Summary. Four experiments were conducted to determine
the effects of temperature, light and leaf extract solutions on the
germination of Giant Parramatta grass [GPG,
Sporobolus indicus (L.) R. Br. var.
major (Buse) Baaijens] collected from a population
on the North Coast of New South Wales. In the first experiment, seeds were
subjected to one of a range of temperature combinations immediately after
collection and again after 8 and 27 weeks. Germination was restricted to a
narrow range of alternating temperatures with a peak at 35°C
day/15°C night when seeds were tested immediately after collection.
More seeds germinated when the samples had been stored, although germination
remained depressed at constant temperatures. These data indicate that freshly
collected GPG seeds are subject to primary dormancy and that few would
germinate in the field immediately after seed fall.
In a second experiment, seeds were buried beneath leaf litter in a pasture
immediately after collection. After 7 months, the seeds were exhumed and
subjected to either constant (20°C) or alternating (35/15°C)
temperatures in either full light, reduced red:far-red (R : FR) light or dark
treatments. Over 95% of GPG seeds germinated when subjected to
alternating temperatures, regardless of light treatment. At constant
temperatures, 97% of seeds germinated under full light, 59% at
reduced R : FR light and <1% in dark treatments.
A germination response to alternating temperatures and/or light treatments
has been reported in pasture weeds and may be an adaptation to detecting gaps
in the pasture canopy. Consequently, the germination of GPG in a pasture may
be manipulated to some extent by altering the amount of pasture cover using
grazing management, mowing and fertiliser applications.
In experiment 3, leaves from a range of coastal grasses were mixed with water
and the solutions were used to germinate GPG seeds. Solutions extracted from
setaria (Setaria sphacelata) leaves completely inhibited
GPG germination while 27% of GPG seeds germinated when imbibed with
kikuyu leaf extract solution. Solution extracted from carpet grass
(Axonopus affinis) leaves had the least effect on GPG
germination.
In experiment 4, the effects of solutions that had been leached from the
leaves of either setaria or carpet grass on seed germination, and root and
shoot lengths of GPG seedlings were compared. Germination was less inhibited
by leachate solutions compared with the extract solutions used in experiment
3. Seedlings in setaria leachates had significantly shorter roots and shoots
than both those germinated in carpet grass leachates and control seedlings.
This may explain, at least in part, why carpet-grass-based pastures are
readily infested with GPG while setaria-based pastures are relatively
resistant to infestation. The potential for allelopathic interactions between
GPG and setaria to be fully utilised to reduce the abundance of GPG in coastal
New South Wales pastures is discussed.