This document discusses plant development and responses to grazing. It covers the morphological and physiological adaptations that allow plants to survive grazing through avoidance, tolerance, and resistance mechanisms. These include growth forms like bunchgrasses and rhizomes, compensatory photosynthesis, resource allocation after defoliation, and structural or biochemical anti-herbivory traits. The key to plant survival under grazing is regrowth ability through meristem protection and remobilization of stored resources.

1. How Plants Grow
Mort Kothmann
Texas A&M University

2. 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.

3. 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

4. Major Plant Groups on Rangelands
Tree
Dicots Monocots
•Grass
Shrub Forb
•Grasslike

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

6. 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.

7. Developmental Morphology
Phytomer Organization Tiller Organization Plant Organization
Blade Ligule
Tiller 1
Phytomer 4
Sheath
Tiller 2
Intercalary
Meristems Phytomer 3
Internode
Tiller 3
Phytomer 2
Node Axillary Phytomer 1
Bud

8. Tiller Cross Section
Leaf Blade Intercalary Meristem
Emerging Tiller
Leaf Sheath
Apical Meristem
Axillary Bud
Adventitious Root

10. Culmless Versus Culmed Tillers
Culmed
Apical Meristem
Culmless
Axillary
Buds

11. Basal Location of Grass Regrowth
in Cumless Tillers

12. Meristematic Contribution to Grass Growth
Contribution to Biomass Production
Intercalary Apical Axillary
Meristems Meristems Buds
Hours Days Weeks
Rate of Growth Following Defoliation
Leaf elongation Leaf production Tiller production
(Cell division & (Activation of dormant
(Cell enlargement)
differentiation) buds)

13. Factors Limiting Plant Growth
• Heat (optimal temperature)
• Below-Ground (roots)
– Water
– Nitrogen and other nutrients
• Above-Ground (shoot)
– Light
– CO2
– Meristems (apical, intercalary, axillary)

14. 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.

15. 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.

16. 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

17. Physiological Responses to Grazing

18. Effects of Grazing on Plants
1. Removal of photosynthetic tissues reduces a plant’s
ability to assimilate energy.
2. Removal of meristems (apical & intercalary) delays
or stops growth.
3. Removal of reproductive structures reduces a
plant’s ability to produce new individuals.
4. Grazing is a natural ecological process and
overgrazing occurred prior to humans.
5. Properly managed grazing is a sustainable
enterprise, but destructive grazing can occur.

19. Compensatory Photosynthesis
120
PN (% of preclipping Ps rate)
110
100
90
80 Control
Moderately clipped
Heavily clipped
70
0 2 4 6 8 10
Time From Clipping (days)

20. Resource Allocation
• Biomass partitioning to roots and sheath is reduced
much more than to leaves following partial
defoliation.
Treatment Total growth Blade growth Sheath growth Root growth
mg mg % total mg % total mg % total
Undefoliated 69 23 33 17 25 20 29
Defoliated 38 20 53 8 21 7 18
Detling et al. 1979

21. Root Responses to Defoliation
50%
70%
90%
All roots
50% of roots stopped
No roots stopped growing for
stopped growing for 17 days
growing 17 days

22. 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.

23. 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?

24. 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.

25. 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.

26. Morphological characteristics
• Primary growth forms of grasses
– Bunchgrasses
– Turf or sod grasses

27. Stolons and Rhizomes
Stolon
Rhizome

28. Variation of the Grass Growth Form
Bunchgrass Intermediate Sodgrass Growth-
Growth-form Growth-form form

30. Bunchgrass Growth Form

31. Herbivory Resistance
Grazing Resistance
(Mechanisms enabling plants
to survive in grazed systems)
Avoidance Tolerance
(Mechanisms that reduce (Mechanisms that increase
the probability of grazing) growth following grazing)
Morphological Biochemical Morphological Physiological
Characteristics Compounds Characteristics Characteristics

32. Anti-quality Factors in Forages

33. Classes of Anti-quality
• Structural plant traits
– Plant parts
• Spines, Awns, Pubescence
– Plant maturity
• Leaf:Stem ratio
• Live:Dead
• Reproductive:Vegetative tillers
– Tensile/shear strength

34. Structural Anti-quality
• Fiber components
– Cell walls
– Lignin
– Silica

35. Anti-quality
Mineral imbalances
• Excess
– Silicon
– Se
– Mo
– NO3
• Deficiency
– N, P, K, Mg (macro minerals)
– Cu, Co, Se, Zn

36. Anti-quality
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

37. 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.