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