Regeneration mechanisms in Swamp Paperbark (Melaleuca ericifolia Sm.) and their implications for wetland rehabilitation


Effects of environmental variables on germination



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5.4.3 Effects of environmental variables on germination 
 
The successful recruitment of M. ericifolia, like other salt-tolerant species, is directly 
related to the ability of the seeds to respond to key environmental cues, particularly 
light and temperatures coupled with salinity levels (Ladiges et al. 1981; Khan and 
Ungar 2001). Progressively lower germination rates at increasing salinity supports 
suggestions made by Ladiges et al. (1981) and Clarke and Hannon (1970) that 
osmotic factors are critical in the inhibition of germination.  The most direct osmotic 
interference is through reduction in the osmotic potential of the aqueous environment, 
reducing water availability and water absorption by the seeds (Khan and Ungar 2001). 
The likely role of osmotic factors in germination is further supported by the finding 
that there was a rapid decrease of germination at higher temperatures and in the light: 
both light and higher temperatures increase osmotic stress on seed and reduce the 
ability of seed to imbibe water (Khan and Ungar 1984) and may have a direct toxic 
effect on the embryo (Zekri 1993).  
 
A wide range of halophytes shows an inhibition of germination through the 
interaction of light, temperature and salinity, including grasses (Khan and Gulzar 
2003), chenopods (Gul and Weber 1999; Khan and Unger 2001) and even the 
ostensibly marine seagrass Zostera marina (Churchill 1983).  Although the interaction 
of these factors on woody plants is poorly studied, the tolerance of several woody 

 
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plants to various levels of salinity alone is well documented (Zekri 1993; Barrett-
Lennard 2003; Krauss et al. 1998; Marcar et al. 2003).  The interaction of light, 
temperature and salinity, and their inhibitory effect on germination, ensures that 
species germinate under conditions that are optimal for recruitment.  Many 
halophytes, including M. ericifolia, show a higher tolerance of salinity and 
temperatures as adult plants than they do as juveniles (Rozema 1995; Ladiges et al
1981; J. Salter unpublished data).  
 
5.4.4 Effects of seed burial and substrate type 
 
The clear preference for germination on or just under the surface confirms 
germination-niche studies carried out by de Jong (2000) and Nicol and Ganf (2000). 
That germination rates are strongly related to depth of burial has been reported in a 
range of plants (McIntyre et al. 1995; Nicol and Ganf 2000); surface or shallow burial 
is often a requirement for successful germination of small seeds (Rotundo and Aguiar 
2004; Eckstein and Donath 2005; Kostel-Hughes et al. 2005; Rotundo and Aguiar 
2005).  Seeds of M. ericifolia are extremely small (~0.0018 mg) and deep burial 
would probably overwhelm the seed’s resources to raise cotyledons to the soil surface.  
It is interesting that deep burial has been shown to inhibit germination and reduce 
viability over time in the related paperbark species, M. quinquenervia (Van et al
1998).  
 
 
 
 

 
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5.4.5 Implications for rehabilitation of coastal wetlands 
 
Episodic recruitment of M. ericifolia at Dowd Morass and at other wetlands along the 
eastern and southern coasts of Australia is likely to be directly related to the spatial 
and temporal prevalence of suboptimal germination conditions and corresponding 
lack of ‘safe sites’.  Periodic flooding with fresh water or large rainfall events flushing 
salt from salinised, but otherwise potential, germination sites may provide the 
conditions required for successful germination.  This study has shown that M. 
ericifolia seeds are tolerant of salinity at the germination stage.  Optimal germination 
however, occurs at 20
o
C with a salinity of < 2 g L
-1
 and with higher overall 
germination in darkness.  If the successful regeneration of M. ericifolia is to be 
achieved in brackish-water wetlands, the sowing of seed must coincide with periods 
and conditions of optimal germination potential.  In south-eastern Australia, the 
optimal temperature regimes (~20
0
C day and ~10-12
0
C night) take place in autumn 
and spring (Bureau of Meteorology 2005).  The effect of darkness on germination 
success indicates that the best germination sites are those protected from sunlight, 
such as the base of other wetland plants or substrates with sufficiently rough surfaces 
to allow for the lodgement of seed in soil pores near the surface.  
 
The conditions for the successful germination of M. ericifolia are relatively narrow, 
with the ideal being recently shed seed sown on or very near the surface, germination 
temperatures around 20
0
C, fresh or near freshwater conditions, and in dark, or at least 
well shaded, conditions.  Re-establishment of this species, through the use of seed, 
would need specific manipulations of the environment or intervention at times of ideal 

 
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natural conditions to achieve success. Even then, the low overall viability of the seed 
would present difficulties to large-scale rehabilitation.  
 
 
 

 
124
Chapter 6 
Effects of environmental conditions on the production of 
hypocotyl hairs in seedlings of Melaleuca ericifolia (Swamp 
Paperbark) Sm. 
 
 
 
Abstract 
 
The production of hypocotyl hairs in the early stages of seedling development can 
strongly influence the success with which plants recruit sexually in harsh 
environments.  Although wetlands are one type of environment in which seedlings 
might be expected to develop hypocotyl hairs, there have been few studies of these 
structures in the woody aquatic plants.  We investigated the production of hypocotyl 
hairs in Melaleuca ericifolia Sm., a small wetland tree widely distributed across 
swampy coastal areas of south-eastern Australia, in relation to water availability, 
salinity, temperature and light regime.  Hypocotyl hairs were about 20 mm long x 30 
μ
m wide; in contrast, root hairs were generally less than 5 mm long and 15 
μ
m wide.  
Hypocotyl hairs were produced only under a narrow range of environmental 
conditions – low salinity, low water availability, moderate temperature, and darkness 
– and seedlings that failed to produce hypocotyl hairs did not survive.  Since the 
conditions under which hypocotyl hairs were produced were at least as, and possibly 
even more, restricted than those required for successful seed germination, it is likely 
that the successful sexual recruitment of M. ericifolia would be rare and episodic 
under conditions existing in most coastal wetlands in south-eastern Australia.  

 
125
 
 
6.1 Introduction 
 
Seedlings have many strategies for improving their survival, particularly in habitats 
that are hostile to the germination of seeds and the establishment of juvenile plants 
(Aronne and De Micco 2004; Nishihiro et al. 2004).  Hypocotyl hairs, single cell 
outgrowths from the base of the hypocotyl not associated with the true root system of 
the plant, are one means by which many plants can increase seedling survival in 
difficult environments.  Despite their likely importance in the sexual recruitment of a 
number of taxa of plants, hypocotyl hairs have been comparatively little studied; they 
are rarely reported in other than a cursory manner in the literature (Hofer 1992; Kuo 
and Kirkman 1992; Kuo 1993) or are not recognized at all as unique entities (Mora et 
al. 2001).  Additional confusion arises from the different names given to these single-
celled structures: coleorhiza (Baranov 1957), hypocotyl epidermal cells (Grierson and 
Schifelbein (2002), or simply hair-like cells or cellular outgrowths on the hypocotyl 
(Kuo and Kirkman 1992; Kuo 1993).   
 
Hypocotyl hairs occur in widely divergent families of both monocotyledons 
and dicotyledons across a wide range of habitats.  The main families of wetland plants 
that produce hypocotyl hairs are the Podostemaceae (Rutishauser et al. 1999), 
Zosteraceae (Churchill 1983; Kuo 1993), Alismataceae and Hydrocharitaceae (Kaul 
1978; Matsuo and Shibayama 2002), Salicaceae (Polya 1961) and Myrtaceae 
(Baranov 1957).  Only two genera, Myrtus (Myrtaceae) and Artemisia (Asteraceae), 
have been identified as possessing hypocotyl hairs in Mediterranean-type ecosystems, 
despite the general harshness of these environments (Aronne and De Micco 2004; 

 
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Young and Martens 1991).  Hypocotyl hairs have been found in several grassland 
plant families, most notably Asteraceae and Caryophyllaceae (Morita et al. 1995). 
 
There is some commonality in the function of hypocotyl hairs across these 
various families and habitats, although differences are apparent as well, usually in 
accord with variations in prevailing environmental conditions.  The role of hypocotyl 
hairs in the submerged aquatic genera MarathrumOtteliaVallisneriaVanroyenella 
and Zostera is believed to be primarily physical, and include anchoring juvenile plants 
to the substratum and facilitating the development of geotropism (Rutishauser et al
1999; Churchill 1983; Kaul 1978).  Hypocotyl hairs in emergent aquatic or 
amphibious species of Alisma,  Callistemon,  Echinodorus,  Limnocharis and 
Lophotocarpus are believed to serve a similar role to those in the submerged aquatic 
taxa, but also facilitate the uptake of water, thereby guarding against desiccation in the 
early stages of seedling development (Kaul 1978; Baranov 1957).  In purely terrestrial 
species, such as Artemisia and Myrtus, hypocotyl hairs also have been shown to 
anchor seedlings to the substratum soon after germination and to assist in the 
development of geotropism (Aronne and De Micco 2004; Young and Martens 1991).  
In terrestrial species, however, hypocotyl hairs may have additional functions in 
protecting the seedling against desiccation and herbivory.  The mucilage produced on 
the hypocotyl hairs of Artemisia  and  Myrtus, for example, may provide additional 
protection against desiccation (Aronne and De Micco 2004; Young and Martens 
1991) and the accumulation of phenolics by hypocotyl hairs in Myrtus, as reported by 
Aronne and De Micco (2004), may deter herbivory. 
 

 
127
What information is available indicates that hypocotyl hairs are produced in a 
similar developmental sequence across the few species that have been examined in 
detail (Aronne and De Micco 2004; Young and Martens 1991; Kaul 1978).  They are 
not only produced before emergence of the radical, secondary or adventitious roots, or 
root hairs, but the production of hypocotyl hairs is a prerequisite to the formation of 
these other structures, as they provide the necessary anchorage for radicles to 
penetrate the substrata and for the seedling to establish clear anisotropic growth.  
Matsuo and Shibayama (2002), for example, reported that Monochoria vaginalis 
seedlings that failed to produce hypocotyl hairs also usually failed to establish roots 
and, if they did develop roots, they failed to penetrate the substrata and the seedlings 
died.   These studies have concentrated largely on non-woody taxa, and the factors 
influencing the production of hypocotyl hairs in woody wetland species are very 
poorly understood.  Indeed, studies on only three woody wetland genera, Callistemon 
(Baranov 1957) and Populus  and  Salix (Polya 1961),  have been reported; both the 
Baranov (1957) and Polya (1961) studies were limited to either simple documentation 
or to an examination of the effects of rapid moisture uptake on hypocotyl hair 
formation.   
 
Hypocotyl hairs have been identified recently, by the authors, in Swamp 
Paperbark (Melaleuca ericifolia Sm.: Myrtaceae), a small wetland tree commonly 
found in freshwater and brackish-water swamps in south-eastern Australia. 
 
Melaleuca ericifolia is the dominant woody plant in many coastal and near-coastal 
swamps, where it forms a vegetation community, commonly known as swamp scrub, 
which is critically important as nesting and roosting habitat for colonially breeding 
water birds such as ibis (Bird 1962; Corrick and Norman 1980; Cowling and Lowe 

 
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1981).  European settlement has caused such major alterations to the extent of M. 
ericifolia communities and their wetland habitats that Bowkett and Kirkpatrick (2003) 
predicted only the largest and most ecologically intact populations in north-east 
Tasmania and the Bass Strait Islands would be likely to survive in the long-term.  
Population of M. ericifolia in Victoria are similar in size to the Tasmanian and Bass 
Strait Island populations, but possibly face an even wider and more intense range of 
disturbances, arising from the greater intensity of human settlement in mainland 
Australia.  Many of the coastal wetlands that formerly accommodated M. ericifolia 
have been subject to severe hydrological modifications, having either been completely 
drained or (less frequently) or inundated almost permanently following the 
construction of levees and other structures (e.g., see East 1935 for an early report on 
the scale of reclamation of Swamp Paperbark wetlands).  The paperbark-dominated 
wetlands that remain are often subject to secondary salinization, or to contamination 
with nutrients and other catchment-derived chemical, including pesticides and other 
toxicants.  
 
Because of the widespread loss of paperbark-dominated wetlands and the roles 
that the remaining ones play in providing a range of vital ecosystem services, there is 
a critical need to understand the environmental factors that control sexual recruitment 
in M. ericifolia (Jeanes 1996; de Jong 1997).  A number of studies have examined the 
effect of environmental conditions on germination of M. ericifolia seeds; these studies 
have shown that effective recruitment from seed is episodic, strongly controlled by 
inundation, salinity, temperature and light, and limited to particular habitats within 
wetland ecosystems having the right combination of environmental conditions (e.g., 
see Ladiges et al. 1981; Jeanes 1996; de Jong 2000; Robinson et al. 2006).  The fate 

 
129
of young seedlings and the factors that control the establishment of young plants are, 
by contrast, far less well understood.  Given the critical role played by hypocotyl hairs 
in herbaceous taxa and in non-woody aquatic species such as seagrasses (Kuo 1993), 
it would appear likely that these structures have important functions also in woody 
wetland taxa such as M. ericifolia.  Accordingly, the aims of this study were two-fold: 
a) to describe the effects of a range of environmental variables on the production of 
hypocotyl hairs in M. ericifolia; and b) to use this information to infer whether the 
environmental conditions that currently exist in coastal, brackish-water wetlands 
would facilitate or inhibit the development of hypocotyl hairs and thus would be 
likely to have an impact on the sexual recruitment of this important species of wetland 
plant.   
 
6.2 Methods 
 
6.2.1 Field site  
 
Dowd Morass is a 1,500 ha brackish-water wetland on the south-western shore of 
Lake Wellington near Sale, Victoria, southern Australia (38
0
07’S 147
0
10’E).  
Vegetation in Dowd Morass is primarily composed of large areas of M. ericifolia-
dominated swamp scrub (~ 500 ha) and extensive beds of the Common Reed, 
Phragmites australis (Cav.) Trin. ex Streud (~ 350 ha); the remaining areas are either 
open water or expanses of bare mudflats, depending on water levels.  Dowd Morass is 
an important and sizable component of the Gippsland Lakes Ramsar site and a 
regionally important site for the breeding of Sacred and Straw-necked Ibis 
(Threskiornis aethiopica and T. spinicollis).  The swamp scrub communities in the 

 
130
wetland provide these birds with their main roosting habitat (e.g., see Cowling and 
Lowe 1981) and a perceived degradation in the extent and condition of M. ericifolia is 
a major concern of the agency responsible for managing the wetland and the larger 
Gippsland Lakes Ramsar site (Parks Victoria 1997).  Dowd Morass is similar to many 
of the brackish-water wetlands that fringe the Gippsland Lakes; a range of previous 
papers have described the hydrology, salinity and vegetation of these areas (Bird 
1962; Ducker et al. 1977; Corrick and Norman 1980; Parks Victoria 1997; Roache et 
al. 2006; Robinson et al. 2006; Salter et al. 2007; Raulings et al. 2007).   
 
The large size and environmental heterogeneity of the Dowd Morass site, 
complicated by variation at the microtopographic scale in soil moisture, salinity, pH, 
organic matter content and elevation, has resulted in the juxtaposition of highly 
contrasting environmental conditions, sometimes within centimetres of each other.  
For example, large areas of the morass are permanently inundated whereas other 
areas, especially around the perimeter, experience alternating wet and dry periods; 
salinities vary from near freshwater (< 1-2 g L
-1
) to over one-half seawater (i.e., > 16 
g L
-1
) according to inundation history and the periodicity of saline intrusions from 
Lake Wellington; light intensities vary widely according to canopy density and crown 
cover.  The composition of potential seedbeds for M. ericifolia seeds varies from fine, 
tight clays, which seeds cannot penetrate, to highly porous hummocks of organic 
matter in which seeds may lodge deeply. 
 

 
131
6.2.2 Life history of M. ericifolia 
 
Melaleuca ericifolia Sm. is a small tree in the family Myrtaceae.  It occurs, often as 
the dominant woody species, widely in coastal and near-coastal freshwater and 
brackish-water wetlands across south-eastern Australia.  Most species of the genus 
Melaleuca are reliant on seed alone for reproduction, but M. ericifolia can also form 
extensive clonal stands through the production of ramets that can become physically 
independent of the parent genet once sexual recruitment has taken place (Bird 1962; 
Ladiges et al. 1981).  
 
Seeds of M. ericifolia are held on the plant in hard woody capsules for many 
years (serotiny) and usually are released on the death of the attached stem.  Seed, once 
released from the capsule, germinates within a few days if conditions are suitable 
(Robinson  et al. 2006). There seem to be no particular triggers (e.g., dormancy 
requirements) for germination, but the conditions that maximise germination success 
are fairly specific: a temperature around 20
o
C, salinity of 2 g L
-1
 or less, and darkness 
(Robinson  et al. 2006). Germination is inhibited by inundation, but once seed has 
germinated seedling are able to survive in water for several weeks by floating on the 
surface (Ladiges et al. 1981). Hypocotyls emerge from the seed first, usually within 3-
5 days, followed a few days later by the cotyledons. The first true leaves emerge after 
approximately 14 days under ideal conditions. Root hairs and secondary roots 
normally emerge after the production of hypocotyl hairs and production of the first 
true leaves, but may emerge earlier if hypocotyl hairs are not produced.   
  

 
132
6.2.3 Seed collection 
 
Seed capsules were collected in April 2004 from 25 genetically distinct plants in a 
large population of M. ericifolia at Dowd Morass.  Seeds were collected from several 
generations of capsules, ranging from 1 to 5 years of age.  Capsules were stored in 
paper bags at 20
0
C for one week.  The bags were lightly shaken to release seed from 
the capsules, and the contents of the bag sieved (mesh size: 1mm) to remove the 
empty capsules and other detritus.  The cleaned seed was placed in paper bags for a 
further three days to remove any excess moisture, then transferred to sealed glass 
containers and stored at 20
0
C in darkness until used.  Experiments began in February 
2005.   
 
6.2.4 Effects of surface sterilisation 
 
Surface-sterilised seeds were soaked in 1 % w/v sodium hypochlorite solution for set 
times (0.5, 1, 2, 5, 10 or 30 min) then rinsed three times in sterile de-ionized water.  
Excess fluid was removed between each step by pipetting seeds onto Whatmans #1 
filter paper under a gentle vacuum.  From each treatment, 100 seeds were plated onto 
0.8 % w/v water agar in four replicate Petri dishes (i.e., 25 seeds per Petri dish).  
Control treatments were subject to the same procedure but soaked in sterile de-ionized 
water only, for either 5 or 30 minutes.   
 
Seeds were incubated with a 20
0
C:10
0
C 16 hr:8 hr day:night cycle and 
shuffled periodically to randomise the possible impact of minor within-cabinet 
variations in environmental conditions.   Following incubation, seedlings were 

 
133
classified into three development classes: i) those with fully developed hypocotyl 
hairs; ii) hypocotyl hairs showing partial or impaired development; and iii) complete 
absence of hypocotyl hairs.  Hypocotyl hairs were classified as being only partially 
developed or having impaired development if they failed to elongate or if only few of 
the hypocotyl hair cells elongated.  Plates were observed for a total of 60 days.  
 

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