How Plants Grow

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How Plants Grow

• Mort Kothmann
• Texas AM University

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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
Phytomer Organization
Tiller Organization
Plant Organization
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Tiller Cross Section
Intercalary Meristem
Leaf Blade
Emerging Tiller
Leaf Sheath
Apical Meristem
Axillary Bud
Adventitious Root
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Culmless Versus Culmed Tillers
Culmed
Apical Meristem
Culmless
Axillary Buds
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Basal Location of Grass Regrowth in Cumless
Tillers
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Meristematic Contribution to Grass Growth
(Activation of dormant buds)
(Cell division differentiation)
(Cell enlargement)
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Factors Limiting Plant Growth

• 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

• 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

• 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

• 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
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Effects of Grazing on Plants

• Removal of photosynthetic tissues reduces a
  plants ability to assimilate energy.
• Removal of meristems (apical intercalary)
  delays or stops growth.
• Removal of reproductive structures reduces a
  plants 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
PN ( of preclipping Ps rate)
Time From Clipping (days)
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Resource Allocation

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

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Root Responses to Defoliation
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

• 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

• 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

• 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

• 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

• Primary growth forms of grasses
• Bunchgrasses
• Turf or sod grasses

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Stolons and Rhizomes
Stolon
Rhizome
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Variation of the Grass Growth Form
Bunchgrass Growth-form
Intermediate Growth-form
Sodgrass Growth-form
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Bunchgrass Growth Form
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Herbivory Resistance
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Anti-quality Factors in Forages
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Classes of Anti-quality

• Structural plant traits
• Plant parts
• Spines, Awns, Pubescence
• Plant maturity
• LeafStem ratio
• LiveDead
• ReproductiveVegetative tillers
• Tensile/shear strength

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Structural Anti-quality

• Fiber components
• Cell walls
• Lignin
• Silica

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Anti-quality Mineral imbalances

• Excess
• Silicon
• Se
• Mo
• NO3
• Deficiency
• N, P, K, Mg (macro minerals)
• Cu, Co, Se, Zn

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Anti-qualityAlkaloids

• 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

• 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.