Abstract:
The effect of various wet and dry day sequences on emergence of seedlings of Astrebla lappacea (Mitchell grass) from both spikelets and caryopses was studied in a glasshouse experiment. Three wet days were required to obtain maximum emergence while periods of 2-4 dry days delayed emergence but did not affect final emergence, provided moisture was re-supplied. The soil reached a water potential of -6 MPa after 2 dry days, which was sufficient to prevent the germination processes from proceeding. Maximum emergence was reached in 8 days for the caryopses and 10 days for the spikelets, so long as each wet day in the period was separated by no more than 2 dry days. At least 40% of the A. lappacea caryopses sown emerged as seedlings from a sowing depth of 60 mm in a sand medium, and from a sowing depth of 45 mm in a clay medium. The maximum depth from which seedlings emerged was 60 mm in the clay medium, and from 80 mm in the sand medium. Both media were maintained at 90% of field capacity. The emergence from single caryopses in sand was greater and more rapid than from clay. Planting depth significantly affected the length of the subcoleoptile internode of A. lappacea during a glasshouse experiment. All seedlings initiated their secondary roots at the soil surface irrespective of sowing depth. Increasing sowing depth retarded the early development of the secondary root system, but by week five, there were no significant differences between the dry weights of secondary roots from plants sown at different depths.