THE EFFECTS OF RAMET DENSITY ON GROWTH AND BRANCHING OF SALVINIA

Single rarnets of the tropical, floating, freshwater, aquatic weed, Salvinia molesta a t its tertiary stage were grown at initial densities of 16 plants, 80 plants and 160 plants per square meter, in a pond. Growth parameters were measured for each plant separately after 6 wks. Results showed that thenumber of nodes and leaves, the mean internodal length, the length of whole stolons, the dry weight of plants, the number of branches and the percentage branching decreased significantly (p<0.05) with increasing initial density of ramets.


INTRODUCTION
Plants are modular, in that they develop through iteration of morphological units termed modules.The iteration of these units, the leaf with its axillary bud and associated internode forms a branching structure,l which depends on the branch angle, the internode lengths and the dynamics of the birth and death of meristems; and hence the arrangement of modules in space.Branching determines the fitness of an organism which depends on the number of descendants that a genet contributes to future generations.Although the branching pattern is genetically determined, its rules may change continually depending on the availability of resources.The response of modules to local environment was demonstrated by N ~b l e , ~ who provided different local nutrient conditions to each module of a single rhizome of Carex arenaria L. Furthermore, light quality (reduced red to far-red ratio) and quantity (reduced light intensity) reduced the branching of Trifolium repens L.3 Factors such as soil moisture, nutrient supply and the season also affect the branching of T. rep en^.^ The presence of both intraspecific and interspecific neighbours changes its local environment (e.g.availability of resources), affecting the growth of individuals in a population both at genet level and modular level.Thus higher densities (intra-specific neighbour effects) may increase the risk of mortality of genet^,^ and on the other hand, they may alter birth and death rates of their modules such as leaves, branches, flowers, fruits, rootlets etc.
The response of plants to the density stress at modular level is expressed in the reduction in size frequently measured in terms of b i o m a ~s .~-~ Reduced size however is mainly due to the effects at modular level and hence due to reduced bran~hing.~JOJlIn clonal plants, density stress may influence modules more than genets.
The clonally spreading, floating aquatic fern Salvinia molesta Mitchell is a problematic weed in tropical fresh water bodies.The plant is sterile12 and therefore population increase depends on vegetative growth and branching.The plant develops branches from lateral buds, which arise in leaf axils on alternate sides of the stolon at successive nodes.Although there are three lateral buds in each axil it is the first rank bud that develops into branches under natural conditions, and the second and third rank buds develop in high nutrient conditions.13 The weed is being biologically controlled by a weevil Cyrtobagous salviniae Calder and Sands.l4 In monitoring control programmes it is essential to know the behaviour of individuals and their parts under different environmental conditions such as availability of resources.The behaviour of individuals may then be interpreted in the context of the population.As the plant's fitness depends on vegetative propagation, studies on the growth and branching are of great importance.Reduced growth and branching due to the effects of damage to leaves and roots andincreased branchingdue to severence of stolons have bein reported.15 The effect of density of plants on its morphological plasticity has also been studied.16 In thick growing mats of S. molesta (i.e. at the tertiary stage) density stress is unavoidable.
The results of experiments designed to study the effect of initial density of ramets (leaf pair with its root and axillary buds) at their tertiary stage1' on growth and branching of S. molesta are reported here.

MIETHODS AND MATERIALS
The pond of 5 x 3 ~1 m in the botanical garden of the University of Kelaniya, was cleaned and layered with mud to 25 cm brought from a place where S. molesta grew densely, to which pond water was added upto 90 cm in height.This level was maintained throughout the experiment by adding pond water.Eighteen wooden frames (quadrats) of 25x25 cm were placed afloat in the tank.The rootlets of all treatments share the same growth medium, although their above-water modules are partitioned by having the wooden frames.
Uniform (in age and size) single node cuttings (ramets) were selected from a healthy clone of S. rnolesta at its tertiary stage, and were used for different treatments.The following treatments were allocated to each frame using six replicate quadrats for each treatment.The lay-out af the experiment was a completely randomized design: After six weeks of growth each individual plant was destructively harvested and number of nodes and leaves on main and primary stolons (stolons arising from main stolon), number of branches on the main stolon, length of main and primary stolons, and the mean internodal length of the main stolon were measured.Five randomly selected plants from T3 treatment quadrats were taken for measurements.Then the plants were washed carefully and the leaves; roots and stolons were separated in each plant and were placedin separate paper bags and oven dried a t 70°C, and the final constant weights were recorded.The percentage branching was then calculated.
The analysis of variance was carried out on the MINITAB data analysis package on a Commodore PC 10 111.Multiple comparison tests were carried out for the comparison between three treatments using Sheffe's method.lREach experiment was carried out three times and the results were similar.Hence the results of the third experiment are reported here.

RESULTS
Table 1 shows the overall values for the parameters for each individual in each treatment.The number of nodes, leaves, the length of stolons, the internode length, the number of branches, percentage branching and dry weights of leaves, roots and stolons were highest in the control, and decreased with increasing density of individuals.The number of nodes on the main stolon of eachindividual plant decreased significantly (p<0.05) by 12% and 18% in the intermediate (T2) and in the highest (T3) density treatments resbectively.The effect on the number of nodes on primary stolons was greater than that on main stolons.The highest number of nodes was found in the control treatment, whilst it was reduced by 69% and 98% in T2 and T3 treatments respectively.The number of leaves on the main stolons were reduced to 89% and 83% ofthe control inT2 a n d ~3 treatments respectively whilst the reductions were 68% and 98% in T2 and T3 treatments respectively on primary stolons.
Control plants had the longest main stolons, and in the other two density treatments, the length decreased by 18% and 25% respectively.The effect was greater in the primary stolons than that found in the main stolon.These reductions were 67% and 98% in T2 and T3 treatments respectively.The mean internode length was also significantly (p<0.05)decreased by 30% and 35% of the control in the intermediate and highest densities respectively.Number of branches on the main stolon was the highest in the control plants and it has reduced by 56% and 83% in the T2 and T3 treatments respectively.The highest percentage branching was found in the control plants and there was a gradual yet, significant (p< 0.05) decrease with increasing density of individuals, and the reductions were 50% and 78% in T2 and T3 treatments respectively.The density treatments affected the dry weights of leaves and roots significantly (p < 0.05).The dry weights were highest in the control treatment, whilst there was a gradual decrease with increasing density.Dry weights of leaves reduced to 75% and 57%, roots reduced to 66% and 48% whilst stolons reduced to 50% in both T2 and T3 treatments.

DISCUSSION
The results show that plants grown at higher densities were less branched and smaller in size.The observedresults are supported by similar observations made by various authors for erect plants with single stems or stages of plants with single stems, e.g.Helianthus annus L.,ll Spergula arvensis L.,1° Trifolium subterraneum L. 19 Clonally spreading plants also have shown less branching at high densities, e.g. at high densities the rate of tiller formation decreased in Loliumperenne L.9 Similar results have been reported for Trifolium rep en^.^^,^'Solangaarachchi and Harperz2 have also shown that in T. repens plants, when grown close to each other branching decreased on interfering sides.At higher densities, plants and their parts compete for the same limited resources (nutrients, water, light and space) available in their immediate environment and this probably influenced the growth and branching of individual plants of 5. rnolesta at higher densities.
At higher densities plants with erect stems elongate their stems (via internode length) which helps keep the leaves exposed to better light.In the present study mean internode length and the length of primary but not main stolons decreased significantly at higher densities.This is probably because stolons grow horizontally and an increase in their extension rate does not place the leaves higher in the canopy.Branching or their further development can continue until all the space is occupied by Resource Depletion Zones (RDZS).~~Thus branching may change according to the availability of resources.Similar results have been found for fungal colonies (J.L. Harper personal com'munication).
The nutrient content in water is probably uniformly distributed and therefore even at higher densities, nutrient limitation may not be a significant factor.Thus in the present experiment reduced branching of each individual at higher densities could have been due to the availability ofless space, which could have influenced the light quantity as well as light quality.At higher densities bud sites are naturally shaded by leaves of individual^:^ whereas at.lower densities the natural shading occurs to a less extent.Thus bud sitesmay not have been well exposed to light at higher densities, and further development of them may have been inhibited, e.g.reduced growth and branching at low light intensities have been reported for Trifolium rep en^.^.^^

Table 1 : The effect of density on branching of S. .moZesta
five ramets per quadrat, T3 -ten ramets per quadrat.Mean 4 se sharing the same letter within each row do not differ significantly (p > 0.05).