How Plants Grow

1. How Plants Grow Mort Kothmann Texas A&M University

2. 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.

3. 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

4. Major Plant Groups on Rangelands Tree Dicots Monocots • Grass • Grasslike Shrub Forb

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

6. 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.

7. Developmental Morphology Ligule Blade Tiller 1 Phytomer 4 Sheath Tiller 2 Intercalary Meristems Phytomer 3 Internode Tiller 3 Phytomer 2 Axillary Bud Node Phytomer 1 Phytomer Organization Plant Organization Tiller Organization

8. Tiller Cross Section Intercalary Meristem Leaf Blade Emerging Tiller Leaf Sheath Apical Meristem Axillary Bud Adventitious Root

10. Culmless Versus Culmed Tillers Culmed Apical Meristem Culmless Axillary Buds

11. Basal Location of Grass Regrowth in Cumless Tillers

12. Meristematic Contribution to Grass Growth Contribution to Biomass Production Intercalary Meristems Apical Meristems Axillary Buds Days Hours Weeks Rate of Growth Following Defoliation Leaf production Leaf elongation Tiller production (Activation of dormant buds) (Cell division & differentiation) (Cell enlargement)

13. Factors Limiting Plant Growth Heat (optimal temperature) Below-Ground (roots) Water Nitrogen and other nutrients Above-Ground (shoot) Light CO2 Meristems (apical, intercalary, axillary)

14. 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.

15. 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.

16. 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

17. Physiological Responses to Grazing

18. Effects of Grazing on Plants 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.

19. Compensatory Photosynthesis 120 110 100 90 Control Moderately clipped Heavily clipped 80 70 6 4 10 8 0 2 PN (% of preclipping Ps rate) Time From Clipping (days)

20. Resource Allocation 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

21. Root Responses to Defoliation 50% 70% 90% All roots stopped growing for 17 days 50% of roots stopped growing for 17 days No roots stopped growing

22. 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.

23. 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?

24. 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.

25. 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.

26. Morphological characteristics Primary growth forms of grasses Bunchgrasses Turf or sod grasses

27. Stolons and Rhizomes Stolon Rhizome

28. Variation of the Grass Growth Form Bunchgrass Growth-form Intermediate Growth-form Sodgrass Growth-form

30. Bunchgrass Growth Form

31. Herbivory Resistance Grazing Resistance (Mechanisms enabling plants to survive in grazed systems) Tolerance (Mechanisms that increase growth following grazing) Avoidance (Mechanisms that reduce the probability of grazing) Physiological Characteristics Morphological Characteristics Biochemical Compounds Morphological Characteristics

32. Anti-quality Factors in Forages

33. 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

34. Structural Anti-quality Fiber components Cell walls Lignin Silica

35. Anti-quality Mineral imbalances Excess Silicon Se Mo NO3 Deficiency N, P, K, Mg (macro minerals) Cu, Co, Se, Zn

36. 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

37. 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.