Family Poaceae - Subfamily Panicoideae
- mostly
warm season grasses that predominate at humid warm temperate to tropical
latitudes and that commonly have a hairy ligule
(we will see one exception in lab of a genus that is cool season)
- distinct
morphologies that are often used in the paleo-ecological
literature to infer warm temperate habitats – silica cells or phytoliths that are predominantly dumbbell-shaped,
as opposed to the sinuous-shaped
silica cells in the leaf epidermis of species of Pooideae,
and leaves with an abundance of microhairs
(figures from L. Watson and M. J. Dallwitz.
1988. Grass Genera of the World. Canberra, Australia: Australian National
University Printing
Service)
- the base chromosome number is
mostly x=10. This means that diploids have somatic chromosome numbers of
20 whereas tetraploids have 40, etc.
Tribe Andropogoneae
1. spikelets
in pairs, one sessile and the other pedicellate (this
pair is often the unit of disarticulation). 2. glumes indurate, both about
equal in length. 3. lemmas papery, commonly inconspicuous. 4. fertile lemma awned. Common genera include:
- Andropogon: spikelet-pairs in spikes, lateral
inflorescence branches bear two or more such spikes.
- Sorghum: spikelet
pairs in panicles, pedicellate spikelet of each
pair well developed and at least staminate.
- Zea:
monoecious, the staminate tassel is a panicle of
spikelet- pairs, pistillate inflorescences are
enveloped by many leaf sheaths.
Tribe Paniceae
1. spikelets
not in spikelet pairs (or if superficially so, the pair not the unit of
disarticulation). 2. glumes papery, the first usually
much shorter than the 2nd, sometimes completely absent. 3. the lemma and palea of the fertile floret boney in texture. 4. fertile
lemma never awned. Common genera include:
- Digitaria: digitately
arranged secund (“one-sided”) spikes
of spikelets, 1st glume
highly reduced to a nubbin of tissue.
- Paspalum:
secund spikes of spikelets
(no really digitately arranged), 1st glume completely absent.
- Echinochloa:
no ligule present (just a rim separating the
sheath from blade on the inside), distinctive glass-like hairs on the
sterile lemma, inflorescence a contracted secund
panicle.
- Cenchrus: inflorescences
with bristles that coalesce and harden around a spikelet or two, and that
disperse with the spikelets as a unit.
- Pennisetum:
inflorescences with bristles that neither coalesce nor harden around the spikelets, but that disperse with the spikelets as a unit.
- Setaria:
inflorescence with bristles that are persistent on the culm
and do not disperse with the spikelets.
- Panicum:
annuals or perennials, not developing a rosette of short, broad basal leaves
during the cool season; plants flowering during the warm season only.
- Dichanthelium: perennials, developing a rosette
of short, broad basal leaves during the cool season; plants flowering
during the cool season and warm season; cleistogamous
spikelets commonly produced on small axillary inflorescences during the late summer and
fall. This is one of the very few C3 genera in the subfamily Panicoideae.
C3 and C4 photosythesis:
CO2 fixation
Of the grass subfamilies we have seen to date, Pooideae
is strictly with C3 photosythesis, whereas Panicoideae is almost entirely with C4.
Background of leaf anatomy in grasses: two major tissue types in
grass leaf are the mesophyll and the bundle sheath cells.
In C3 grasses, the mesophyll is loosely packed (with
lots of air spaces) and the bundle sheath cells lack chloroplasts
(photosynthesis takes place in the mesophyll). In C4
grasses, the mesophyll is tightly packed and radiately arranged (no air spaces) around the bundle sheath
cells, which contain the bulk of the chloroplasts (photosynthesis occurs in
this tissue and not in the mesophyll).
1. C3 cycle ('dark reaction' or Calvin cycle). In C3
grasses, this cycle occurs in the mesophyll. CO2 is
initially fixed by RuBP carboxylase
(rubisco) and joined together with Ribulose bisphosphate (RuBP, 5-carbon - RuDP is the old
name for RuBP). The product of this reaction is
unstable and immediately breaks down into 2 molecules of phosphoglyceric
acid (PGA, 3-carbon - hence the
name C3). The reaction continues on to ultimately produce starch and regenerate
RuBP (the original 5-carbon compound).
2. C4 cycle. In C4 grasses, CO2 fixation occurs in the mesophyll and is transported to the bundle sheath cells
where the C3 cycle occurs. CO2 is fixed by phosphoenolpyruvate
carboxylase (PEP
carboxylase) and joined with phosphoenolpyruvate
(PEP; 3-carbon) in the mesophyll cells to form a stable 4-carbon compound, Malate or Aspartate (hence the
name C4). This compound is then transported into the bundle sheath cells where
the CO2 is released and then immediately fixed by rubisco
as part of the C3 cycle restricted to the bundle sheath cells.
Implications: PEP
carboxylase has a much higher affinity for CO2 than
does rubisco. At low CO2 pressures, rubisco doesn’t distinguish well between O2 and CO2
and if combined with O2, both a 3-carbon and 2-carbon compound are produced and
the 2-carbon compound degrades to CO2. This is a process known as
photorespiration (O2 in and CO2 out). Even though rubisco
can fix this newly generated CO2, much energy is used with no net gain. The C4
cycle reduces photorespiration by maintaining the bundle sheath cells at
relatively high CO2 levels. PEP carboxylase works well at temperatures above 30 degrees,
but not optimally below this temperature. Thus the reason why C4 grasses are
referred to as warm season grasses, and why they don’t compete well with
C3 grasses at cooler temperatures. C4 grasses have an edge in dry warm sites or
open sunny sites as they can keep leaf stomata closed during mid-day and
extract every last CO2 molecule in the leaf. In contrast, C3 grasses that keep
stomata closed in dry sunny sites undergo high amounts of photorespiration. In
summary, C3 grasses are favored habitats that are not C02-limited, such as
those characterized by some combination of cool, shady, and moist, or where
stomata don’t have to be closed much and CO2 thus taken in. In contrast,
C4 grasses are favored in habitats that are CO2-limited, or with some combination
of warm, sunny, and dry, where stomata have to be closed and thus CO2 levels
reduced inside the leaf tissue. Spartina pectinata (Chlorideae and
thus C4) is restricted to shallow water of ponds and swamps. But in keeping
with its C4 metabolism, such moist habitats of this species are at low warmer
elevations and in open sunny sites, and such saline inhabiting species probably
do not use much of the water in which they are growing. Similarly, Koeleria macrantha
(Aveneae, and thus C3) occurs in about every grassland
of western North America, from Montana and adjacent Canada south to Arizona. In
northern habitats, this species grows throughout the summer at a broad range of
elevations, where as in the southernmost habitats it is either restricted to
the higher elevations or puts on most of its growth during the early growing
season.
Comparison of the two major types of C4 photosynthesis found
in Panicoideae (figures from L. Watson and M. J. Dallwitz. 1988. Grass Genera of the World. Canberra,
Australia: Australian National
University Printing
Service)
First, compare the leaf anatomies of the following two
C4 types with that of C3, where mesophyll
tissue predominates and bundle sheaths are small.
A. Type C4PS. Aspartate is the
C4 compound, found in some Paniceae but most commonly
in the subfamily Chloridoideae, which will be introduced
in the coming week.
- The large bundle
sheath cells (Kranz anatomy) are derived
from the same tissue as the mesophyll. This type
of bundle sheath is called the parenchyma sheath (PS)
- Photosynthesis (the C3 cycle)
occurs in the parenchyma (bundle) sheath only
- The mesophyll
cells are densely packed and radiate outward from the bundle sheath; all mesophyll cells are intimately associated with the
vascular bundle by compartmentalization.
- The few genera of Panicoideae
with this type are most common in arid areas in and around the latitudes
of the Tropics of Cancer and Capricorn
B. Type: C4MS. Malate
is the C4 compound, - found in all genera of Andropogoneae
and many genera of Paniceae.
- The large bundle
sheath cells (Kranz anatomy) are derived
from the same tissue as the vascular tissue. This kind of bundle sheath is
called the mestome sheath (MS)
- Photosynthesis (the C3 cycle)
occurs in the mestome (bundle) sheath only
- The mesophyll
cells are loosely packed and with air spaces (similar to Pooideae and other C3 grasses); however, all mesophyll cells are intimately associated with the
vascular bundle by proliferation of the vascular strands, and thus mestome bundle sheath.
- the genera of Panicoideae with this type (i.e., most of the
subfamily) predominate in tropical and warm temperate regions with high
humidity
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