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

taxonomic patterns of protein amino acids in leaves and caryopses of grasses, but provided only aggregrate data at the tribal
level. Seeds of Stipeae are high in Asx (aspartic acid or asparagine), glycine, methionine and arginine and low in glutamic acid,
proline, alanine and leucine. They are nutritionally richer than seeds of Danthonieae, Aristida, Chloridoideae and Panicoideae,
have protein contents equivalent to cultivated cereals and superior nutritional value to wheat, sorghum and maize varieties. The
“Stipa” spp. analysed (including Nassella spp.) in aggregate are very similar to Oryza in leaf protein pattern except that Stipeae
are lower in alanine and higher in valine. Stipeae, Oryza and Ehrharteae have the lowest free Glx (glutamic acid or glutamine)
and the highest free Asx levels in the Poaceae. The variation in leaf amino acids are “unlikely” to be of any nutritional
importance for mammals but may influence insect herbivory.
Investigations have recently been undertaken by the Victorian Department of Primary Industries in an attempt to identify
molecular fingerprinting techniques for identification of Nassella spp. (David McLaren, DPI, pers. comm.) but results have not
been published.
Breeding system
N. neesiana is self fertile (Connor et al. 1993) but can cross pollinate. It produces both chasmogamous and cleistogamous panicle
seeds, along with concealed cleistogamous seeds on the stem nodes (Slay 2002c) (see discussions above on floral morphology).
Such a breeding system is well suited to colonising grassland habitats (Evans and Young 1972).
Cleistogamy is more common in grasses than any other plant family and occurs in c. 19% of genera and 5% of species (Groves
and Whalley 2002). Culley and Klooster (2007) found that it had been reported in 326 grass species, including 41 spp. of Stipa
and 5 of Nassella. Two types of cleistogamy occur in Nassella, complete and dimorphic. Plants that are completely
cleistogamous produce only cleistogamous flowers. Plants with dimorphic cleistogamy have both chasmogamous and
cleistogamous flowers, the latter characterised by prominent differences in floral morphology including reduction in size or
number of floral parts. In the cleistogamous flowers of grasses, the anthers, pollen sacs, stamens, stigmas, and lodicules are much
reduced in size and the flowers develop no further than the bud stage (Brown 1952). The different flower types can be separated
spatially on the plant, or temporally (often seasonally), or both, and in some plant species the ratio of flower types can vary
between individuals and populations (Culley and Klooster 2007). The proportion of cleistogamous and chasmogamous florets in
grass species is often under environmental control, e.g. Amphicarpum purshii produces few cleistogenous seeds after fire, but
larger numbers as the likelihood of fire becomes high (Groves and Whalley 2002, citing Quinn 1998). Brown (1952) found that
such facultative or ecological cleistogamy occurred in Nassella leucotricha in response to low availability of soil water. The
distribution of cleistogamous and chasmogmaous fruits in the panicle depended on the soil water potential at the time of floral
initiation.
Chasmogamy allows for gene exchange between individuals via pollen, and has population and evolutionary advantages where
there is greater environmental variability, or the prevailing genotypes produce phenotypes that are poorly adapted. Cleistogamy,
on the other hand, ensures self fertilisation with consquent high uniformity of genotype frequencies, so maintains the existing
frequencies of locally adapted phenotypes and genotypes (Groves and Whalley 2002, Culley and Klooster 2007). Meiosis occurs
in both gametes of the cleistogamous flower, so gene resorting does occur (Groves and Whalley 2002), enabling continued
“repatterning of the gene pool, differing only in degree from [that in] outbreeding populations” (Evans and Young 1972 p. 235).
Cleistogamous seeds may be energetically less expensive to produce because of their greater rate of fertilisation and savings in
pollen (Connor 1986), so the seeds can be larger (Culley and Klooster 2007). However in N. neesiana they are generally
smaller,with much reduction in the size of the appenedages. Cleistogamy has disadvantages, including increased inbreeding
depression and the aforementioneddecreased genetic variation (Culley and Klooster 2007). Dimorphic cleistogamy requires that
each flower/seed type offers specific selective advantages. In grasses, the most important of these may be the differential
dispersability of the seed types. Variation in the incidence of cleistogamy in the panicle of N. neesiana does not appear to have
been studied, nor have potential environmental influences of facultative cleistogamy.
Reproduction in N. neesiana is probably entirely sexual; apomixis, often evident by the production of twin seedlings from a
single seed (Groves and Whalley 2002), does not seem to have been reported (e.g. Puhar 1996).
Seed production
In comparison to most other grasses the panicle seed of N. neesiana is large, and relatively few are produced per plant. In
pastures at Waipawa, New Zealand, 793 ±128 culms m -2 were present in a pure ungrazed sward in pasture, and potential seed
yield per panicle was 38, indicating a potential annual panicle seed yield of c. 30,000 m -2 (Slay 2001). In dense, ungrazed
infestations on the Northern Tablelands of NSW Gardener et al. (1999 2003a) recorded maximum production of 1,584 panicle
seed m -2 in 1995 (the preceding year and spring being as dry) and 22,203 panicle seed m -2 in 1996 (above average rainfall),
determined from the number of infloresences m -2 and the number of glume pairs per inflorescence. The variation between the
drought year and the wet year resulted from changes in the number of panicles m -2 .
Gardener’s (1998) figure of 22,000 has subsequently been widely quoted (e.g. “20,000” in Snell et al. 2007). However the spring
of 1995 was actually exceptionally wet (see Gardener 1998 Fig. 3.2 on p. 22) and may have resulted in a mast seeding event,
accounting for the c. 14 fold increase in the estimated seed production over the previous year, and a c. 3.7 fold higher production
than 1997. Masting has been defined as “synchronous highly variable seed production among years by a population” (Kelly et al.
2008) and may have evolved as a a seed-predator satiation strategy (Kelly et al. 1992). The prime characteristic of masting
events is that a very high proportion of the population reproduce massively in mast years and poorly in non-mast years, cued by
weather events (Kelly et al. 2008). Masting has been described in a number of long-lived tussock grasses including Stipa
tenacissima, and appears to result from the build up of stored reserves and rapid responses to high water availability during the
growing season (Haase et al. 1995). Conversely, poor seed set is a feature of dry years and drought periods. Bountiful weather is
required for mass-seeding, but predator satiation, in which a much larger proportion of the seed crop survives in mast years than
non-mast years, accounts for the evolutionary benefits of the phenomenon, at least in New Zealand Chionchloa grasses (Kelly et
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