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

The anatomy of the awn of Stipa sens. lat.. (including Nassella) "differs from ... most other grasses ... although there have been
attempts to explain the twisting and untwisting motion, more work could be usefully done on the subject. The awn in transection
consists mainly of thickened cells (fibres) with two lateral pockets of chlorenchyma. The chlorenchyma breaks down with age
and, apparently, the awn does not twist until this breaking-down process has been initiated” (Vickery et al. 1986 p. 11). Murbach
(1900: studies on the eastern USA sp. Piptochaetium avenaceum (L.) Parodi, formerly Stipa avenacea L.) found that the awn of
panicle seed has an outer layer of sclerenchyma cells with a spiral structure to the cell wall, a central fibro-vascular bundle and a
band of chlorphyllous tissue on each side of the bundle. The two outer layers consist of thick-walled mechanical cells with a very
small lumen oriented on the inner side of the cell and surrounded by spirally arranged cellulosic material. Wetting and drying of
cells on opposite sides of the awn produce forces that act in opposite directions, producing torsion, and both the outer and middle
layers are responsible. The awns therefore straighten when wetted and twist as they dry out (Whittet 1969, Groves and Whalley
2002, personal obs.). The awn twists in an anticlockwise direction (Slay 2002a).
The awn columns of adjacent seeds often become intertwined (Connor et al. 1993, Edgar and Connor 2000) or twisted together
at maturity (Walsh 1994) forming a tangled mass (Liebert 1996), and fall as an aggregate, losing the dispersal ability of lone
seeds (Gardener et al. 2003a). Such seed masses may be retained by the plant long after the seeds mature (Groves and Whalley
2002), or may be “easily detached en masse attaching themselves to clothes, machinery and animals” (Slay 2002a p. 10).
Callus - an extension of the lemma formed by the oblique articulation of the lemma with the rachilla (Hitchcock and Chase
1971), attaching the seed to the stem (Bourdôt and Ryde 1986); sharp (Walsh 1994, Barkworth 2006), particularly sharply
pointed (Bourdôt and Hurrell 1989b), “extremely sharp ... when dry and mature” (Slay 2002c p. 8); approximately one third the
length of the seed (Martín Osorio et al. 2000); 2-4 mm (Moraldo 1986, Walsh 1994, McLaren, Stajsic and Iaconis. 2004), 2.5-3.5
mm (Jessop et al. 2006), 2.8-3.5 mm (Baeza et al. 2007), 3-4 mm (Jacobs et al. 1989), c. 4 mm (Verloove 2005), 2.5.-3.2 mm
(Burkart 1969), 2-4.5 mm (Barkworth 2006), 3-5.5 mm long (Barkworth and Torres 2001), strigose (Barkworth 2006), oblique
(Jacobs et al. 1989); with silky white hairs 1-2 mm long (Martín Osorio et al. 2000), or 1.5 mm long (Baeza et al. 2007), bearded
(Barkworth and Torres 2001), hairy (Bourdôt and Ryde 1986), covered with white appressed hairs (Hayward and Druce 1919),
villous (Burbidge and Gray 1970) or hidden by a tuft of white hairs (Verloove 2005), the hairs antrorse (Gardener et al. 2003a) and
up to 4 mm long (measured from their lowest position on the callus to their furthest extent) (Jacobs et al. 1989).
All the seed hairs are retrorse, i.e. pointed away from the callus tip.
Cleistogenes: “clandestine seeds” (Bourdôt and Hurrell 1992 p. 102), “floreted cleistogenes” on “clandestine axillary spikelets”
(Connor et al. 1993); formed at nodes “towards the base of flowering culms” (Jacobs et al. 1989, Edgar and Connor 2000), on all
(Connor et al. 1993) or able to be produced on all culm nodes (Gardener et al. 2003a); subtended by slender transparent
prophylls (Connor et al. 1993, Jacobs et al. 1989); 1-3 flowered, with 2 awned glumes about half the length of those in panicle
florets (Jacobs et al. 1989, Connor et al. 1993), lower glume 1-3 nerved, upper glume with up to 7 nerves; lemma, corona and
palea as in aerial florets (Jacobs et al. 1989), except reduced in size, awn to 25 mm, caryopsis to 4 mm, callus about 0.5 mm
(Jacobs et al. 1989, Edgar and Connor 2000); 2.5 x 1.4 mm (Jacobs et al. 1989, Connor et al. 1993); anthers reduced to 1 fertile
and 2 small sterile anthers, cayopsis round or planoconvex to 4 mm long (Jacobs et al. 1989); stem cleistogenes of two types – 4-
5 mm, turbinate, long-callused and 1.4-2.5 mm and plump (Slay 2002a); Descriptions rarely have the clarity and detail necessary
to appropriately describe the variation in cleistogenes. Cleistogenes in general are more rounded than panicle seed (Bourdôt and
Hurrell 1992); lack a large hygroscopic awn (Gardener et al. 2003a) and have a less well-developed lemma than panicle seed
(Hurrell et al. 1994). Upper nodes produce larger numbers, a maximum on the third and fourth nodes and an average of c. 7 per
tiller (Gardener et al. 2003a); up to 5 “unsheathed” cleistogenes on the second and third nodes (Slay 2001 p. 28); each node
above the basal with the potential to produce a few seeds (Gaur et al. 2005); a potential total of 13 cleistogenes (including the
basal) per culm (Slay 2001). Thin stems may have more cleistogenes than thick ones, possibly indicating a compensatory effect
for reduced panicle seed production of thin stems (Julio Bonilla pers. comm.). Mean mass of upper node cleistogenes 2.0 mg,
maturity on average 4 weeks after maturation of panicle seed (Gardener et al. 2003a). Basal cleistogenes initiated as a bud at the
base of the culm, present even in small 3-tillered plants (D. McLaren, 26 October 2006, based on observation of Shiv Gaur);
solitary (Burkart 1969, Gaur et al. 2005), “always singular” on the Northern Tablelands of NSW but “often in multiples” in
Argentina (Gardener et al. 1996b), successively develop one on top the other, so accumulate upwards (D. McLaren, 26 October
2006, based on observation of Shiv Gaur); 1-3 (Slay 2001), or 1-2 (Connor et al. 1993), less than half the basal nodes producing
one cleistogene on average (Gardener et al. 2003a); occurring beneath the soil surface (Gaur et al. 2005) or often below ground
(Gardener et al. 2003a); nut-like, 2..5 mm long, 1.5 mm wide, yellow to dark brown depending on age (Slay 2002a), light dull
yellow when newly formed, becoming brown and thin as they mature (Gaur et al. 2005), lemon to fawn colour, plump, with the
top “‘Turkish roof’ shaped” (Slay 2001 p. 42); mean mass 3.3 mg (Gardener et al. 2003a).
Roots: fibrous, crown thickened (Muyt 2001), not rhizomatous (Barkworth 2006).
In the context of the flora of Victoria, identification to species from macroscopic vegetative characters alone is unreliable, but
can possibly be achieved by microscopic examination of epidermis morphology and distribution of stomata and silica bodies
(Walsh 1998). The detail provided by Watson and Dallwitz (2005) indicate this is possible, at least for distinguishing the Nassella
species present in Australia. Identification using leaf phytolith character assemblages also appears possible (see below).
Evolved changes, which may be reflected in morphology, and perhaps enhanced growth, can be expected in introduced
populations of exotic plants, but these changes can be difficult to distinguish from genotypic plasticity (Cox 2004). There is a
need to determine if there are any morphological differences between exotic and native populations and between exotic
populations in different areas. Possibly some evidence for differences exists in the morphological descriptions above. No
morphological descriptive publications examined provides sample sizes or standard deviations for any measurements, nor any
evidence that the material described is truly representative of the populations. Evolved changes are likely to be evident in the
diaspore and other reproductive characteristics (Cox 2004). For example populations of Pinus concorta ssp. latifolia at the
exanding edge of their post-glacial invasion front have smaller, better dispersing seed than the core population (Rejmánek and
Richardson 1996 see their citation). A comparison of the seed morphology of core and fringe populations would be of interest:
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