Structural defenses

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Structural defenses In addition to obvious
structural defenses like thorns, spines
(both cactus and porcupine), and others, some
structural defenses are more subtle. One of
these is the structural difference accompanying
different mechanisms of photosynthesis (C3
versus C4). The mesophyllof C3 plants contains a
relatively uniform concentration of starch, a
loosely packed collection of chloroplasts, and
is continuous with the vein transporting
photosynthate. In C4 plants only the initial step
of photosynthesis, the fixing of carbon into
carboxylic acid, occurs in the mesophyll. It is
void of starch and chloroplasts. The initial
fixation product, a C4 acid from which the
system gets its name, is transported into the
bundle sheath where, following enzymatic
conversion, normal Calvin-Benson photosynthesis
occurs.
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The bundle sheath is highly lignified and
frequently contains deposited silica. Thus, it
is hard to chew. Yet starch (food for herbivores)
is formed only there. The chloroplasts are
densely packed into these cells, and pass
photosynthate rapidly into veins. Therefore,
starch concentrations in 'photosynthetic tissues
are lower in C4's. In addition, comparisons seem
to indicate that C4 plants have lower
proportions of non-structural carbohydrates, and
more structural (cell wall) carbohydrates
(lower food value, harder to digest), and lower
concentrations of both nitrogen and phosphorus.
C4 cross section C3 cross section
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C3
C4
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In sum, that means C4 plants have lower food
quality per gram successfully consumed. Lower
digestibility (or its impact) has been observed
in a number of studies. Caswell and Reed (1977)
found highly significant differences in the
demography of grasshoppers fed diets of solely C3
grass (wheat) and a mixture of native prairie
grasses which included about half C4.
Diet
Wheat
Native grasses nymphal survival
86 26
adult survival (40 d) 80-90
7 fecundity (eggs/female) 138-207
7 The fecal pellets of
Melanoplus grasshoppers indicate the reasons for
the 'relative starvation' the grasshoppers face.
The pellets include long strings of unbroken
bundle sheath cells. The toughness of the
structure has defeated the grinding action which
dismembers other tissues in their midguts. No
such undigested cells remain on a ration of C3
plants.
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The teeth of many herbivores indicate the
difficulty they have in chewing and breaking
down their foods. Many grassland grazers have
hypsodont teeth (teeth that can be replaced when
worn down), and others have differing tooth
structure. One example of tooth structure is
within the genus Microtus. Microtus
pennsylvanicus, extends over a range from
prairies in the midwestern U.S. (Kansas and
Nebraska) dominated by C4 grasses to meadows in
the east (New York, Pennsylvania) where C4's are
rare if present. Closely related congeners range
further north (sub-boreal communities) and into
the alpine areas of the Rocky Mountains (e.g.
Montana). The relative contribution of C3 and C4
grasses varies from dominance in tall grass and
short grass prairies in the midwest to moderate
presence in Montana to virtual absence in
Pennsylvania. How do the teeth of Microtines vary
in response to this gradient?
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Heres the tooth as a tool in taxonomy to
distinguish mammals
This is the keying molar
The molar teeth of microtus have grinding ridges
on their surfaces Which are important keying
characters within the genus. These ridges are
arranged into triangles. The more complex the
tooth surface (i.e. the more triangles present)
the better equipped the tooth is for grinding.
Its complexity varies from having 3-4 triangles
in animals captured in Pennsylvania (and 4-5 in
M. montanus captured in alpine meadows in the
Rockies) to 6-7 triangles in prairie populations
(and 5-7 as the molar character used to separate
M. ochrogaster, the prairie vole, from the meadow
vole).
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The same cline has been observed through
evolutionary time. The Kansan equus beds contain
many representatives of fossil teeth of
precursors of the contemporary vole. Primitive
species had 3 triangles on molar teeth at a time
when climates and evolution had not led to
northward range extension of C4's into present
day prairies from their site of origin, believed
to be high plateau grasslands of central
Mexico. At the same time palynology suggests the
increasing presence of C4's in Kansas
grasslands, the tooth surfaces of voles became
increasingly complex, slowly increasing the
number of triangles towards present day counts.
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Finally, plants may be able to regulate the
reproduction of their herbivores. Negus and
Pinter (1966) showed that supplementation of a
standard laboratory diet with 1g of wheat
sprouts per day (or an acetone extract from the
sprouts of seeds) led to females in the lab
population having more and larger litters
extract extract control
greens greens control total young 188
166 209
149 total litters 47 38
45 38
litters/female 3.9 3.2
3.8 3.2 young/female 15.7
13.8 17.0
12.4 litter interval 28 37
30 35 litter
loss 21 30
16 32
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In the spring adults feed on a diet with large
amounts of plant sprouts as plant growth begins
reproduction is stimulated. The active chemical
stimulating reproduction is 6-methoxybenzoazolino
ne. A decline in estrogenic activity of plant
tissues and extracts late in the growing season
(and possibly the presence of actively inhibitory
chemicals) would lead to a cessation of
reproduction as plant physiology changes at the
transition from fall into winter. In the field
Negus and Berger (1977) demonstrated that they
could trigger reproduction using wheat sprouts
in a population otherwise reproductively
inactive in mid-winter. Following 2 weeks of
daily addition of wheat sprouts the diet of part
of an island population, every female captured
in the supplemented area was pregnant, while no
control animals became pregnant until onset of
spring, and the sprouting of native grasses.
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Inhibitory chemicals were also documented. Berger
and Negus grew Wheat from sprouts to a height of
10cm (adult leaves under lab conditions) and
extracted natural phenols from the leaves.
Inhibitory effects seemed traceable to
derivatives of cinnamic acid (ferrulic acid and
p-coumaric acid) which were modified by the
extraction procedure to a vinylphenol. Animals
fed extracts or the pure identified compounds
had reduced ovarian weights. Tests began with
17-18 day juvenile females and treated them for
7 days, during which ovarian maturation normally
occurs Uterine
wt. mg Control
13.7 1.9
Extract 10.5 2.1
Vinylphenol
9.8 1.5 ferrulic acid
12.5 3.3 control vinylpheno
l treated follicles maturing 17.7 3 5.7
2.2
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Inhibitory effects in natural field populations
(all differences are significant)
litters/female young/female
breeding control 3.45
.74 15.09 4.28 90.9
p-coumaric acid 2.35 1.2 9.06 5.2
58.8 To function as seasonal
inhibitors, the concentration of the compounds
must change seasonally. Sure enough, they do!
Sprouts contain virtually none of the inhibitors.
During active vegetative growth of adult leaves,
wheat tissue contains about 1mg/g dry weight of
ferullic acid and p-coumaric acid. At the time of
senescence (post-flowering, brown and drying) the
content increases to 3 mg/g dry weight. That late
increase corresponds to the time of annual
reproductive cessation in M. montanus.