INTRODUCTION

Myriophyllum spicatum L. commonly known as watermilfoil is a rooted perennial, widely distributed in Kashmir Himalaya and can be found in almost all kinds of freshwater habitats including lakes, ponds, irrigation ditches and rivers (Arshid et al., 2011). It is native to Europe, Asia and North America (Couch & Nelson, 1985). From the initial point of introduction in the North America, M. spicatum has spread to 46 states and three Canadian provinces of North America (Jacono & Richerson, 2003). It is categorized among the five most ubiquitous, noxious and the most widely managed aquatic weed in lake ecosystems (Bartodziej &Ludlow, 1998; Marsden & Hauser, 2009).The importance of seeds in the propagation of Myriophyllum spicatum may lie in their ability to remain viable following various disturbances serving as an effective long-term mechanism of survival and regrowth.

Although seed germination of terrestrial species has been extensively studied, few studies have examined submerged macrophytes (Baskin & Baskin, 1988; Hartleb et al., 1993; Lal & Gopal, 1993; Ke & Li, 2006; Hay et al., 2008; Jarvis & Moore, 2008). This may be due to the fact that most submerged macrophytes show dominant clonal reproduction, and their seeds are rarely observed to grow into mature plants in nature. However, seeds of submerged macrophytes have been found to play important roles in the establishment of new genotypes in existing populations (Kimber et al., 1995), movement of populations into new regions (Arnold et al., 2000; Figuerola & Green, 2002; Harwell & Orth, 2002) and re-establishment of populations after episodic declines (McMillan & Jewett-Smith, 1988; Titus & Hoover, 1991; Parveen et al., 1995; Jarvis & Moore, 2008). A recent study on the genetic pattern of Potamogeton malaianus has suggested that bird mediated seed dispersal could greatly influence gene movements among lakes (Chen et al., 2009). Germination requirements and dormancy characteristics of species are often assumed to be adaptations to the particular habitats in which the species occur (Angevine & Chabot, 1979; Meyer et al., 1990). Germination at the right time and in the right place is important to determine the probability of a seedling surviving to maturity (Thompson, 1973). Previous studies have indicated that temperature (Hartleb et al., 1993; Ke & Li, 2006; Hay et al., 2008; Jarvis & Moore, 2008) and light (Coble & Vance, 1987; Lal & Gopal, 1993) are important ecological triggers in the seed germination of submerged species. Xiao et al. (2010) have examined the effect of temperature, water level and burial depth on seed germination of Myriophyllum spicatum and Potamogeton malaianus. Madsen & Boylen (1989),while studying the seed germination of M. spicatum from an oligotrophic and eutrophic lake, have concluded that in oligotrophic lake, the seeds do not contribute to the spread of the population due to the low seed set and low rate of germination, while in eutrophic lake there was a potential for seeds to contribute to annual propagation and expansion of the population due to relatively high seed set percentages and germination rates. In this background, understanding the seed set, viability, and germination characteristics of M.spicatum may reveal a mechanism for species invasion.

In the present study, we have studied the fruit and seed morphology, seed set, seed viability and quantified the effect of different physical as well as chemical treatments on percentage seed germination what have allowed us to explore the spread potential and/or seedling recruitment mechanism in the species.

MATERIAL AND METHODS

In the present study, fresh and one year old seeds of Myriphyllum spicatum L. were collected in October, 2010,from three standing water sites namely Dal Lake (DL), Mansbal Lake (ML) and Hygam Wetland (HW). The two running water sites, namely Shalimar Stream (SS) and Chanderhama Irrigation Canal (CIC), did not produce any seed.One part of the seed lot was used one year later in 2011 for germination studies. The seed quality parameters were studied along the following lines,

RESULTS

DISCUSSION

The fruit and seed of Myriophyllum spicatum have been variously described by different researchers and they were given to different names. Fruit has been given the name as nut and seed as nutlet (Orchard, 1986). But according to the present investigation, it cannot be called as nut because nut is a single seeded indehiscent fruit. Based on the present stereo and scanning electron microscopic investigations the fruit is a schizocarp with four deep longitudinal sutures from which it splits into four individual seeds called nutlets. The schizocarpic fruits are intermediate between dehiscent and indehiscent fruits. In Myriophyllum spicatum the fruit instead of dehiscing rather splits into a number of segments each containing one or more seeds. Our observations are strongly supported by the findings of Aiken (1981).

During the present investigation, it was observed that Myriophyllum spicatum produces a copious amount of seeds and the seed set percentage ranged from 70.98% to 77.91% across different populations. The seed set depends upon various factors, including inflorescence size, position of inflorescence with respect to water surface, meiotic behavior and habitat. The standing water populations possess large inflorescences, which are always above the water surface and have more than thirty flowers per inflorescence. The pollen ovule ratio is high and the stigmas remain receptive for long duration along the whole length of the spike. All these features including normal meiotic behavior favor effective pollination and in turn high seed set. The small amount of variation observed in seed set between different standing water population’s viz. Dal Lake, Mansbal Lake and Hygam wetland may be due to the eutrophic environment of the Dal Lake if compared to Mansbal Lake and Hygam wetland. This is supported by the work of Madsen &Boylen (1989) on Myriophyllum seed ecology from an oligotrophic and eutrophic Lake. The plants inhibiting running waters did not produce any seeds. Probably the spikes in such populations do not withstand the pressure of running waters and pollen grains are washed away without causing effective pollination. Seed germination in Myriophyllum spicatum was found to be significantly influenced by various factors such as physical and chemical treatments, age of seeds and light conditions. The seeds in which seed coat was not removed (intact seeds) did not germinate even when treated with different concentrations of GA3, IAA and also after removal of epicarp and mesocarp. The study of Patten (1955), that shows that partial removal of stony endocarp of the Myriophyllum spicatum seeds is effective in causing seed germination, is in close agreement with the present study, however, the removal of epicarp and mesocarp causes seed germination is not in agreement with the present study.

The cold stratification treatment to some extent improved seed germination. This confirms that dormancy is due to the mechanical barrier of the seed coat, mainly because of hard stony endocarp of the seed. Low temperature of the seeds causes rupturing of the seed coat leading to breakdown in their dormancy (Patten, 1955). Similar results have been obtained by Teltscherova & Hejny (1973); Baskin & Baskin (1998) and Hay et al.(2008) in different species of Potamogeton. Under natural conditions, the freezing winter temperatures of the Kashmir valley may have promoter effect on seed germination. Cold stratification is an effective way of alleviating dormancy in many species especially those from temperate regions (Baskin & Baskin, 1998; 2004) which ensures that germination does not occur until spring commences when water temperature is more favorable to the vegetative growth of aquatic plants (Hay et al., 2008). The seed age has a significant effect on its germination. The present data confirms that M. spicatum seeds do survive desiccation. The freshly collected seeds as well as those dry stored up to one year germinate provided the seed coat is cut. Percent germination, however, declines with the aging of the seed. Similar results were obtained by Hay et al. (2000, 2008) in the case of Potamogeton. These observations indicate that no after ripening period is needed by the embryo of M. spicatum and the seeds are an important means of propagation in the habitats where water dries up for some period during the year. These observations are in agreement with the work of Kadano (1984), who observed that Potamogeton distinctus overcomes the long lasting drought in irrigation reservoirs under warm temperature conditions by the formation of seeds.

The seed germination varied significantly under continuous dark and alternate light and dark conditions. The percentage germination was found to be more in alternate light and dark conditions if compared to the seeds Kept in continuous dark conditions in all the treatments. These observations are in agreement with the results of Coble & Vance (1987). However, for aquatic plants effects of quality of light are less clear, although some quantitative effects are known (Spencer et al., 1971; Sharma & Gopal, 1978). The seeds did not germinate in trays provided with sediment and water. The reason for this is the hard seed coat that restricts the water absorption and low temperature needed by the seeds for their germination, causing prolonged seed dormancy.

CONCLUSIONS

Myriophyllum spicatum produces a significant amount of seeds in standing water bodies of Kashmir. However, the seed germination of the species is influenced by various factors such as light, chilling, age of seedsand even the under sediment environment of these water bodies. The successful germination of seeds due to various physical and chemical treatments under lab conditions is an indication of the fact that some part of the species population might also be contributed by sexual reproduction (asexual reproduction being the dominant wayof reproduction) through seed germination in nature.

ACKNOWLEDGEMENTS

The second author is highly thankful to CSIR, New Delhi, for providing financial assistance in terms of JRF Fellowship during this work. We are also thankful to the Director of USIC for providing SEM facilities.