Thạc Sĩ Variation of the germinable soil seed banks in a pasture infested by parthenium hysterophorus l. at

ii
ACKOWLEDGEMENTS
I would sincerely like to thank Professor Steve Adkins (principal supervisor), Dr. Chris
O’Donnell (associate supervisor) and Dr. Doug George (associate supervisor) - who offered
endless hours of patience, support and guidance - for their valuable roles in discussing
ideas and problems associated with the project and for reading many drafts. Special thanks
to Dr. Sheldon Navie who played a great role in identification botanical species and to Mr.
Allan Lisle who assisted me with data analysis. Many thanks also to all of the staff of the
School of Land, Crop and Food Sciences - The University of Queensland - who supported
me with equipment and materials. The land owner, Mr. Syd Smith, was kind enough to
provide me with the property-related information and to let me set up sites on his property.
Thanks to Mr. Shane Noon, a Kilcoy officer, for helping locating the study sites.
Special thanks to the UQ’s fellows, who accompanied with me on field trips and
who made my life in Australia pleasurable.
I greatly appreciate the financial support of the Australian Development Scholarship
for my study at the University of Queensland.
Finally, I gratefully acknowledge the support of my family members, who cared and
encouraged me during the project. iii
ABSTRACT
Seed germination method was used to determine the germinable soil seed bank at
two contrasting sites (one in a gully and another at the top of a ridge) within a sub-tropical
pasture at Kilcoy, south-eastern Queensland, in January 2008 (mid-summer) and July 2008
(mid-winter). These two sites demonstrated differences in topography, soil type and
moisture content and standing vegetation composition. During the period of study, the
germinable soil seed bank at the two sites varied between 11,526 to 23,252 seeds/m 2 in the
gully and between 10,136 to 12,799 seeds/m 2 at the top of the ridge. Parthenium weed
(Parthenium hysterophorous L.) exhibited a moderately abundant and persistent seed bank
in the gully (varying from 358 to 821 seeds/m 2 ) and at the top of the ridge (from 863 to
1,189 seeds/m 2 ). The weed was not the dominant species at either site, representing 1.5 to
7.1% of the total germinable seed bank in the gully and 8.5 to 9.3% of the total seed banks
at the top of the ridge. The species richness and the species diversity of the germinable seed
bank were high at both sites with no significant difference showing up between them or
between summer and winter. This management plan, designed around chemical control,
had not yet been in operation long enough to have the the desired effect of reducing the
seed bank to below a desirable maximum level. The disproportional low percentage of
highly desirable pasture species (including legume, palatable perennial grass and other
native species) coupled with the presence of parthenium weed in the germinable soil seed
bank indicates that the pasture is still in a degraded state. iv
TABLE OF CONTENTS
DECLARATION . i
ACKOWLEDGEMENTS . ii
ABSTRACT . iii
TABLE OF CONTENTS . iii
LIST OF TABLES . vii
LIST OF FIGURES viii
LIST OF PLATES . ix
CHAPTER 1 1
INTRODUCTION 1
CHAPTER 2 3
LITERATURE REVIEW . 3
2.1. Scope of the review 3
2.2. Parthenium weed (Parthenium hysterophorus L.) . 3
2.2.1. Taxonomy and description of parthenium weed . 3
2.2.2. The history of spread and present distribution . 5
2.2.3. Habitat . 7
2.2.4. Growth and development . 8
2.2.5. Biology of parthenium weed seed 9
2.2.6. Parthenium weed and its significance 12
2.2.7. Parthenium weed management . 16
2.3. Soil seed bank 23
2.3.1. Soil seed bank concept 23
2.3.2. Soil seed bank study approaches . 27
2.3.3. Composition of the soil seed bank in grasslands 31
2.3.4. Factors affecting the soil seed banks in gr azed communities 32 v
2.3.5. Soil seed bank of parthenium weed 35
2.4. Kilcoy climate/land use of the study site . 36
2.4.1. Kilcoy location 36
2.4.2. Kilcoy climate . 36
2.4.3. Land use of the study site . 38
2.5. Conclusion . 38
CHAPTER 3 40
MATERIALS AND METHODS . 40
3.1. Study sites 40
3.2. Sampling method . 40
3.3. Analytical method 41
3.4. Statistical analysis 43
CHAPTER 4 44
RESULTS . 44
4.1. Temporal variation in seed density and composition 44
4.2. Species richness and diversity . 53
4.3. Life history . 54
4.3.1. Annuals/biennials versus perennials 54
4.3.2. Life form 56
4.3.3. Introduced species versus native species 57
4.4. Germination rate 57
CHAPTER 5 61
DISCUSSION 61
5.1. Temporal variation in seed density and composition 61
5.2. Species richness and diversity . 66
5.3. Life history . 67
5.4. Germination rate 69
CHAPTER 6 71
CONCLUSION 71vi
REFERENCES 72
APPENDICES . 82
Appendix 1. Temperatures ( 0 C) and humidity (%) at Kirkleagh (latitude 27.2’S, 82
Appendix 2. Mean (■), max(▲) and min (♦) temperature of the first 20 days of
the germination trials in the glasshouse in January (A) and in July (B). . 83
Appendix 3. Above ground vegetation species versus seed bank species at the
gully 84
Appendix 4. Above ground vegetation species versus seed bank species at the top
ridge . 88vii
LIST OF TABLES
Table 2.1. Recommendations on seed bank sample sizes from the literature . 27
Table 2.2. The main advantages and disadvantages of the germination and the seed
extraction methods for analysing soil seed banks. . 30
Table 2.3. Temperatures ( 0 C) and humidity (%) at Kirkleagh (latitude 27 o 2’S,
longtitude 152 o 34’E) for the period 1978 – 1993. 37
Table 4.1. Temporal variation in the germinable soil seed bank at the gully of the Kilcoy
pasture. . 45
Table 4.2. Temporal variation in the germinable soil seed bank at the top ridge of the
Kilcoy pasture . 49
Table 4.3. Species richness and diversity (Shannon-Weiner Index) of the seed banks at
the gully and the top ridge of the Kilcoy pasture. 54
Table 4.4. The spatial variation in the seed densities of annuals and perennials. . 55
Table 4.5. Spatial variation in the seed densities of native and introduced species 57
Table 5.1. The contribution of difference plant groups to the viable seed banks in
difference grassland types . 63viii
LIST OF FIGURES
Figure 2.1. A map of parthenium weed distribution in Australia based on herbarium
records, where red dots represent the presence of parthenium weed 6
Figure 2.2. Diagrammatic representation of the four seed bank types 24
Figure 2.3. Long-term average rainfall at Lindfield (latitude: 26 o 84’S, longitude:
152 o 58’E, 14km away from the study sites). 37
Figure 3.1. Monthly rainfall (blue bars) in the year 2008, compared with long-term
rainfall (black bars) averages of the period from 1988 to 2008 at Lindfield
(14 km away from the study sites). 41
Figure 4.1. The germinable seed proportion of parthenium weed in comparison with
that of other plant groups in the germinable soil seed banks at the gully in the
Kilcoy pasture . 48
Figure 4.2. The proportion of parthenium weed in comparison with other groups of
species in the germinable soil seed banks at the top of a ridge in the Kilcoy
pasture. 52
Figure 4.3. Life forms of species detected in the germinable soil seed banks from
samples collected at the gully and the top ridge of the Kilcoy pasture during
the entire study period . 56
Figure 4.4. Seedling emergence of parthenium weed (○) and all other species (■) during
the first 20 days after wetting the soil samples collected in January from a
gully (A) and a top ridge (B) of the Kilcoy pasture. . 58
Figure 4.5. Cumulative seedling emergence of parthenium weed (○) and all other
species (■) during the first 20 days after wetting the soil samples collected in
January from a gully (A) and a top ridge (B) of a Kilcoy pasture 58
Figure 4.6. Seedling emergence of parthenium weed (○) and all other species (■) during
the first 20 days after wetting the soil samples collected in July from a gully
(A) and a top ridge (B) of a Kilcoy pasture. . 59 ix
Figure 4.7. Cumulative seedling emergence of parthenium weed (○) and all other
species (■) during the first 20 days after wetting the soil samples collected in
July from a gully (A) and a top ridge (B) of a Kilcoy pasture. 60
LIST OF PLATES
Plate 2.1. Parthenium weed (Parthenium hysterophorus L.). (a) Early seedling growth
with hairy true leaves and alternate leaf arrangement; (b) Late seedling
growth with rosette growth habit and deeply lobed leaves . 4
Plate 2.2. Mature plant. Note the small white flower heads and much-branched terminal
panicles . 51
CHAPTER 1
INTRODUCTION
Parthenium weed (Parthenium hysterophorus L.), an annual herbaceous weed belonging
to the Asteraceae family, is a noxious and invasive weed, which is native to the Americas
including the United States of America, Mexico, Cuba, and the West Indies (Evans 1997;
Mahadevappa 1999). Often accidentally introduced into new locations as seed, it has now
spread over many regions of East and South Africa, parts of South East Asia, to certain
Pacific Islands, to India, Pakistan, and Australia. It has spread at an alarming rate
achieving the status of a major weed within a relatively short period of time (Adkins &
Navie 2006; Kohli et al. 2006; Mahadevappa 1999; Navie et al. 1998b; Picman & Picman
1984; Reddy et al. 2007; Singh et al. 2004). In Australia, parthenium weed was declared
one of the 20 “Weeds of National Significance” (WONS), mainly due to its potential to
spread rapidly and its adverse impacts on the economy, the environment and society
(Thorp & Lynch 2000).
Parthenium weed is a serious problem in many agricultural production systems as
well as in non-agricultural areas, especially those that are sparsely covered or disturbed
(Brooks et al. 2004; Navie et al. 1998b; O'Donnell & Adkins 2005; Reddy et al. 2007).
Parthenium weed infestations result in the reduction of productivity, increasing production
costs and damage to the environment. Its aggressive growth habit reduces the diversity and
size of other plant populations (Sridhara 2005 cited in Dhileepan 2007) as well as their seed
banks (Navie et al. 2004), and it reduces the productivity of many kinds of pastures
(Adkins & Navie 2006; Chippendale & Panetta 1994; Haseler 1976). Parthenium weed has
also been reported to aggressively colonize several kinds of arable systems leading to crop
yield loss of as much as 70% (Channappagoudar et al. 1990; Tamado & Milberg 2004).
Additionally, the weed acts as an alternative host for a wide range of crop pests, which
include certain kinds of nematodes, mycoplasma and black scarab (Pseudoheteronyx sp)
(Basappa 2005; Navie et al. 1998b). In addition, severe parthenium weed infestations are to
be found along roadsides, over wastelands, and in urban settings (Brooks et al. 2004; 2
Haseler 1976). Nonetheless, it is hardly ever a problem in areas that are ecologically healthy
and well covered with desirable plants (Brooks et al. 2004; O'Donnell & Adkins 2005).
Parthenium weed poses various detrimental effects upon human and animal health.
Upon contacting parthenium weed plant parts or its pollen, people may suffer severe
allergic reactions such as itching, alopecia, dermatitis, hay fever or asthma (McFadyen
1995; Navie et al. 1998b). Likewise, parthenium weed is poisonous to grazing animals and
the meat and milk production from livestock eating the weed can be tainted (Kadhane et al.
1992; Navie et al. 1998b).
The wide distribution of parthenium weed and its rapid increase in population size
may be due to a number of factors including its prolific seed production, allelopathic
properties, or lack of effective management practices (Haseler 1976; Navie et al. 1998b;
Reddy et al. 2007).
Understanding the size and composition of soil seed banks is of great importance in
monitoring plant community structure and function. The buried seed population has a role
in re-establishing a population or altering its composition (Viragh and Gerencser 1988;
Coffin and Lauenroth 1989; Rice 1989; Navie et al. 2004). Knowledge of the germinable
seed bank is an integral part of any study on the ecology of communities and the
recruitment of species into those communities (Coffin & Lauenroth 1989; Navie et al.
2004; Rice 1989). In turn, the composition and density of the soil seed bank is influenced
by the seed-production capability of above-ground vegetation (Coffin & Lauenroth 1989).
The main objective of this study is to measure the size and temporal and spatial
variation in the germinable seed bank of parthenium weed in a grazed pasture at Kilcoy,
south-eastern Queensland. The composition of the entire germinable seed bank was also
determined in order to characterize the structure and dynamics of the seed banks of
communities infested by this invasive noxious plant species. This will give an estimate of
future plant population abundance and biodiversity. Further work outside of the scope of
this present project will monitor change as further weed management practices are applied. 3
CHAPTER 2
LITERATURE REVIEW
2.1. Scope of the review
This review focuses on the biology of parthenium weed (Parthenium hysterophorus L.) and
its management and the soil seed banks. The first part of the review is devoted to reviewing
the basic biology of the weed and its management. This includes a description of
parthenium weed, its growth, spread and distribution, and its significance to agricultural
production, pastoral industries, human health, and the environment. The various approaches
that are used to manage the weed are also mentioned. The second part of the review stresses
the importance of studying the soil seed bank in relation to pasture management and briefly
reviews factors affecting the composition and size of soil seed banks. An overview of
parthenium weed soil seed banks is included as well.
2.2. Parthenium weed (Parthenium hysterophorus L.)
2.2.1. Taxonomy and description of parthenium weed
The following description of parthenium weed is based on Everist (1974); Haseler (1976);
Navie (2002); Navie et al. (1998b, 2004); and Wilson et al. (1995).
Parthenium weed is a plant species belonging to the tribe Heliantheae of the
Asteraceae (Daisy family), an extremely diverse family distributed worldwide. The weed is
commonly named parthenium weed in Australia but various alternative names are used
abroad. For example, in India, it is known as bitter weed, carrot weed, broom-brush and
congress weed; in the Caribbean, it is called whitetop, escobar amarga and feverfew; and
known as false ragweed or ragweed parthenium in the United States of America. 4
Parthenium weed is a profusely branched, erect and fast-maturing annual herb, which can
flower within 4 weeks. The weed may emerge, grow and flower vigorously all year round if
conditions are favourable. Its life cycle is divided into two main stages namely juvenile
(rosette) stage and mature (adult) stage.
Juvenile stage: Parthenium weed seed leaves are hairless, broadly elliptic with short
stalks (about 1.5 mm) while the first two true leaves are hairy, egg-shaped and apex
rounded (Plate 2.1a). In its early growth stage, parthenium weed forms a rosette with
large, hairy, dark green and radial leaves which are deeply divided into narrow pointed
lobes. The leaves are up to 20 cm long and 12 cm wide. The large lower leaves are spread
on the ground like a carpet (Plate 2.1b).
Plate 2.1. Parthenium weed (Parthenium hysterophorus L.). (a) Early seedling growth
with hairy true leaves and alternate leaf arrangement; (b) Late seedling growth
with rosette growth habit and deeply lobed leaves (Wilson et al. 1995).
Mature stage: Mature plants are erect, reaching up to 2.5 m in height although most plants
do not exceed 1.5 m. Upon stem elongation, smaller, narrower and less divided leaves, in
comparison to the radial leaves, are formed along the upper stem. The stem is hairy, rigid
and longitudinally grooved. Both leaves and stem of parthenium weed are coated with fine
soft hairs. The whole plant is bluish or grayish-green in colour. Flower heads, which
produce five black seeds, are creamy-white, about 3 to 5 mm across and are clustered on
large branched stalks arising from the leaf axils (Plate 2.2). The achene complex, i.e. seed,
a b 5
is shed gradually or retained on the inflorescence until the stem senescences. Seeds are
black and 1 to 2 mm in length.
It is easy to confuse parthenium weed with species
such as annual ragweed (Ambrosia artemisiifolia
L.), perennial ragweed (A. psilostachya DC.), burr
ragweed (A. confertiflora DC.) or lacy ragweed (A.
tenuifolia Sprengel), especially when only the
vegetative growth stage is seen. Parthenium weed
can be distinguished from the above species by the
following distinct characteristics: (i) in its early
vegetative growth stage, parthenium weed does not
possess opposite leaves; (ii) parthenium weed has a
distinguishing longitudinally grooved stem; (iii)
flower heads of parthenium weed are small, white
and borne on much-branched terminal panicles
while those of Ambrosia species are monoecious,
unobtrusive and predominantly green in colour.
2.2.2. The history of spread and present distribution
Parthenium weed, naturally growing in the tropical and subtropical Americas, from the
southern USA to the southern Brazil and northern Argentina (Dale 1981), has been
accidentally introduced into many regions around the world and achieved major weed status
within a relatively short period (Evans 1997; Navie et al. 1998b). Until 1977, it did not have
any place in the list of the world’s worst weeds (Holm et al. 1977). Within the last few
decades, it has become one of the most dreaded weeds of the world (Mahadevappa 1999).
The introduction of parthenium weed into Australian has occurred on at least two
occasions: one in south-east Queensland, near Toogoolawah, in the 1940s (presence first
Plate 2.2. Mature plant. Note the small
white flower heads and much-
branched terminal panicles
(Wilson et al. 1995).6
recorded in the 1950s) and the other in central Queensland, near Clermont, in 1958 (Haseler
1976; Navie et al. 1998b). The most serious introduction was the second one, when
parthenium weed was imported in a contaminated pasture seed lot from Texas (USA)
(Haseler 1976). This second introduction was not noticed until 1973, when a series of mild
winters and high rainfall summers favoured the rapid spread of the plant (Navie et al.
1998b). The weed then rapidly spread through central Queensland and infested 170,000
km 2 of prime grazing lands or 10% of the entire state (Chippendale & Panetta 1994). The
weed gained the status as a serious pest in Queensland in 1974 (Navie et al. 1998b) and as a
weed of national significance in 2000 (Thorp & Lynch 2000). Although parthenium weed
is not yet a major weed in Australian
cropping systems and lands, it is a major
pest of grazing areas and is often
dominant along roadsides (Navie et al.
1998b; Williams & Groves 1980).
Presently, parthenium weed occurs in
Queensland, New South Wales, Victoria,
Western Australia and the Northern
Territory (Figure 2.1) but has been noted
to have significant potential to spread
throughout all of the warm and
temperate, humid and sub-humid areas of
Australia (Navie 2002). However, it seems
to be confined to the areas not experiencing
extreme temperatures (<5 0 C or >40 0 C)
(Dale 1981) and heavy shading (>80%
shade) (Williams & Groves 1980).
Parthenium weed has been introduced into India between the 1950s and 1960s
through importation of food grains from the USA. It was first noticed in 1955 in Pune
(Maharashtra) (Kohli et al. 2006) and has since spread throughout the country, infesting
about 5 million ha of land (Adkins & Navie 2006; Mahadevappa 1999). Parthenium weed
Figure 2.1. A map of parthenium weed
distribution in Australia based on
herbarium records, where red dots
represent the presence of parthenium
weed. Map sourced from Australia’s
Virtual Herbarium, 23rd December,
2008, <http://www.anbg.gov.au/avh/>7
has spread gradually from one place to another becoming common along the highways, petrol
bunks, railway tracks, bus stops on road sides and other waste lands (Mahadevappa 1999).
Parthenium weed has also spread into Trinidad, Guyana, Jamaica, Nepal, Israel,
South Korea, Taiwan, North Vietnam, Bangladesh, Sri Lanka, southern China, certain
Pacific islands (Vanuatu, New Caledonia, Tahiti and Hawaii) and Indian islands (the
Mascarenes, the Seychelles, Rodriguez, Bourbon and Mauritius) (Mahadevappa 1999;
Navie 2002). It has reached Africa, being recorded in Kenya, Madagascar, Mozambique,
South Africa, Zimbabwe, Ethiopia, and may become more prominent in other parts of the
world in the near future (Adkins & Navie 2006; Evans 1997; Kohli et al. 2006;
McFadyen 1992).
2.2.3. Habitat
Parthenium weed is a weed of a wide range of habitats throughout the world, including
wastelands, pastures, agricultural areas, forest nurseries, open urban areas, roadsides, public
lawns, railway track sides, petrol bunks, bus stops, along the edge of canals, new
construction sites and along streams and rivers, and even on rooftops (Mahadevappa 1999;
Navie 2000). However, its preferred habitat varies depending on different geographical
locations. For instance, in the Americas and India, parthenium weed commonly occurs in
cultivated areas, being recorded in a wide range of crops; whereas in Australia, it is a serious
problem in pastures, with very little adverse impacts in cultivated areas though having been
detected in some crops (Navie 2000).
Parthenium weed grows vigorously and prolifically in areas that are disturbed, both
naturally and deliberately (i.e. those disturbed by the traffic of vehicles and/or livestock)
(Haseler 1976; Holman 1981). It is particularly troublesome along floodplains where soil
deposition and erosion occur (Navie et al. 1999). The weed exists in a wide range of soil
types but favours soils of high fertility such as black, alkaline and cracking clay soils (Dale
1981). In other soil types, more severe soil disturbances are required for the establishment of
parthenium weed but extensive stands of the weed are rarely found on heavy black soils 8
(Dale 1981). Parthenium weed can adapt to the areas with a wide range of annual rainfall,
varying from 200 mm to 1,150 mm (Dale 1981). It has been reported that the major
parthenium weed infestation areas in Australia are in the sub-coastal regions of central
Queensland, where annual rainfall ranges between 500 and 700 mm with a dominant summer
incidence (Haseler 1976). Parthenium weed particularly favours areas where brigalow
(Acacia harpophylla F.Muell. ex Benth.) and gidyea (Acacia cambagei R.T.Baker) low
open forests have been cleared (Dale et al. 1978). Recently cleared lands used for livestock
are the common areas with heavy parthenium weed infestations (Holman 1981). This is the
case in Australia where the main parthenium weed-infested areas are beef feeding pastures
(Anon. 1985a cited in Navie et al. 1998).
2.2.4. Growth and development
Although parthenium weed reproduces only by seed, it is able to successfully compete with
the growth of other species owing to its substantial abilities to establish and grow vigorously.
Parthenium weed can germinate, grow and flower over a wide range of photoperiods and
temperatures if moisture is favourable (Haseler 1976). It germinates more rapidly than other
plant species (Navie et al. 2004). In its early growth stage, parthenium weed has a rosette
growth habit and spreads radially close to the ground, suppressing the growth of any
vegetation underneath (Mahadevappa 1999). Parthenium weed can flower within 4 weeks
after germination (Haseler 1976), seed prolifically and form an enormous seed bank in the
soil (Navie et al. 1998b, 2004). The mature plant is erect (up to 2.5 m in height), much-
branched and may form axillary branches down the stem when it gets older (Navie et al.
1998b). Furthermore, a long tap root allows the weed to extract deep water within the soil
profile for its vigorous growth and to reserve energy for fast regrowth if the weed is slashed
or grazed (Navie et al. 1998b). Besides, parthenium weed seeds, which are small and light,
are easily dispersed for quick colonization in new areas.
The aggressive invasiveness of parthenium weed in various parts of the world has
been attributed to its allelopathic properties, which enable the weed to successfully
suppress the growth of other crop, pasture and tree species. Parthenium weed affects the 9
emergence, early growth and development of other species by releasing phenolic acids and
sesquiterpane lactones from fresh plant parts (Adamson 1996; Mersie & Singh 1987;
Picman & Picman 1984; Tefera 2002; Wakjira et al. 2005). The autotoxic effects of
parthenium weed were also reported as inhibition of pollination and fruit set of
neighbouring species (Evans 1997; Navie et al. 1998b).
From Haseler’s observation (1976), soil moisture seems to be the major factor limiting
the emergence and growth of parthenium weed, thus its main growth season in Australia is
summer, coinciding with the abundance of rainfall. Soil moisture is also a contributing
factor affecting the duration of flowering (Navie et al. 1998b).
Although temperature is not the limiting factor controlling the growth and
development of parthenium weed in Australia, it affects the duration of the vegetative phase
prior to flowering (Williams & Groves 1980). Parthenium weed flowers earlier under the
day/night temperature regime of 27/22 0 C in comparison to that of 21/26 0 C and 33/28 0 C.
However, flowering does not require any specific day length (William & Groves 1980).
2.2.5. Biology of parthenium weed seed
Seed production: Seed is the only means of parthenium weed reproduction. It is a prolific
seed producer, producing up to 25,000 seeds per plant or 300,000 seeds per m 2 given that
sufficient moisture is available to produce a high density of vigorous plants (Navie et al.
1998b). In abandoned fields in India, it was estimated that about 200,000 seeds per m 2 were
present in the seed bank (Joshi 1991). In good seasons, there may be two cohorts of
parthenium weed, making the number of parthenium weed seeds produced even greater
(Navie et al. 1998b). In Queensland’s pastures, the number of germinable parthenium weed
seeds buried in the soil was recorded to be varying between 1,500 and 34,000 seeds/m 2
(Navie et al. 1998b).
The aggressive invasiveness of parthenium weed in various parts of the world has
been attributed to its allelopathic properties, which enable the weed to successfully 10
suppress the growth of other crop, pasture and tree species. Parthenium weed affects the
emergence, early growth and development of other species by releasing phenolic acids and
sesquiterpane lactones from fresh plant parts (Adkins & Sowerby 1996; Mersie & Singh
1987; Picman & Picman 1984; Tefera 2002; Wakjira et al. 2005). The autotoxic effects of
parthenium weed were also reported as inhibition of pollination and fruit set of
neighbouring species (Evans 1997; Navie et al. 1998b).
Seed dispersal: Parthenium weed seed is black, light (0.045-0.049 mg) and flanked by a
pair of floats which aids dissemination by wind and water (Joshi 1991). Wind does not
transport parthenium weed seeds far from the plant (only just a few metres away) but
whirlwinds can disperse numerous seeds to considerable distances (Haseler 1976). Water is
thought to be an important means of parthenium weed dispersal, assisting the spread of
enormous numbers of parthenium seeds along the waterways in central Queensland (Navie et
al. 1998b). Moreover, parthenium weed seeds are dispersed by livestock, native and feral
animals (Navie et al. 1998b). They are likely to be transported long distances in mud and
debris (Haseler 1976). In most cases of long distance dispersal, parthenium weed seeds are
transported on vehicles, machinery, or livestock, or with crop and pasture seeds or in fodder
(Navie et al. 1998b). Consequently, parthenium weed may spread to new areas that are
thousands of kilometres away from the nearest plants.
Seed dormancy: It has been noticed that parthenium weed seed exhibits more than one
dormancy mechanism (Navie et al. 1998a). McFadyen (1994) assumed that the germination
of parthenium weed seeds was not controlled by primary dormancy mechanisms. In contrast,
Picman and Picman (1984) suggested that water soluble germination inhibitors
(sesquiterpene lactones, parthenin and coronopilin) present in parthenium weed seeds,
especially freshly shed seeds, need to be leached before maximum germination is attained.
This explained why the germination rate of newly shed seeds was lower than that of seeds
buried for several months and would, therefore, result in the formation of a more persistent
seed bank in the soil (Navie et al. 1996b; Navie et al. 1998a). It was also noticed that the
germination rates of parthenium weed seeds increased when the distance between them (i.e. 11
seed density in the germination dish) and the washing period preceding the germination
increased (Picman & Picman 1984).
Parthenium weed seed may be induced into a state of conditional physiological
dormancy by ambient environmental conditions, as is the case with many other plant
species that possess light requirements when buried (Baskin & Baskin 1989 cited in Navie
et al. 1998b). It is likely that when buried in the soil, parthenium weed seeds exhibit a form
of conditional dormancy which would result in the formation of more persistent seed banks
(McFadyen 1994).
Seed longevity: There are varying reports on parthenium weed longevity from different
researchers. While Butler (1984)) reported that the germination rate decreased from 66%
after burial for 1 week to 29% after 2 years of burial; the viability of parthenium weed
seeds was greater in the work of Navie et al. (1998a) (74% after 2 years of burial) and
Tamado et al. (2002) (more than 50% after burial for 26 months). Surface-lying seeds,
however, remained viable for periods no longer than 6 months (Navie 2002). These findings
suggested that seed incorporation into the soil - which is enhanced by some forms of soil
disturbance, such us flooding, cultivation or animal activities - is important for the long-term
persistence of parthenium weed (Navie 2002). Some field observations showed that
parthenium weed seed buried in the soil can remain viable for at least 6 years and will
germinate when brought to the soil surface (White 1994 cited in Navie et al. 1998a).
Seed germination: Parthenium weed seeds are able to germinate over a wide range of
temperatures (from 9 to 30 o C) but optimum germination occurs between 22 and 25 o C
(Navie et al. 1999). Very warm (above 30 o C) or very cool (below 5 o C) temperatures may
limit the germination, particularly its rate (Williams & Groves 1980). The total germination
was not affected by light but the germination rate showed some effects. The germination
rate declined when the differential of day/night temperature increased from 5 to 11 o C at low
mean temperatures (Williams & Groves 1980). 12
The germination limiting factors in the field appear to be soil moisture availability
(Williams & Groves 1980). Germination plummeted from 91% to 50% and 12% when the
soil moisture decreased from the field capacity to -0.07 MPa water potential and -0.24
MPa, respectively. No germination was recorded in the soil kept at -0.9 MPa (Williams &
Groves 1980).
The germination rate of parthenium weed seed was affected by the length of time
between shedding and the start of germination (Navie et al. 1998a). During the first month
after shedding, no seedling emergence was recorded though considerable rainfall was
available (31 mm) but 30% of the surface sown-seeds germinated in the succeeding 3
months after the first notable rainfall event (> 10 mm). By the end of the fourth month, a
total of 51.4% of seeds had emerged and no more seedling emergence was detected after
the fifth month. However, (Tamado et al. 2002) observed no germination from the depth
below 5 cm in the soil. Seeds of parthenium weed reached maximum germinability in the
field, 1 to 6 months after shedding.
2.2.6. Parthenium weed and its significance
Parthenium weed is regarded as one of the world’s seven most dreaded weeds
(Mahadevappa 1999). It has appeared as a serious problem in many regions around the
world imposing deleterious effects on crop production, animal husbandry, human health
and biodiversity.
Crop production: With allelopathic properties, parthenium weed has great potential to
aggressively colonise a wide variety of croplands, including those used for pasture grasses,
cereals, vegetables, oil crops, nut crops, sugar cane, cotton, other weeds and even tree
species, by inhibiting their germination and growth (Adkins & Sowerby 1996; Evans 1997;
McFadyen 1992; Mersie & Singh 1987; Navie 2002; Navie et al. 1998b). The germination
of cowpea (Vigna unguiculata (L.) Walp.) and bean (Phaseolus vulgaris L.) was inhibited
to the extent of 50 and 60%, respectively (Mahadevappa 1999). Water soluble phenolics
(caffeic acid, ferulic acid, vanicillic acid, anisic acid and fumaric acid) and sesquiterpene 13
lactones (parthenin and coronopilin) exuded from the weed parts also exhibit an inhibitory
effect on the growth and nodulation of legumes, thus inhibiting nitrogen fixation and the
growth of nitrifying bacteria (Kanchan & Jayachandra 1981).
Pollen grains of parthenium weed were claimed to impose negative effects upon the
chlorophyll content of leaves with which they come into contact and can interfere with the
pollination and fruit set of surrounding species (Evans 1997; Navie et al. 1998b). It was
evidenced that the presence of parthenium weed pollen on the stigmatic surface of maize
(Zea mays L.) resulted in a decrease in the grain-filling by up to 40%. Poor fruiting of
leguminous crops smooth crotalaria (Crotalaria pellida L.) and Asian ticktrefoil
(Desmodium heterocarpon (L.) DC var. van Meeuwen) was also recorded in plants covered
by parthenium weed pollen grains. The presence of parthenium weed in agricultural crops
not only suppresses the yields but also contaminates the harvested products thus reducing
their marketability (Navie 2002).
Another adverse impact parthenium weed has on crop systems is its role as an
alternative host for a wide variety of crop pests, acting as an inter-season reserve or sources
of inoculum. Examples of this include the scarab beetle Pseudoheteronyx sp. on sunflower
(Helianthus annuus L.) in central Queensland; certain kinds of plant parasitic nematodes in
the USA; the American serpentine leafminer (Liriomyza trifolii (Burgess)) on bell pepper
(Capsicum annuum L.) in Texas; the polyphagous lepidopteran Diacrisia obliqua Walker
(Bihar hairy moth), an important pest of agriculture and forestry; the black bean aphid
(Aphis fabae Scopoli) on black bean (Castanospermum australe A.Cunn & C.Fraser ex
Hook.) in India, etc (Basappa 2005; Evans 1997; Navie et al. 1998b). Furthermore,
parthenium weed may function as a secondary host of some plant pathogens, including
Xanthomonas campestris cv. phaseoli, Pseudomonas solanacearum, potato virus X and Y,
tomato leaf curl virus and crop virus transmitted by the cotton whitefly Bemisia tabaci
Gennadius (Evans 1997).
The yield losses caused by uncontrolled parthenium weed populations in various
crops in India were well-documented by Mahadevappa (1999), who noted a reduction to the extent of 40% and 50% in tomato (Solanum lycopersicum L.) and ragi crops (Eleusine
coracana L.), respectively. In irrigated sorghum (Sorghum bicolor (L.) Moench) in India, a
30% decline in sorghum grain weight causing a 35% reduction in grain yields was recorded
(Channappagoudar et al. 1990). Tamado and Milberg (2004) warned of a severe yield loss
of up to 75% of grain sorghum in an unweeded control.
Animal husbandry: Parthenium weed poses detrimental effects on livestock production,
animal health, and quality of meat and milk. The presence of parthenium weed in pastoral
regions may lead to a monoculture of parthenium weed pastures which are non-nutritious
and unable to sustain grazing animals (Chippendale & Panetta 1994). This results in