Germination and seedling emergence in Diplachne fusca: a semi-aquatic weed of rice fields.
The native grass D. fusca has become an important weed in rice fields in temperate Australia since rice cultivation began there in 1922. Field and glasshouse experiments were conducted to examine the population dynamics of D. fusca in rice fields and to develop strategies for its control. D. fusca can develop a persistent seed bank, with 48 000 seeds/m2 (0-6 cm depth) measured at one site 18 months after the last seed input. No seedlings emerged from depths >10 mm whereas 60% of seedlings arose from seed lying on the soil surface. Emergence was strongly inhibited by the presence of litter or living pasture plants. The highest seedling density was on bare ground (1480/m2), but this represented <5% of the seed bank. The patterns of seedling emergence were similar to those observed after experimentally storing seeds for 7.5 months in a range of semi-natural and artificial storage conditions. High dormancy levels were found in seed which had been stored dry and exposed to natural weather conditions. Storage in water gave the highest germination, but a single drying cycle reduced germination from 44 to 10%. Darkness inhibited germination of non-dormant seed. The development of dormant seed reserves in the soil and the dependence on open areas for regeneration are features that D. fusca shares with many arable weeds and colonizing species. These features as well as the loss of seed dormancy with prolonged flooding, are interpreted as adaptations to intermittent wetlands, the natural habitat of D. fusca prior to its invasion of rice fields.