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

are the seeds larger/smaller, longer/shorter awned, less/more hairy, and how might this relate to increased dispersal efficiency? Is
there evidence of higher cleistogene production in Australia, e.g. does Burkart’s (1969) ‘solitary’ indicate a true difference
between populations in the core native range and with the Australian and New Zealand populations?
Phytoliths
The epidermis of grasses includes specialised small cells known as silica cells which occur in pairs with cork cells amongst the
unspecialised ground cells, and which manufacture variously-shaped silica (SiO 2 ) bodies (Esau 1977). Silica deposition in these
cells was thought to occur “by a passive nonmetabolic mechanism” and the silica cells lose their protoplast (Esau 1977 p. 86).
However Si appears to be actively taken up by plant roots as monosilic acid and concentrated in the shoots where it is
polymerised (Reynolds et al. 2009). The microscopic particles of hydrated silica, deposited in intracellular and/or intercellular
spaces are known as opaline phytoliths in the older literature, or simply phytoliths, and are particularly abundant in Poaceae, in
which a multiplicity of phytolith forms occur (Gallego and Distel 2004, Parr et al. 2009). Phytoliths are found in plants of other
familes including Equisetaceae, some gymnosperms and dicotyledons, and other plants in the Poales, but they are most abundant
and diverse in Poaceae (Thomasson 1986). Most phytoliths consist of hydrated SiO 2 (Honaine et al. 2006), but they also occlude
plant carbon at the time of creation (Iriarte 2006, Parr et al. 2009). The occluded C is probably derived from internal cytoplasmic
material and is highly resistant to decomposition, in some cases for >10,000 years (Parr et al. 2009). Higher taxa of grasses can
be readily distinguished by the types of leaf phytoliths they produce (Piperno and Sues 2005) and each grass species has a
characteristic phytolith assembage, determined by identification of the relative frequency of the phytolith morphotypes (Gallego
and Distel 2004). These have so far been classified by shape and size into over 50 forms, including dumb-bells, crosses, elongate
types and hairs (trichomes).
Zucol (1996) defined leaf phytolith assemblages in eight Argentinean Nassella spp., including N. neesiana, using cluster and
principal component analyses and 49 morphological characters, and found that N. neesiana was grouped with N. hyalina.
Gallego and Distel (2004) were able to define a group including N. tenuissima that contained phytoliths with a high frequency of
dumb-bell shapes with a short central portion, straight ends, rectangular, smooth elongate hooks and and a sharp-pointed apex.
Species differences within the group could also be identified and the possibility of species level identification. Honaine et al.
(2006) found that Nassella spp. and Piptochaetium spp. have “abundant Stipa-type dumb-bells” and “great quantities” of rondels
(truncated cones), and formed a clear group separate from the other grasses examined. They illustrated typical N. neesiana
phytoliths: a prickle hair, prickle, dumb-bell with a long central portion and convex ends, simple lobate dumb-bell with a short
central portion and convex ends, Stipa-type dumb-bell with a short central portion and convex ends, and dumb-bell with a spiny
central portion. Honaine et al. (2009) illustrated N. neesiana rondells. Honaine et al. (2006) provided a chart of the relative
frequency of phytolith morphotype groups for the species examined. Inter alia, N. neesiana was found to have an exceptionally
high frequency of dumb-bells with a long central portion and convex ends, and of simple lobate dumb-bells, compared with the
other stipoids examined. Whether there is a distinctive stipoid phytolith ‘profile’ has yet to be determined, “Stipa-type” dumbbells
also being present in members of Pooideae and Arundinoideae (Honaine et al. 2006 pp. 1160-1).
Phytoliths are incorporated into soil after degradation of plant tissue, and because of their resistance to decay can be used to
reconstruct former plant communities and ecological history (Honaine et al. 2006 2009) and long-term prehistorical grassland
dynamics (Iriarte 2006). They also survive and are concentrated by animal digestion, so can be extracted from dung to determine
animal diets (Piperno and Sues 2005, Prasad et al. 2005). As fossils, they are more resistant to oxidisation than pollen (Iriarte
2006), and more taxonomically informative than grass pollen, which generally has few useful identification characters and is
very similar to pollen of some other families, including Restionaceae (Thomassen 1986).
Phytoliths and silica cells undoubtedly play a major role in deterrence of herbivory (Stebbins 1986, Reynolds et al. 2009), both
vertebrate and invertebrate, and the silica bodies are “among the few substances capable of inducing morphological changes to
animal mouthparts” (Piperno and Sues 2005 p. 1128). Silicon defenses reduce the palatability and digestibility of plant tissue and
increase its abrasiveness and hardness (Reynolds et al. 2009). Soluble Si is also involved in induced chemical defences against
insect herbivore attack (Reynolds et al. 2009).
The phytolith profile of different plant organs (leaf, culm, root, inflorescence) differs markedly in a species (at least for
Paspalum quadrifarium) (Honaine et al. 2009), suggesting adaptation to defend against the differing ranges of predators to
which these organs are susceptible. Since the earliest known grasses contain a range of phytolith morphotypes similar to modern
taxa, extant grasses produce much larger quantities of phytoliths than other plant taxa (Prasad et al. 2005) and chemical antipredator
defences are generally uncommon in the family, it is clear that these silica defences have played a very important role in
the long history of coevolution of grasses and their predators (Piperno and Sues 2005, Reynolds et al. 2009). The relationships
between particular plant predators and particular silica-based plant defences have been partially explored (Reynolds et al. 2009),
with some emphasis on the functioning of trichomes in defence against insects.
Cytology
Chromosome numbers in grasses range from 2n = 4 to 2n = 263-265 (Hunziker and Stebbins 1986, Groves and Whalley 2002),
with n = 6 or n = 7 most likely the primitive condition (Stebbins 1972, Tsvelev 1984), probably the former (Hunziker and
Stebbins 1986). More than 80% of grasses are polyploid in origin, a larger proportion than any other large plant family, and
polyploid series are common within species (Hunziker and Stebbins 1986, Groves and Whalley 2002). Hybidisation resulting in
polyploidy has been very important in the diversification of stipoid taxa: Hunziker and Stebbins (1986) calculated that 91% of
250 Stipa spp. were polyploid, while Barkworth and Everett (1986) and Vásquez and Barkworth (2004) considered all Stipeae to
be probably of polyploid origin.
Chromosomes in Stipeae are reportedly “fairly small” and “usually aneuploid” (Tsvelev 1984 p. 848), i.e. the diploid
chromosome number is usually not an exact multiple of the haploid number (Johnson 1972). N. neesiana material from
Argentina, Bolivia, Chile and Uruguay was found by Bowden and Senn 1962) to have diploid chromosome numbers of 28 and
was considered tetraploid, except for one sample from Limache, Chile (Bowden and Senn 1962). The exceptional material,
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