How Plants GrowMort KothmannTexas A&M University

Plant Development and Responses to GrazingObjective 1Review 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 PlantsAnnual (grass, forb)Perennial (grass, forb) WoodyDeciduous or evergreenSprouting or non-sprouting (basal)Cool season or warm seasonAnti-herbivoryChemical Physical

Major Plant Groups on RangelandsTreeShrubForbGrassGrasslike

DicotsMonocots

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 OrganizationTiller OrganizationPlant Organization

Tiller Cross SectionLeaf BladeLeaf SheathAdventitious RootEmerging TillerApical MeristemAxillary BudIntercalary Meristem

Culmless Versus Culmed TillersCulmlessCulmedApical MeristemAxillary Buds

Basal Location of Grass Regrowth in Cumless Tillers

Meristematic Contribution to Grass Growth(Cell enlargement)(Cell division & differentiation)(Activation of dormant buds)

Factors Limiting Plant GrowthHeat (optimal temperature)Below-Ground (roots)WaterNitrogen and other nutrientsAbove-Ground (shoot)LightCO2Meristems (apical, intercalary, axillary)

Resources and MeristemsIntercalary 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 ResourcesPlants 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 ConceptsN uptake is with water; if water is limiting, N will be limitingHigher levels of available N increase water use efficiencyLevel of available NO3 in the soil affects the species composition of the vegetationWeeds require higher levels of NO3 than do climax grasses

Physiological Responses to Grazing

Effects of Grazing on PlantsRemoval 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.

Compensatory PhotosynthesisPN (% of preclipping Ps rate)Time From Clipping (days)

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

Root Responses to DefoliationNo roots stopped growing50% of roots stopped growing for 17 daysAll roots stopped growing for 17 days 50%70%90%

*Classic investigation from mid1950s; lead to take half leave half rule.

Root Responses to DefoliationRoot 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 GrowthThe 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 RegrowthCarbohydrate 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 ExponentialThe 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 characteristicsPrimary growth forms of grassesBunchgrassesTurf or sod grasses

Stolons and RhizomesStolonRhizome

Variation of the Grass Growth FormBunchgrass Growth-formIntermediate Growth-formSodgrass Growth-form

Bunchgrass Growth Form

Herbivory Resistance

Anti-quality Factors in Forages

Classes of Anti-qualityStructural plant traitsPlant parts Spines, Awns, PubescencePlant maturityLeaf:Stem ratioLive:DeadReproductive:Vegetative tillersTensile/shear strength

Structural Anti-qualityFiber componentsCell wallsLigninSilica

Anti-quality Mineral imbalancesExcessSiliconSeMoNO3DeficiencyN, P, K, Mg (macro minerals)Cu, Co, Se, Zn

Anti-qualityAlkaloidsWestern plantsLargest class of secondary compoundsFound in 20-30% of plant speciesHighly toxicEastern plantsErgot alkaloidsFescue pasturesDallisgrassPerennial ryegrass

Toxicity of anti-herbivory compoundsPlants 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.

*Classic investigation from mid1950s; lead to take half leave half rule.