How Plants Grow

Mort KothmannTexas 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.– Explore the mechanisms that convey grazing

resistance to plants.

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

Major Plant Groups on RangelandsTree

Shrub Forb•Grass

•Grasslike

Dicots Monocots

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

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.

Developmental MorphologyPhytomer Organization Tiller Organization Plant Organization

Phytomer 4

Phytomer 2

Phytomer 3

Phytomer 1

Tiller 1

Tiller 2

Tiller 3

Axillary Bud

Blade

IntercalaryMeristems

Node

Ligule

Sheath

Internode

Tiller Cross Section

Leaf Blade

Leaf Sheath

Adventitious Root

Emerging Tiller

Apical Meristem

Axillary Bud

Intercalary Meristem

Culmless Versus Culmed Tillers

Culmless

Culmed

Apical Meristem

Axillary Buds

Basal Location of Grass Regrowth in Cumless Tillers

Meristematic Contribution to Grass Growth

Contribution to Biomass Production

Rate of Growth Following Defoliation

Intercalary Meristems

Apical Meristems

Axillary Buds

Hours Days Weeks

Leaf elongation Leaf production Tiller production

(Cell enlargement) (Cell division & differentiation)

(Activation of dormant buds)

Factors Limiting Plant Growth

• Heat (optimal temperature)• Below-Ground (roots)

– Water– Nitrogen and other nutrients

• Above-Ground (shoot)– Light– CO2

– Meristems (apical, intercalary, axillary)

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.

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.

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

Physiological Responses to Grazing

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.

Compensatory PhotosynthesisP N

(% o

f pre

clip

ping

Ps

rate

)

Time From Clipping (days)

120

110

100

90

80

70

0 2 4 6 8 10

ControlModerately clippedHeavily clipped

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

Root Responses to Defoliation

No roots stopped growing

50% of roots stopped

growing for 17 days

All roots stopped

growing for 17 days

50%

70%

90%

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.

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?

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.

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.

Morphological characteristics

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

Stolons and Rhizomes

Stolon

Rhizome

Variation of the Grass Growth Form

Bunchgrass Growth-form

Intermediate Growth-form

Sodgrass Growth-form

Bunchgrass Growth Form

Herbivory ResistanceGrazing Resistance

(Mechanisms enabling plants to survive in grazed systems)

Avoidance(Mechanisms that reduce the probability of grazing)

Tolerance(Mechanisms that increase growth following grazing)

MorphologicalCharacteristics

BiochemicalCompounds

MorphologicalCharacteristics

PhysiologicalCharacteristics

Anti-quality Factors in Forages

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

Structural Anti-quality

• Fiber components– Cell walls– Lignin– Silica

Anti-quality Mineral imbalances

• Excess– Silicon– Se– Mo– NO3

• Deficiency– N, P, K, Mg (macro minerals)– Cu, Co, Se, Zn

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

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.