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How Plants Grow. Mort Kothmann Texas A&M University. Plant Development and Responses to Grazing. Objective 1 Review the developmental morphology and growth form of grass plants. Objective 2. Evaluate some major physiological and morphological plant responses to grazing. Objective 3.

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How plants grow l.jpg

How Plants Grow

Mort Kothmann

Texas A&M University


Plant development and responses to grazing l.jpg
Plant Development and Responses to Grazing

Objective 1

Review the developmental morphology and growth form of grass plants.

Objective 2.

Evaluate some major physiological and morphological plant responses to grazing.

Objective 3.

Explore the mechanisms that convey grazing resistance to plants.


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Functional Categories of Plants

Annual (grass, forb)

Perennial (grass, forb)

Woody

Deciduous or evergreen

Sprouting or non-sprouting (basal)

Cool season or warm season

Anti-herbivory

Chemical

Physical


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Major Plant Groups on Rangelands

Tree

Dicots

Monocots

  • Grass

  • Grasslike

Shrub

Forb


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Surviving plants have strong drought resistance and well developed chemical or structural anti-herbivory.


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Grassland with scattered shrubs and small trees on upland. Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.


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Developmental Morphology Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Ligule

Blade

Tiller 1

Phytomer 4

Sheath

Tiller 2

Intercalary

Meristems

Phytomer 3

Internode

Tiller 3

Phytomer 2

Axillary

Bud

Node

Phytomer 1

Phytomer Organization

Plant Organization

Tiller Organization


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Tiller Cross Section Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Intercalary Meristem

Leaf Blade

Emerging Tiller

Leaf Sheath

Apical Meristem

Axillary Bud

Adventitious Root


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Culmless Versus Culmed Tillers Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Culmed

Apical Meristem

Culmless

Axillary Buds


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Basal Location of Grass Regrowth in Cumless Tillers Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.


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Meristematic Contribution to Grass Growth Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Contribution to Biomass Production

Intercalary Meristems

Apical Meristems

Axillary Buds

Days

Hours

Weeks

Rate of Growth Following Defoliation

Leaf production

Leaf elongation

Tiller production

(Activation of dormant buds)

(Cell division & differentiation)

(Cell enlargement)


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Factors Limiting Plant Growth Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Heat (optimal temperature)

Below-Ground (roots)

Water

Nitrogen and other nutrients

Above-Ground (shoot)

Light

CO2

Meristems (apical, intercalary, axillary)


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Resources and Meristems Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Intercalary meristems are primarily involved with cell enlargement which requires primarily CHO and has low N requirement.

Axillary meristems are sites of cell division and differentiation. Cell division requires N; thus N availability will limit the number of active meristems.

N content of leaves is generally 2X that of roots; thus, low N results in less shoot growth relative to root growth.


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Allocation of Plant Resources Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Plants allocate resources (phytosynthetate) with the priority towards acquiring the most limiting resource(s).

If water is limiting, allocation is shifted towards root growth over shoot growth.

If leaf area is limiting, allocation is shifted towards leaf growth over shoot growth.


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Key Concepts Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

N uptake is with water; if water is limiting, N will be limiting

Higher levels of available N increase water use efficiency

Level of available NO3 in the soil affects the species composition of the vegetation

Weeds require higher levels of NO3 than do climax grasses


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Physiological Responses to Grazing Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.


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Effects of Grazing on Plants Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Removal of photosynthetic tissues reduces a plant’s ability to assimilate energy.

Removal of meristems (apical & intercalary) delays or stops growth.

Removal of reproductive structures reduces a plant’s ability to produce new individuals.

Grazing is a natural ecological process and overgrazing occurred prior to humans.

Properly managed grazing is a sustainable enterprise, but destructive grazing can occur.


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Compensatory Photosynthesis Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

120

110

100

90

Control

Moderately clipped

Heavily clipped

80

70

6

4

10

8

0

2

PN (% of preclipping Ps rate)

Time From Clipping (days)


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Resource Allocation Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Biomass partitioning to roots and sheath is reduced much more than to leaves following partial defoliation.

Treatment Total growth Blade growthSheath growthRoot growth

mg mg % total mg % total mg % total

Undefoliated69 23 33 17 25 20 29

Defoliated 38 20 53 8 21 7 18

Detling et al. 1979


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Root Responses to Defoliation Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

50%

70%

90%

All roots stopped growing for 17 days

50% of roots stopped growing for 17 days

No roots stopped growing


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Root Responses to Defoliation Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Root growth decreases proportionally as defoliation removes greater than 50% of the plant leaf area.

Frequency of defoliation interacts with defoliation intensity to determine the total effect of defoliation on root growth.

The more intense the defoliation, the greater the effect of frequency of defoliation.


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Consequences of Reduced Root Growth Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

The net effect of severe grazing is to reduce:

Total absorptive area of roots.

Soil volume explored for soil resources e.g. water and nitrogen.

How may this alter competitive interactions?


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TNC Contribution to Shoot Regrowth Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Carbohydrate reserves exist and they provide a small amount of energy to contribute to initial leaf growth following severe grazing or leaf damage e.g., fire, late spring freeze.

Current photosynthesis is the primary source for growth of new shoots.


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Growth is Exponential Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

The initial or residual amount of plant tissue is very important in determining the rate of plant growth at any point in time.

The total amount of root and shoot biomass is more important than the concentration of reserve CHO.


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Morphological characteristics Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Primary growth forms of grasses

Bunchgrasses

Turf or sod grasses


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Stolons and Rhizomes Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Stolon

Rhizome


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Variation of the Grass Growth Form Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Bunchgrass Growth-form

Intermediate Growth-form

Sodgrass Growth-form


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Bunchgrass Growth Form Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.


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Herbivory Resistance Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Grazing Resistance

(Mechanisms enabling plants to survive in grazed systems)

Tolerance

(Mechanisms that increase growth following grazing)

Avoidance

(Mechanisms that reduce the probability of grazing)

Physiological

Characteristics

Morphological

Characteristics

Biochemical

Compounds

Morphological

Characteristics


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Anti-quality Factors in Forages Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.


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Classes of Anti-quality Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Structural plant traits

Plant parts

Spines, Awns, Pubescence

Plant maturity

Leaf:Stem ratio

Live:Dead

Reproductive:Vegetative tillers

Tensile/shear strength


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Structural Anti-quality Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Fiber components

Cell walls

Lignin

Silica


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Anti-quality Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.Mineral imbalances

Excess

Silicon

Se

Mo

NO3

Deficiency

N, P, K, Mg (macro minerals)

Cu, Co, Se, Zn


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Anti-quality Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.Alkaloids

Western plants

Largest class of secondary compounds

Found in 20-30% of plant species

Highly toxic

Eastern plants

Ergot alkaloids

Fescue pastures

Dallisgrass

Perennial ryegrass


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Toxicity of anti-herbivory compounds Competition is for light and soil resources. Fire is a major determinant of the dominant vegetation. Grazing tolerance is more important than anti-herbivory.

Plants with highly toxic compounds do not allow animals to learn from negative post-ingestive feedback.

Plants with less toxic compounds allow animal to learn and develop aversions.

When nutritious forage is limited, positive feedback may override negative feedback and animals will consume toxic plants.