail address: radams@deakin.edu. au

0140}1963/99/120437#12 $30.00/0 ( 1999 Academic Press

438 R. ADAMS

is only weakly serotinous, few seeds are stored on the adult trees. Recruitment istherefore dependent on a regular input of seeds coinciding with conditions favourablefor germination, which in turn must coincide with suitable establishment conditions.The absence of seed dormancy and the likelihood that most seeds of Callitris germinateon the soil surface (Adams, 1982), increase seed exposure to unfavourable temperaturesand to short-term fluctuations in soil moisture availability.

For many arid species, the presence of water soluble inhibitors in the seed coat(Koller, 1955; Koller & Negbi, 1959) prevents germination under such unfavourableconditions, until sufficient rain has fallen to leach the inhibitor and ensure thatadequate moisture is present in the soil to enable seedling establishment. Many otherarid zone species also lack complex dormancy mechanisms, and once the water andtemperature requirements for germination are satisfied, germination begins. The role oftemperature, and the relationship between decreasing germination rate and percentagegermination with increasing water stress, are generally well established (Koller & Hadas,1982) for a wide range of grasses, herbs, and shrubs (Freeman, 1973; Mott, 1974; Piatt,1976; Hagon & Chan, 1977; Meidan, 1990), but relatively few tree species have beeninvestigated (Bonner, 1968; Edgar, 1977; Kaufmann & Eckard, 1977), and even fewerarid zone tree species. In addition, in environments where rainfall events may befrequent but small, seeds are likely to become partially hydrated, or undergo periodichydration and subsequent dehydration without completing germination. The ef-fects of partial hydration of seeds on viability, germination rate, and germinationpercentage are variable and not well understood.

Following the release of Callitris seeds over summer, seeds are in contact with soilswith high temperatures and low water potentials. The increasing frequency of lightshowers ((5 mm) as the cooler months approach increases the probability that seeds aresubjected to natural hydration}dehydration cycles, or that they become partially hy-drated (primed) and persist in that condition for some time. Koller (1955) showed thathydration}dehydration cycles can increase germination, Barbour (1968) reported noincrease in actual germination of Larrea divaricata, although he noted an increasedgermination rate, and Mott (1974) found a decrease in the abilities of three desertannuals to germinate following such cycles. The effects of hydration}dehydrationcycles are cumulative (Baskin & Baskin, 1982), and the variations in species’ responsesto these cycles are probably due to the degree of metabolic activity initiated during theperiod(s) of hydration (Morohashi & Shimokoriyama, 1977; Hegarty & Ross, 1978;Dubrovsky, 1996).

Ross & Harper (1972) have argued that it is the relationship between the number ofemerged seedlings and the number of subsequently emerging seedlings that is importantin terms of successful establishment. However, it is the absolute time of emergence whichis of prime importance for seeds that have to cope with unpredictable climatic conditions.Prolonging germination, or germination during summer, increases the risk of the develop-ing root being unable to keep pace with a retreating soil-moisture front (Leslie, 1965), andany strategy which reduces the time lag between the beginning of seed imbibition and seedgermination is likely to be critical for successful seedling recruitment.

The impacts of temperature, moisture stress, priming, and cycles of hydration}dehydration on germination of Callitris seeds were investigated experimentally, as anunderstanding of these factors is crucial for the successful management of these long-lived semi-arid species.

Materials and methods

Seeds of Callitris verrucosa and Callitris preissii were collected from Wyperfeld NationalPark, Victoria (latitude 35317@, longitude 140351@). Seed viability is variable, but isgenerally between 30}35% (Adams, 1982). All treatments consisted of three replicates

GERMINATION OF CALLITRIS SEEDS 439

of 20 seeds of each species, and seeds were considered germinated as soon as the radicleemerged. Ungerminated seeds were assessed for viability by cutting and examining theembryo. Germination was expressed as the percentage of viable seeds germinated.Germination was recorded for 40 days except where specified. All percentage data werearcsin transformed for analysis. Statistical analysis was carried out using KWIKSTATsoftware (TexaSoft, 1996).

Temperature

Replicates were placed for germination in the dark at 5, 10, 16, 18, 20, or 253C constanttemperatures, and two alternating temperatures of 32/133C and 23/93C. The latter wereon 12-hourly cycles and represented approximate mean maximum and minimumambient temperatures for summer and spring/autumn.

Osmotic stress

Replicates were exposed to osmotic solutions of 0 (distilled water), !0 ) 15 (&fieldcapacity),!0)5,!1)0,!1)5 (&wilting point), and!2)0,!2)5 or!3)0 MPa, preparedusing polyethylene glycol 6000 (Michel & Kaufmann, 1973; Emmerich & Hardegree,1990; Hardegree & Emmerich, 1990). Germination in the dark at the eight osmoticpotentials at two constant temperatures (183C, 123C) was recorded for 50 days.

Hydration}dehydration cycles

Replicates were subjected to 0, 1, 2, or 3 cycles of 24 hours of hydration at 183Cfollowed by 24 hours dehydration before being sown, two treatments of three hydration}dehydration cycles plus 7 days delay (3`7) before sowing, and two hydration}dehydration cycles plus 14 days delay (2`14) before sowing. Dehydration was carriedout by removing the lid of the petri dish and allowing the contents to dry to their originalweight in natural light at 183C (Vincent & Cavers, 1978). Seed moisture content was notdetermined. Delays were achieved by keeping the dehydrated seeds on dry filter paper inclosed petri dishes at 183C. Samples were sown onto Whatman seed test filter paper inpetri dishes at 183C and watered with distilled water.

Seed priming

Replicates were imbibed in a polyethylene glycol 6000 solution (Michel & Kaufmann,1973) at an osmotic potential of!1)5 MPa at 183C for 14 days, then washed, sownonto Whatman seed test filter paper in petri dishes at 183C and watered with distilledwater. Replicates germinated in distilled water without prior priming were used ascontrols.

Results

Temperature

No germination was recorded for either species at 5, 25, or 32/133C. The minimum timeto the onset of germination (Tmin) for both species was approximately 9 days at 183Cconstant temperature (Table 1). Significant increases in this time occurred at higherand lower temperatures, but at any given temperature the minimum germinationtime was not significantly different between species. The spring/autumn alternating

Table 1. Ewect of temperature on seed germination of Callitris verrucosa (cv)and Callitris preissi (cp). Tmin"mean ($S.E.) minimum time to onset of germina-tion; T50"mean ($S.E.) time to 50% viable seeds germinated; Tmax"mean($S.E.) maximum time to completion of germination; Max %"mean ($S.E.)maximum % viable seeds germinated. For each attribute the mean values with thesame letters are not signixcantly diwerent at 5% level of probability (Newman-

Keuls Multiple Comparisons Test)

Attribute Species 103C 163C 183C 203C 23/93C

Tmin cv 16 )7$0 )3 10 )3$0 )3 8 )7$0 )3 10 )7$0 )7 7 )3$0 )3(days) e d bc d a

cp 16 )7$0 )3 11 )0$0 )6 9 )0$0 )0 10 )7$0 )7 7 )7$0 )3e d c d ab

T50 cv 20 )3$0 )8 11 )7$0 )6 11 )5$0 )5 13 )0$0 )3 8 )8$0 )4(days) e ab ab b a

cp 18 )7$0 )2 14 )7$1 )0 12 )0$0 )7 16 )3$1 )3 12 )7$1 )5de bc ab cd b

Tmax cv 28 )3$3 )2 24 )3$3 )9 15 )0$0 )6 22 )0$0 )0 15 )3$2 )3(days) bcd abc a ab a

cp 32 )7$3 )0 34 )7$2 )6 19 )0$2 )5 37 )0$2 )1 27 )0$0 )0bcd cd ab d bcd

Max % cv 97 )7$2 )3 97 )3$2 )7 100 )0$0 93 )0$3 )6 96 )0$2 )1ab ab ab ab ab

cp 93 )3$6 )7 98 )0$2 )0 100 )0$0 100 )0$0 86 )7$2 )3ab ab a a b

440 R. ADAMS

temperature (23/93C) significantly reduced the minimum germination time from thatrecorded at 183C constant temperature by about 1)5 days for both species.

T50 for Callitris verrucosa at 183C was 11)5 days, and only showed a significantincrease at 103C. T50 for Callitris preissii at 183C was 12 days, and showed significantincreases in time as the temperatures increased and decreased from 183C. Thespring/autumn alternating temperature (23/93C) did not significantly alter T50 for eitherspecies (Table 1).

Germination temperature affected the time to maximum germination (Tmax).The shortest time from sowing to Tmax occurred at 183C for both species, and temper-atures higher or lower than this increased the time (Table 1). The increase was onlysignificant between species at 203C, where Tmax time for C. preissii was increasedsignificantly more than that for C. verrucosa.The spring/autumn alternating temperature(23/93C) did not substantially change Tmax for either species.

The final percentage of viable seeds germinating (Max %) at constant temperatureswas not significantly different for either species (Table 1). However, Max % for C.preissii at 23/93C decreased significantly from that obtained at 183C and 203C constanttemperatures. There was no significant change in Max % for C. verrucosa.

Osmotic stress

Constant temperature 183C

Osmotic potentials to !0)5 MPa did not alter either the minimum or maximum time togermination for either species. Osmotic potentials less than !0)5 MPa increased both

GERMINATION OF CALLITRIS SEEDS 441

the minimum and maximum time required for germination of both Callitris verrucosaand Callitris preissii (Table 2). At osmotic potentials less than !1)0 MPa, no germina-tion for either species was recorded up to 40 days after sowing.

The most dramatic effect of imposed water stress was on the maximum percent-age of viable seeds germinating. All viable seeds of both species germinated at osmoticpotentials greater than !0)5 MPa, but at !1)0 MPa C. verrucosa germination wasreduced from 100% to 82%, and that of C. preissii from 100% to 43%.

Constant temperature 123C

Osmotic potentials lower than!1)0 MPa significantly delayed germination for bothspecies, and potentials lower than!0)5 MPa significantly increased the maximumgermination time (Table 3).

Compared to the control, there was no significant interspecific difference in thefinal percentage germination at osmotic potentials of!1 )0 MPa or above. However, atthis lower temperature, seeds of both species were able to germinate at lower waterpotentials (Table 3). At!1)5 MPa and!2)0 MPa the percentage of C. verrucosa seedsable to germinate was significantly greater (p(0)05) than that for C. preissii. At!2)5 MPa, 35% of C. verrucosa seed was still able to germinate, but no germination ofC. preissii seed was recorded.

Hydration}dehydration cycles

Compared to the control, Tmin for Callitris verrucosa and Callitris preissii was unaf-fected until seeds were treated to at least three hydration}dehydration cycles (Table 4).Cycles with delays before sowing did not alter minimum germination times from thoseachieved from the equivalent non-delayed cycle.

Tmax was significantly increased by one and two cycles, but three cycles and cycleswith delays did not alter the maximum germination time from that achieved with nohydration}dehydration cycles.

Table 2. Ewect of osmotic stress on seed germination of Callitris verrucosa (cv)and Callitris preissii (cp) at 183C. Tmin"mean ($S.E.) minimum time to onset ofgermination; Tmax"mean ($S.E.) maximum time to completion of germination;Max %"mean ($S.E.) maximum % viable seeds germinated. For each attributethe mean values with the same letters are not signixcantly diwerent at 5% level of

probability (Newman-Keuls Multiple Comparisons Test)

Attribute Species 0 MPa !0 )15 MPa !0 )5 MPa !1 )0 MPa

Tmin cv 8 )0$0 )0 8 )0$0 )0 11 )7$0 )3 18 )7$0 )7(days) a a a b

cp 8 )0$0 )0 10 )0$1 )0 11 )0$0 )0 21 )1$2 )0a a a c

Tmax cv 17 )3$0 )7 14 )3$1 )9 16 )7$0 )7 31 )0$2 )1(days) ab a ab c

cp 19 )0$2 )5 16 )7$1 )8 24 )7$3 )4 32 )0$1 )5ab ab b c

Max % cv 100$0 100$0 100$0 82 )3$0 )5a a a b

cp 100$0 100$0 97 )4$2 )6 43 )1$0 )4a a a c

Table 3. Ewect of osmotic stress on seed germination of Callitris verrucosa (cv) and Callitris preissi (cp) at 123C. Tmin"mean ($S.E.)minimum time to onset of germination; Tmax"mean ($S.E.) maximum time to completion of germination; Max %"mean ($S.E.)maximum % viable seeds germinated. For each attribute the mean values with the same letters are not signixcantly diwerent at 5% level of

probability (Newman-Keuls Multiple Comparisons Test)

Attribute Species 0 MPa !0 )15 MPa !0 )5 MPa !1 )0 MPa !1 )5 MPa !2 )0 MPa !2 )5 MPa

Tmin cv 14 )0$0 )0 14 )0$0 )0 14 )0$0 )0 16 )7$0 )7 26 )7$1 )8 30 )7$2 )9 38 )3$2 )4(days) a a a a b bc d

cp 14 )0$0)0 14 )0$1 )3 14 )0$0)0 16 )0$0 )0 26 )7$1 )8 33 )3$1 )3 noa a a a b c germination

Tmax cv 16 )7$0 )0 17 )3$0 )7 24 )7$2 )7 28 )0$0 )0 37 )3$2 )7 40 )0$0 )0 46 )0$0 )0(days) a a ab bcd def ef f

cp 22 )7$0 )7 24 )0$4 )2 28 )7$2 )4 33 )3$2 )7 36 )0$2 )0 37 )3$2 )7 noab abc bcd cde de def germination

Max % cv 100$0 )0 100 )0$0 )0 100 )0$0 )0 76 )5$1 )0 66 )0$0 )1 49 )5$1 )1 34 )6$0 )7e e e d cd bc ab

cp 100$0 )0 99 )4$0 )6 97 )0$0 )7 72 )4$1 )0 37 )4$0 )1 20 )2$0 )3 noe e e d ab a germination

442R

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Table 4. Ewect of hydration}dehydration cycles on seed germination of Callitris verrucosa (cv) and Callitris preissi (cp). Tmin"mean($S.E.) minimum time to onset of germination; T50"mean ($S.E.) time to 50% viable seeds germinated; Tmax"mean ($S.E.) maximumtime to completion of germination; Max %"mean ($S.E.) maximum % viable seeds germinated. For each attribute the mean values with

the same letters are not signixcantly diwerent at 5% level of probability (Newman-Keuls Multiple Comparisons Test)

Attribute Species 0 cycles 1 cycles 2 cycles 3 cycles 2#14 delay 3#7 delay

Tmin cv 8 )0$0 )0 6 )0$0 )0 6 )0$0 )0 5 )0$0 )0 7 )7$0 )7 4 )7$0 )4(days) bc ab ab a bc a

cp 10 )3$1 )5 10 )0$1 )0 9 )3$0 )9 3 )7$0 )9 7 )7$0 )7 6 )0$0 )0d d cd a bc ab

T50 cv 10 )7$0 )9 9 )7$0 )3 12 )5$1 )9 7 )5$0 )3 10 )2$0 )7 6 )2$0 )9(days) ab ab b ab ab a

cp 17 )2$1 )3 21 )3$0 )9 18 )7$2 )3 10 )5$}1 )8 13 )3$0 )9 9 )8$0 )9c c c ab b ab

Tmax cv 17 )3$0 )7 28 )0$4 )2 28 )3$1 )3 14 )7$1 )3 15 )0$0 )0 11 )7$1 )3(days) a bc bc a a a

cp 20 )7$2 )4 32 )7$2 )0 32 )0$3 )2 14 )7$1 )3 21 )0$1 )0 19 )7$3 )5ab c c a ab ab

Max % cv 99 )0$1 )0 98 )5$1 )5 100$0 100$0 98 )4$1 )6 98 )9$1 )1a a a a a a

cp 99 )3$0 )7 98 )8$1 )2 98 )6$1 )4 95 )9$1 )1 100$0 96 )6$0 )9a a a a a a

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444 R. ADAMS

For Callitris verrucosa T50 was not significantly different for any treatmentcompared with the control. Cycles with a delay before sowing were not significantlydifferent from the equivalent cycle with no delay (dehydration phase) implying thatthere is a hydration memory. For Callitris preissii T50 was significantly reduced from thatof the control for three cycles and both cycles with a delay. Only the 2#14 day delaywas significantly different from the equivalent non-delayed cycle.

The hydration}dehydration cycles, and cycles with delays before sowing, did notaffect seed viability as there was no significant reduction in the final percentage ofviable seeds germinating (Max %) (Table 4).

Seed priming

Seeds primed at !1)5 MPa at 183C resulted in significant reductions in both theminimum and maximum time taken for germination for both species (Fig. 1) whencompared with the control. T50 was also significantly reduced by priming. Callitrisverrucosa reduced from 13)0$0)3 to 6)3$1)3 days (p(0)05) and Callitris preissii from16)3$1)9 to 7)5$1)6 days (p(0 ) 05). Seed priming for 14 days did not affectseed viability as there was no significant reduction in the final percentage of viable seedsgerminating for either species.

Discussion

No complex dormancy mechanisms have been found for either Callitris species (Adams,1982), and like many species exhibiting a characteristically ‘Mediterranean’ germinationresponse (Thompson, 1973), both Callitris verrucosa and Callitris preissii germinatedreadily at temperatures between 16}203C, with germination inhibited above 203C andbelow 103C. Complete germination of all viable seeds occurred most rapidly at 183C forboth species. In the field, this relatively narrow range of favourable germination temper-atures occurs during autumn and spring, and is likely to restrict the germination of bothspecies to these seasons. However, periods of suitable germination temperatures maynot necessarily coincide with the occurrance of other favourable conditions for seedlingestablishment (Schupp, 1995). The two species are found on distinctly differentsoil types throughout their range. Callitris verrucosa grows on deep, slightly acidic sands,while Callitris preissii grows on relatively fertile, calcareous sandy loams (Adams, 1982).However, C. verrucosa has lower upper and lower rainfall limits (350 mm and 170 mm)than does C. preissii (690 mm and 250 mm). Nevertheless, for both soil types soilmoisture stress is generally low in the months following autumn germination, while itincreases rapidly and remains high through late spring and summer.

Most reported species are restricted to germination at water potentials greater than!1)0 MPa (McWilliam et al., 1970; Mott, 1974; Edgar, 1977; Hagon & Chan, 1978),although a few species, such as Trifolium subterraneum (Mott, 1974), Lolium (McWil-liam et al., 1970), and Agropyron cristatum (Briede & McKell, 1992), are able togerminate at !1)5 MPa. More negative water potentials, especially when coupled withincreasing or decreasing temperature, usually result in reduced levels of germination(Kaufmann & Eckard, 1977). However, even at potentials as low as !2 )0 MPa, somespecies such as Quercus pilastrus (Bonner, 1968), Brassica oleracea and Lepidium sativum(Hegarty & Ross, 1978) are still able to achieve some degree of seed germination. ForCallitris seeds, the most rapid germination of seeds occurred at 183C at water potentialsbetween 0 MPa and !0)5 MPa, and as osmotic stress increased beyond !1)0 MPagermination became inhibited in both species. At 123C, although the maximum percent-age of seeds able to germinate showed the expected decrease with decreasing osmoticpotential, the tolerance of seeds to moisture stress increased. Callitris preissii was able to

Figure 1. Percentage of viable seeds of Callitris verrucosa (a) and Callitris preissii (b) germinatingover time after priming at !1 ) 5 MPa for 14 days (}j}); control (}r}).

GERMINATION OF CALLITRIS SEEDS 445

achieve approximately 20% germination at !2)0 MPa, while 35% of C. verrucosa seedsgerminated at !2)5 MPa. This effect of temperature on osmotic response is great,and has previously been reported. From a comparison of pearl millet and sorghumresponses to temperature and water stress, Smith et al. (1989) suggested that the abilityof a species to germinate under drought conditions and at sub-optimal temperaturescould convey an establishment advantage in environments where ideal germinationconditions are rare.

However, although Callitris germination can occur at water potentials less than!1)0 MPa, Tmin is approximately doubled for both species (8 days to 16 days at 183Cand 14 days to 27 days at 123C), thus increasing the likelihood of the available soil

446 R. ADAMS

moisture becoming depleted before seedling emergence (Adams, 1982). For Callitris,the combination of temperature and osmotic potential most suitable for germination islikely to be encountered in autumn and spring. However, seedlings resulting fromautumn germinants are more likely to encounter favourable conditions than are seed-lings from spring germinants, as during late spring and early summer the soil dries andthere is a greater chance of the soil moisture front retreating beyond the root zone ofestablishing Callitris seedlings.

Under field conditions, the rate of seed germination and establishment is critical(Kaufmann & Eckard, 1977). Hydration}dehydration cycles and seed priming havebeen successfully used to increase germination percentage and rate in crop (Hegarty,1978; Brocklehurst & Dearman, 1983) and weed seeds (Vincent & Cavers, 1978), andhydropriming has been shown to result in the earlier germination of desert cacti(Dubrovsky, 1996) and Allium porrum (Ahsraf & Bray, 1993). Numerous processesmay be implicated in the hydropriming and desiccation tolerance responses of seeds(Hegarty, 1978), including the activation of DNA repair processes (Boubriak et al.,1997) and the formation of desiccation-stable DNA at low water potentials (Osborne& Boubriak, 1994). DNA damage occurs during dormancy, and its repair occurs withinthe first few hours of imbibition (Boubriak et al., 1997). Incremental repair of damagedDNA during hydration phases, and the stability of repaired DNA during dehydrationphases may effectively shorten the Tmin for germination when seeds eventuallyreceive sufficient moisture to complete germination.

Both hydration}dehydration}rehydration cycles and seed priming are likely to beimportant in natural seed populations by reducing the time lag between the occurrenceof favourable germination conditions and actual germination (Bai & Romo, 1995;Dubrovsky 1996). Primed and hydrated}dehydrated seeds have the ability to retain thephysiological changes induced by the hydration phase (Vincent & Cavers, 1978; Dub-rovsky, 1996; Boubriak et al., 1997). Thus, small precipitation events may have a cumu-lative effect, resulting in final germination after only a small rainfall event. Theretention of these physiological changes during dehydration periods is seen in the seedsof Callitris, where hydration}dehydration cycles followed by delays before sowingshowed no loss of ‘hydration memory’ (Dubrovsky, 1996), but resulted in similarreductions in the time lag to germination as cycles with no time lag. Similarly, primingseeds to just below threshold hydration resulted in no loss of viability, and significantreductions in the time taken for radicle extension when finally sown. The capacity ofCallitris seeds for incremental germination, by utilizing small amounts of available waterover a period of time, is likely to increase overall germination success in a low rainfallenvironment.

The differences in germination abilities of the two Callitris species at highosmotic stress, and the ability of C. verrucosa to germinate and reach T50 more rapidlythan C. preissii after one and two hydration}dehydration cycles, and priming, suggeststhe former species is better adapted for germination under limiting soil moistureconditions, and may partially explain the edaphic distributions of the species. Thecapacity of C. verrucosa to germinate at water potentials less than !1)0 MPa isconsistent with its more frequent occurrences on very sandy soils. Immediately followingrain, sandy soils generally retain moisture at a higher matric potential than the fine-textured topsoils occupied by C. preissii, but they also dry more rapidly, reducing soilwater potential to a greater degree. Therefore, even minor soil texture changes maycreate disproportionately effective ecological barriers to seed germination for thesespecies. However, rapid germination may be possible if seeds are able to utilize soilmoisture from numerous light showers. Water taken up at low osmotic potentials maynot result in actual field germination, but it may contribute to the partial hydration ofseeds and the accumulation of the physiological changes leading to germination. Rapidgermination of these partially hydrated seeds is likely to occur when the soils becomerecharged after rain.

GERMINATION OF CALLITRIS SEEDS 447

Summary

Experimental germination of seeds of Callitris occurs readily at temperatures andosmotic potentials likely to approximate those found in the field during autumn andspring. However, recruitment events for these, and many other semi-arid species, maybe quite rare if germination does not coincide with suitable seedling establishmentconditions. The capacity of both species to imbibe small amounts of soil water duringcycles of light rainfall followed by soil drying, and to accumulate the resulting physiolo-gical changes without loss of viability, is likely to significantly reduce Tmin and T50 andconfer a considerable advantage for subsequent seedling establishment. Species withthese germination strategies, in a low rainfall environment, may have more frequentsuccessful recruitment events than would otherwise be predicted from the overallpatterns of rainfall and temperature.

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