Literature review: Impact of Chilean needle grass ... - Weeds Australia

Dormancy mechanisms allow the seeds to persist in the soil for “many years” (Gardener et al. 1999 p. 10). Dormancy can be
broken by scarification and de-hulling (removal of the lemma) (Puhar and Hocking 1996) but is not broken by stratification
(chilling) at 3ºC for periods of 24, 72 and 168 hours (Puhar 1996). Heating of seeds to 60 ºC in an oven for 30, 60 or 240 minutes
signficantly increased germination (Puhar 1996). Gardener et al. (2003b) found that de-hulling of seeds stored for 8 months
increased germination from 48.5% to 82% at 15-25ºC with a 12 h light/dark cycle. Andersen (1963) found that germination of
naked caryopses of the closely related N. leucotricha was usually complete after 21 days in light at 15-25 ºC and in darkness at
15 ºC, and that seed with intact glumes germinated best when sown upright in soil and sand for 28-35 days at 15-25 ºC. Chilling
had little effect on germination.
An intact lemma may prevent germination by restricting the embryo or by acting as a barrier to water and oxygen. The lemma
protects the embryo from dessication and other harsh environmental conditions. Germination occurs only when the lemma is
broken or degraded by weathering (Puhar 1996, Slay 2002c) or the actions of decomposers. This may require 3-4 months in the
soil, so germination of panicle seed is delayed until mid-winter (Slay 2002c). Germination rate increases with duration of ground
burial: 90% of de-awned seeds buried in the ground for 2 y in terylene cloth bags germinated in the laboratory compared with
48% of laboratory-stored seeds, but seed viability did not vary significantly with burial depth (0-10 mm vs. 10-20 mm)
(Gardener et al. 2003b). In New England Tablelands pastures, only a small proportion of the seed bank germinates over 2 y
(Gardener et al. 2003b).
Dyksterhuis (1945) found that cleistogenes of N. leucotricha commonly failed to germinate within a year of their production and
were usually not wetted by rains because of the tight wrapping of the leaf sheath. Disintegration of the sheath was required for
germination. Few cleistogenes appeared to germinate on living plants. Basal cleistogenes reporrtedly gave rise to seedlings
especially in old, closely grazed, dead tussocks. Here they were protected from frost heave, which caused major mortality of
seedlings of panicle seed origin that germinated in areas cleared of litter and outside of tussocks. Seedling numbers were much
higher in bare areas than areas with accumulated litter.
Germination
The panicle seeds of N. neesiana, like most of the awned grass species tested by Peart (1984), are probably adapted to germinate
after lodgement in suitable sites at the surface with their lemmas only partially buried, and, contrary to the opinions above, to
have no dormancy but to react whenever moisture conditions and temperatures are suitable. In vitro, fresh seed germinates after
about 10 days at constant temperatures between 18 and 25ºC under a 16 h photoperiod, but not at constant temperatures of 18, 20
or 25ºC in continuous darkness, although germination is stimulated in darkness by a temperature fluctuation of 10-20ºC,
“suggesting a mechanism for pasture gap detection” via both light and heat (Bourdôt and Hurrell 1992 p. 101).
The timing of seed germination is regulated by rainfall (Bourdôt and Hurrell 1992). Germination occurs mainly in spring and
autumn but can happen at other times of the year if adequate soil moisture is available and temperatures are suitable (Bourdôt
and Ryde 1986, Duncan 1993, Gardener et al. 1999, Britt 2001, Slay 2001). In New Zealand pastures, there are two distinct
peaks in autumn or early winter, and in spring or late spring, and high winter rainfall may delay spring germination. Germination
is probably limited by low winter temperatures, and almost certainly by summer drought (Bourdôt and Hurrell 1992). Similar
patterns are apparent in Australia. Germination occurs predominantly in autumn and spring on the New England tablelands
(Gardener et al. 2003b).
According to Muyt (2005 p. 4), germination is “likely ... in response to the death of adult plants”. Seeds reportedly germinate
only in bare areas (Gardener et al. 1996a) or when gaps are created in pastures (Gardener et al. 1999).
Establishment of seedlings and juvenile plants
Many characters of seedling grasses may be useful taxonomically (Sendulsky et al. 1986), but those of N. neesiana do not appear
to have been systematically described, nor has the development of seedlings and juvenile plants been adequately documnted.
Internodes of grass seedlings are initially meristematic but new roots form mainly at the nodes, and the production of more,
larger, adventitious roots from the increased stem surface is enabled as the stem increases in diameter (Clark and Fisher 1986).
Later the thickening process of the stem changes to an elongation stage, with nodes forming on the stem at the leaf insertion
points where there is no elongation. Unlike most other plants, the most proximal nodes and the upper sections of the internodes
mature first, leaving an intercalary meristem, capable of cell division and growth, at the base of the internode, surrounded by the
base of the leaf sheath (Clark and Fisher 1986). The seedling gradually transforms into a juvenile plant that produces new leafy
shoots (tillers) from basal buds, each with their own roots. In tillering grasses the initiation of the inflorescene in the culm
generally corresponds with cessation of new vegetative tiller production, which may resume after flowering (Clark and Fisher
1986).
Peart (1979 1984) established that grasses with hygroscopic awns and a barbed callus like N. neesiana, have a distinct seedling
recruitment strategy, involving lodgement in an upright position on the soil surface with c. half the lemma exposed and no
dormancy. Seeds that fail to lodge by the callus produce seedlings in which the radicle is less likely to successfully penetrate the
soil. These seeds lie on the surface and were found to be destroyed by fire.
Recruitment of N. neesiana seedlings is often high. Slay (2001) recorded 1108 seedlings m -2 in winter in densely infested pasture
in New Zealand. Earlier germinating cohorts have better survival rates than later cohorts (Gardener et al. 2003b). On bare
ground, 78% of seedlings survived over 20 months (Gardener et al. 2003b). Seedling emergence from a natural seed bank on
bared ground varied from 5-136 m -2 in any 6 month period (Gardener et al. 2003b). Seedlings emerging from de-awned panicle
seeds deliberately placed, callus-downwards, 2 cm apart, at shallow depth (0-10 mm) had lower survival rates than seedlings
from seeds buried at 10-20 mm depth (Gardener et al. 2003b).
Where there is a seed bank, areas bared with herbicide “generally produce a large germination ... within 12 months” (Duncan
1993) and are ‘quickly reinvaded’ (Bourdôt and Ryde 1986). Cover and abundance data from surveys at Derrimut Grassland
Reserve, Victoria, suggested that seedling establishment is uncommon in areas of dense T. triandra (Lunt and Morgan 2000). In
experimentally bared ground (glyphosate application) emergence ceased after the regrowth of surrounding vegetation (Gardener
et al. 2003b), suggesting that disturbance that creates bare ground and sunlight are germination triggers (Gardener et al. 1996a).
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