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.

  1. 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)
  2. Photosynthesis (the C3 cycle) occurs in the parenchyma (bundle) sheath only
  3. 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.
  4. 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.

  1. 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)
  2. Photosynthesis (the C3 cycle) occurs in the mestome (bundle) sheath only
  3. 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.
  4. 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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