Studies were conducted to investigate seed longevity, seedling emergence and seed production of the weed Phalaris minor in wheat in northern India. The longevity of P. minor seeds buried in bags in the field was often limited to less than 1 year, although many seeds buried at 30 cm depth in a rice-wheat rotation remained viable for longer. The application of direct seeding in wheat reduced the seedling emergence rate of P. minor, when compared with conventional ploughing and sowing. However, a larger P. minor seedbank in the upper soil layer in plots under direct seeding partly reversed this positive effect in one of the two studies. Besides differences in relative distribution of weed seeds through the soil profile, it was likely that other factors such as reduced soil disturbance and soil characteristics associated with the application of direct seeding were also involved in regulating the emergence rate. Mature P. minor plants in wheat were found to invest a stable part (27%) of their aboveground biomass in seed, so that total seed weight was strongly linearly correlated with the aboveground biomass of the mother plant. Individual seed weight, however, was little affected by the weight of the mother plant.
: Two populations of Echinochloa crus-galli (R and I) exhibited resistance to quinclorac. Another population (X) exhibited resistance to quinclorac and atrazine. The R and I populations were collected from monocultures of rice in southern Spain. The X population was collected from maize Ðelds subjected to the application of atrazine over several years. The susceptible (S) population of the same genus was collected from locations which had never been treated with herbicides. The quinclorac value (dose causing 50% reduction in shoot ED 50 fresh weight) for the R and I biotypes were 26-and 6-fold greater than for the S biotype. The X biotype was 10 times more tolerant to quinclorac than the S biotype and also showed cross-resistance to atrazine, being 82-fold more resistant to atrazine than the R, I and S biotypes. Chlorophyll Ñuorescence and Hill reaction analysis supported the view that the mechanism of resistance to atrazine in the X biotype was modiÐcation of the target site, the DI protein. Quinclorac at 20 mg litre~1 did not inhibit photosynthetic electron transport in any of the test biotypes. The quinclorac values (herbicide dose needed for 50% Hill I 50 reaction reduction) of the S population was over 50 000-fold higher than the atrazine value for the same S population, indicating that quinclorac is not a I 50 PS II inhibiting herbicide. Propanil at doses greater than 0É5 kg ha~1 controlled all the biotypes.
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