Ecological Studies on Environmental Adaptation in Weeds

1. The Effect of Density on Growth and Seed Productive Structure of Large Crabgrass and Common Purslane

Abstract

As a part of studying the mechanism of environmental adaptation in weeds, this experiment was carried out to investigate the effect of planting density on growth and seed production of two species of weeds.
Large crabgrass (Digitaria sanguinalis var. marginata Fernal.) or common purslane (Portulaca oleracea L.) was planted in a/2, 000 pot to provide the four levels of planting density, i. e. large crabgrass; 5, 20, 50 and 100 plants/pot, common purslane; 5, 20, 40 and 65 plants/pot.
1. In common purslane, the increase of mortality up to about 10% was observed with the increase of planting density during the growing period. On the other hand, no mortality was observed in large crabgass.
2. Tolal dry matter production per plant decreased almost linearly with increasing the planting density up to 65 plants in common purslane and 100 plants in large crabgrass, and in both weeds the differences between maximum and minimum density were larger at the harvesting time than at the vegetative stage.
3. As for the seed productions and their components, the number of heads or pyxis per plant, and seed production per plant decreased in both species with the increase of planting density.
Seed production per head or pyxis, however, showed the highest value at 20 plants-plot in large crabgrass and 5 plants-plot in common purslane respectively.
4. Phenotypic plasticity on each character of both weeds was showed by regression coefficient to planting density, and it was recognized that common purslane had a high phenotypic plasticity as compared with large crabgrass.
5. Crude reproductive efficiency (CRE) of both weeds was calculated according to Harper & Ogden's method. It was showed clearly that large crabgrass had a high CRE value as compared with common purslane and also in common purslane, CRE value decreased with the increase of planting density.
6. The individual variation within plot was compared with respect to some characters measured, and it was commonly observed in both weeds that the individual variation within plot increased with the increase of planting density.
7. It was suggested from above results that both weeds have the different reproductive strategy under the stress condition caused from the increase of planting density.
The high reproductivity in large crabgrass seems to be derived from keeping the number of plants per unit area, their uniform growth and seed production. On the other hand, in common purslane it seems to be derived from the mutual supplement in reproduction among the plants which survived under the stress condition.