VARIATION IN SEED CHARACTERS OF TERMINALIA CHEBULA RETZ. FROM THE SOUTHERN WESTERN GHATS OF TAMIL NADU

Sahaya Anthony Xavier, G. Department of Botany, St.Xavier's College (Autonomous), Palayamkottai – 627002. E.mail: saxsxc@gmail.com Seed is an important aspect of a plant’s life history. The successful regeneration of a plant in its habitat and the ability of the plant to disperse across ecosystems depend upon the seed. The number of seeds produced and the size of the seed are factors which affect the survival of a plant. The size of the seed determines the amount of reserve food available to the developing seedlings. In stressful environments, larger seeds may have a higher establishment rate, as they may provide more reserve food for the seedlings. From the Terminalia chebula populations in the Southern Western Ghats of Tamil Nadu, 100 seeds each were collected from twelve populations. The seed weight, seed length and seed width were noted individually. The seed characters were noted and compared between the populations. The mean values were also calculated and compared. The inter-population variation was studied with respect to the seed characters of Terminalia chebula.


INTRODUCTION
Seed size is an important aspect of the reproductive biology of a plant. It is traditionally considered that seed mass within a species is a remarkably constant characteristic (Harper et al., 1970;Silvertown, 1981). Schaal (1980) has shown that there is considerable variation in the seed mass between the individuals of a species or the seed mass of an individual plant can vary greatly (Schaal, 1980). It has also been recognized that seed size is correlated with habitat (Salisbury, 1942). Seed size affects the seedling biomass. Usually the seedlings from larger seeds are larger than those from small seeds, especially in the early stages of growth (Schaal, 1980). Evolution has led to variation in the seed weight between or within plant species and populations. The variation in seed size and weight are a means to produce a large number of seeds which can ensure the fitness of the plant populations. A greater allocation of the resources from the mother plant also ensures a greater chance of establishment of the resulting seedling (Zhang, 1998).
There is however, a quid pro quo in that the massive production of seeds and allocation of resources to individual seeds is dependent on the resources available with the maternal plant. Thus the compromise between the two different strategies results in the variation in the seed weights even within a species. Various biological and environmental factors play a crucial role in determining the seed weight. Thus seed weight is dependent on the genetic constitution of the maternal plant and the environmental and evolutionary processes operating in the environment (Zhang, 1998). Plant species in closed or dry habitats produce larger seeds than those in open or moisturerich habitats (Salisbury, 1942;Herbert, 1972). The seed weight also varies with the height, growth, form and mode of dispersal (Zhang, 1998). Therefore the study of the relationship between seed characters and other traits in natural populations is important (Sivakumar et al., 2002).
This study investigated the variation in seed characters in natural populations of Terminalia chebula of the Southern Western Ghats in Tamil Nadu. Therefore the seed length, seed width and seed mass (seed weight) of the above species was investigated.

Plant material
Terminalia chebula Retz. is a tropical tree, 15-20m high, which has yellow-white flowers. The fruit is a dry drupe. It is widely distributed in the greater part of India and Burma in mixed deciduous forests of comparatively dry types. It grows well in laterite, clayey and sandy soils. In peninsular India, it is found in mixed deciduous forests to dry deciduous forests and extends up to an elevation of 3000'. It survives well with the temperature between minimum 30° -60° F. and 100° -180° F and rainfall from 30" to 130". It is fairly hardy against frost as well as drought. It withstands fire well and shows good recovery from burning. The fruits, known as myrobalans, contain 3.5% chebulic acid, 37% fatty oil, 27-39% tannin and ellagic acid. The drug is highly astringent (Keys, 1976). The fruits are used for their astringent, anti-diarrhoeal and haemostatic properties.

Seed collection and data analysis
Seeds of Terminalia chebula were collected randomly from natural populations and labeled. The seeds were dried in shade in the lab. One hundred seeds were taken randomly from each population to represent the population. Two collections were made at Chanan Parai and Veerapuli forests from two different populations. The seed length and width were measured using a Vernier calliper and the seed weight was measured using an electronic balance (SHIAMDZU Model BL-6205). The data was tabulated and analyzed.

RESULTS AND DISCUSSION
The results of the study are given in Table 1-4 and Fig. 1-3.

Seed length
The maximum seed length among all the populations was observed to be 3.98 cm (S1) and the minimum seed length was 1.27 cm (S10). The average seed length was found to be maximum at 3.21 am in Chanan Parai I population (S1) and the average seed length was minimum in Kadukkatheri population (S9) (2.73). The mean seed lengths of the populations varied from 2.73 cm to 3.21 cm although individual seeds showed a much higher variation. However as a population the mean seed weight does not show much variation.

Seed width
The maximum average seed width was 1.94 cm in Chanan Parai I population (S1) and the minimum average seed width was 1.48 cm represented by two populations (Courtallam (S5) and Kadukkatheri, Courtallam (S9). The maximum seed width was observed in two populations (2.94 cm width at S6 and S10). The minimum seed width was observed in Mylar with seed width of 1.00 cm.
The mean seed width too shows a variation of 1.48cm to 1.94 cm. The range of variation too is not very high for the seed width. Therefore the mean seed width too does not show much variation.

Seed weight
The average maximum seed weight was 3.66g at Chanan Parai II (S2) and the minimum average seed weight was 2.91g at Kadukkatheri (S9). The minimum seed weight was 1.12 g at Courtallam (S5). The maximum seed weight was 6.62 g at Chanan Parai II (S2). The mean seed weight varied from 3.15g to 3.66 g although individual seeds did show quite a large variation. When considered as a group however, the variation between the groups is not significant.
There is variation in the seed sample collected from the given area. However, the results of the ANOVA show that the variation in the seed length, seed width and seed weight are not statistically significant. This could be because the seeds have been collected over a small geographical area.
The difference in seed size may have important ecological implications. The variation in seed mass within a species may affect the seed germination (Weis, 1982) and the germination rate (Zhang and Maun, 1990). Large seeds may have more reserve food which could enable them to have a greater percent germination than smaller seeds (Hendrix, 1984). On the other hand, smaller seeds may germinate more quickly than larger seeds. They may be better able to exploit the given conditions in an environment and may thus have a competitive edge over larger seeds (Howell, 1981). Seedlings from smaller seeds, particularly of fast-growing species, would be able to cope with mild drought by morphogenetic and physiological plastic response in a better way than those from large seeds. However, seedlings from large seeds had greater survival than those from smaller seeds under intense water stress. (Ekta and Singh, 2004).
For a plant, decreased seed weight may be a disadvantage as small seeds are often associated with lower germination percentage and smaller seedlings would lead to decreased chances of seed germination and seedling establishment (Stanton, 1984;Krannitz et al., 1991). However, Stamp (1990) found that with increase in the size of the seed, there is a decrease in the seed germination rate. Hendrix (1984) reported that the increase or decrease in rate of germination with increase in seed size was dependent on the environmental conditions. Several studies have documented the variation in seed weight both within and between plant populations (Hawke and Maun, 1988;Michaels et al., 1998, Zhang andHamill, 1996;Cordazzo, 2002). Populations with different seed weights are expected to have evolved under different selection pressures (Westoby et al., 1990;1992). Moreover, seed traits do not evolve in isolation but are shown to be Table 1. Seed characters in Terminalia chebula seeds.
correlated with other plant traits such as plant height and growth form (Mazer, 1989;Leishman et al;1995). Variability studies are the prerequisite for genetic improvement of any tree species under various agro climatic conditions, (Sharma et al., 1994). In case of Grewia optiva, the seed length and the 100 seed weight were found to be the best predictors of germination (Tyagi et al., 1999). -Veerapuli II; S9 -Kadukka theri, Courtallam; S10 -Foothills of Kadukka theri, Courtallam; S11 -Servalar; S12 -Mylar.

CONCLUSION
Apart from the variations in the seed characteristics such as seed size, seed length, seed breadth and seed weight a number of other factors too play a crucial role in the establishment of a seed. These include ecological factors such as seed dispersal, seed dormancy and viability, seed predation and the ability of a seed to uptake water. The requirements of a seed may also vary from site to site (Winn, 1985). Thus the variation in seed size is of great ecological significance in the establishment and maintenance of plant populations. The seed also plays an important role in the perpetuation of populations in harsh environments which in turn are important to determine the evolutionary success of a species. Further studies should be conducted on a wider scale to understand the variation in seed characteristics of Terminalia chebula.