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.
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Texas A&M University
Review the developmental morphology and growth form of grass plants.
Evaluate some major physiological and morphological plant responses to grazing.
Explore the mechanisms that convey grazing resistance to plants.
Annual (grass, forb)
Perennial (grass, forb)
Deciduous or evergreen
Sprouting or non-sprouting (basal)
Cool season or warm season
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.
Contribution to Biomass Production
Rate of Growth Following Defoliation
(Activation of dormant buds)
(Cell division & differentiation)
Heat (optimal temperature)
Nitrogen and other nutrients
Meristems (apical, intercalary, axillary)
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.
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.
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
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.
PN (% of preclipping Ps rate)
Time From Clipping (days)
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
All roots stopped growing for 17 days
50% of roots stopped growing for 17 days
No roots stopped growing
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.
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?
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.
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.
Primary growth forms of grasses
Turf or sod grasses
(Mechanisms enabling plants to survive in grazed systems)
(Mechanisms that increase growth following grazing)
(Mechanisms that reduce the probability of grazing)
Structural plant traits
Spines, Awns, Pubescence
N, P, K, Mg (macro minerals)
Cu, Co, Se, Zn
Largest class of secondary compounds
Found in 20-30% of plant species
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.