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 Rangelands Tree Dicots Monocots •Grass Shrub Forb •Grasslike
Surviving plants have strong drought resistance andwell developed chemical or structural anti-herbivory.
Grassland with scattered shrubs and small trees onupland. Competition is for light and soil resources. Fire isa major determinant of the dominant vegetation. Grazingtolerance is more important than anti-herbivory.
Basal Location of Grass Regrowth in Cumless Tillers
Meristematic Contribution to Grass Growth Contribution to Biomass Production Intercalary Apical Axillary Meristems Meristems Buds Hours Days Weeks Rate of Growth Following DefoliationLeaf elongation Leaf production Tiller production (Cell division & (Activation of dormant(Cell enlargement) differentiation) 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 Plants1. 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 Photosynthesis 120PN (% of preclipping Ps rate) 110 100 90 80 Control Moderately clipped Heavily clipped 70 0 2 4 6 8 10 Time From Clipping (days)
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 % totalUndefoliated 69 23 33 17 25 20 29Defoliated 38 20 53 8 21 7 18 Detling et al. 1979
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
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 StolonRhizome
Variation of the Grass Growth Form Bunchgrass Intermediate Sodgrass Growth- Growth-form Growth-form form
Bunchgrass Growth Form
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 PhysiologicalCharacteristics Compounds Characteristics Characteristics
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
Anti-quality Mineral imbalances• Excess – Silicon – Se – Mo – NO3• Deficiency – N, P, K, Mg (macro minerals) – Cu, Co, Se, Zn
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
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.