Botany Department, Obafemi Awolowa University, Ile-Ife, Nigeria
We have monitored a large population of tetraploid Oryza punctata Kotschy ex Steud. found in Ipetumodu, Nigeria (7 deg 30'N, 40 deg 45'E) since 1986. Nwokeocha (1992) reported chromosome mean configuration of 21.7 ring IIs plus 2.3 rod IIs, without any multivalents from 500 pollen mother cells. This indicated autosyndetic pairing as in an amphidiploid. The genetic uniformity observed in the population was consistent with this observation, although the possibility of occasional allosyndetic pairing was suspected.
Paired soil samples of 50cm X 25cm with a depth of 20cm were taken from three locations (A, B and C) from a pond (Fig. 1). One sample was used to estimate the total number of seeds in two strata, 0-10cm and 10-20cm, and to examine seed germination; the other was watered to induce germination and the resulting seedlings were monitored for 10 weeks, after which the residual seeds in the soil were counted.
Fig. 1. Collection sites in the Ipetumodu population of Oryza punctata. A
and C represent locations of high density, while B is in an open submerged
location. Clusters of Oryza punctata line the bank and extend far into the
bush. C extends far beyond the field of view. The locations are generally
open, but some shade is provided by trees, especially by Bambusa vulgaris L.
The surface of water is covered by Nymphaea species.
About 93% of the total seeds were found in the 0-10 cm stratum of the soil. The approximate seed densities were estimated to be 768/m2 for A, 216/m2 for B, and 1824/m2 for C. Germination was 51% for A and 53% for C which had high seed densities, and 78% for B where seeds were held under mud. These germination estimates yielded recruitment values of 11.5% for A, 56% for B, and 10% for C. The high germination and recruitment estimated for B seem to suggest the presence of seeds at a shallow depth. As discussed by Harper (1977, p. 83-147), high germination may be explained by the preservative effect of mud.
The proportions of dormant seeds were 47% for A, 19% for B, and 46% for C (Fig. 2). The residual seeds in the soil germinated, but could not emerge. This is consistent with the results of germination tests which showed that dormancy was not important for seeds in this population. Since germination requirement is met for all the seeds and only about 11% of them are recruited into seedlings in a season, no seeds may be left under the soil. Observations over years have shown that recruited seedlings compete poorly with plants regenerating from rootstocks, and in the flooded open location like C, seedlings are difficult to be established. A fire incidence in December 1991 destroyed the rootstocks considerably and created open space for the establishment of recruited seedlings.
These observations raise a poor prospect for genetic variability which can be engendered by occasional allosyndetic pairing and maintained by seedling recruitment and survival. The population dynamics expected from the observations will lead to erosion of genetic resources (Fig. 3). Fire seems to encourage seedling recruitment, but threatens the stabilizing role of the vegetative bud component of the seed bank. This population, like other similar ones, is subjected to keen competition with Coix lacrima-jobi, Panicum maximum, Urena lobata, Alternanthera sessilis, and Eclipta alba, the last two being aggressive colonizer weeds. The occurrence of tetraploid Oryza punctata in stable locations under shade (Sano 1980)
Fig. 2. Histograms showing distributions of seeds and seedlings in the sample
sites.
Fig. 3. A model for the population dynamics of Oryza punctata.
seems to be largely due to the protection of the rootstocks which hold the ground from season to season.
Acknowledgement. This work is supported by grant 1425MB of the Obafemi Awolowo University.
References
Harper, J. L., 1977. Population Biology of Plants. Academic Press, London.
Nwokeocha, C. C., 1992. Botanical and cytogenetic studies of some African accessions of a wild rice (Oryza punctata Kotschy ex Steud.). M. Sc. Thesis.
Sano, Y., 1980. Adaptive strategies compared between the diploid and tetraploid forms of Oryza punctata. Bot. Mag. Tokyo 93: 171-180.