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. – 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 ShrubForb Grass Grasslike 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 Morphology Phytomer Organization Tiller Organization Plant Organization Phytomer 4 Phytomer 2 Phytomer 3 Phytomer 1 Tiller 1 Tiller 2 Tiller 3 Axillary Bud Blade Intercalary Meristems 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 – CO 2 – 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 NO 3 in the soil affects the species composition of the vegetation – Weeds require higher levels of NO 3 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 Photosynthesis P N (% of preclipping Ps rate) Time From Clipping (days) Control Moderately clipped Heavily clipped

Resource Allocation Biomass partitioning to roots and sheath is reduced much more than to leaves following partial defoliation. TreatmentTotal growthBlade growthSheath growthRoot growth mg mg % total mg % totalmg % total Undefoliated Defoliated 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 Resistance Grazing Resistance (Mechanisms enabling plants to survive in grazed systems) Avoidance (Mechanisms that reduce the probability of grazing) Tolerance (Mechanisms that increase growth following grazing) Morphological Characteristics Biochemical Compounds Morphological Characteristics Physiological 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

Structural Anti-quality Fiber components – Cell walls – Lignin – Silica

Anti-quality Mineral imbalances Excess – Silicon – Se – Mo – NO 3 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.