• Rgn12

    According to our observation in India and Thailand (Morishima et al. 1980). the habitats of the wild rice are either roadside ditches (2-6 m wide) or depressions including lowland Fields and ponds (10-60 m in one-side size). The perennial types tend to be in deep swamps which retain water throughout the year, while the annual types grow in temporal swamps which are parched in the dry season. The habitats are disturbed by man and cattle to varying degrees. In addition, the wild plants would be subjected to competition with neighboring plants in favorable sites, and to various environmental stresses in less-favorable habitats.

    The occurrence of the wild rice is sporadic. There are many sites seemingly with similar ecological conditions, but the wild rice is found only in some of them. Where it occurs, it is often a dominant component of the plant community. This suggests that the wild rice has a low regenerating capacity as compared with other ruderal weeds. During our trips, the degree of dominance of the wild rice in each plant community was recorded in terms of percentage of total plant cover. The percentages ranged from 5% to 100% with a mean of 75% in India (13 sites), and from 38% to 100% with a mean of 64% in Thailand (14 sites).

    The number of weed species growing together was 78. Among them. a perennial grass, Leersia hxandra Sw. had the highest relative frequency (32%) in both India and Thailand, followed by Fimbristylis mileacea Vahl. (23%) and Echinochloa species (20%). The biomass of L.. hexandra was strongly negatively correlated (r =-0.81) with the biomass of wild rice, in the field. It was also found that annual weeds tended to be frequent in the habitats of annual wild rice, and perennial weeds were predominant in the habitats of perennial wild rice (cf. Oka 1988, p.31).

    To look into the mode of competitive interaction, a perennial (W120. from India) and an annual (W630. from Burma) strain of the wild rice and a strain of L.. hexandra (from Taichung) were tested for two successive seasons (1982 to 1983) with the plants grown in pots either singly or in 1:1 mixture. This experiment was conducted at Taichung in three conditions, i.e.. shallow water (0-2 cm), deep water (10-12 cm), and fertilized (6-5-5g/m ² NPK) shallow water (details in Oka 1994).

    In the first year. the effect of vegetative competition was positive for the annual wild rice and negative for L.. hexandra. In winter, the annual plants died and the perennial plants deteriorated. Regenerating success in the second season depended on the establishment of seedlings or new plantlets. It was found that both the perennial and annual wild rices as well as L. hexandra had the largest plant number and stem number per pots in their pure stands, and were affected negatively by the presence of alien plants (Table 1 ). The annual wild rice was greatly reduced in number when it was grown with perennial wild rice or L. hexandra. It did not occur in deep water. The perennial wild rice regenerating by ratooning was suppressed by the presence of L. hexandra strongly, and to some extent by the presence of annual wild rice. The L. hexandra plants which regenerate mainly by cespitose tillering were relatively insensitive to co-existing wild rices (Oka 1994, Table 1).
 
 

Table 1. Effects of vegetative competition (1982 July: single plant dry weight, g) and regenerating competition
        (1983 July: stem number per pot) on a perennial wild rice (P). an annual wild rice (A) and Leersia hexandra
        (L) as shown by data for pure stand and 1: 1 mixture (mean for 2 plots)
 

    For the purpose of comparison, the soils sampled from a paddy Field and those from under L. hexandra and some other plants in Taichung were kept dry in laboratory. cleaned by removing plant roots and other debris, and were used for germination tests with the same lot of wild-rice seeds. It was found that the soils where L. hexandra grew gave much lower germination than those from paddy, even when the soil samples were brought to Mishima for tests (Table 2: Oka 1992). The soils taken beneath a wild-rice stand also gave reduced germination. This was reconfirmed in an experiment conducted at Taichung in which the seeds of an annual strain (W630) collected from the field at Pingtung in February were tested in April (Table 2. 1986 experiment).

    These data suggest that the growing plants exude some phytotoxic substance which is retained in soil for a period and it exerts an allelopathic effect on seed germination. The allelopathic soil-ingredient would have a negative effect on the growth of plantlets produced by vegetative regeneration, even if to a lower degree than for seed germination. It may be suggested that the interrelationships among component species of a plant community are complex.
 
 

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