plant growth and development

• The root cap is the first part to grow, followed by the epidermis.
• root apical meristem and shoot apical meristem are responsible for primary growth of plans and elongation along thier axis
• in dicotyledonous plant and gymnosperms lateral meristems: vascular cambium and cork cambium appear later in life
• cork cambium and vascular cambium increase girth in places theyr active
• period of growth is divided into 3 phases meristamatic , elongation and maturation
• cells in root and shoot apex possess large conspicuous nuclie are rich in protoplasm and thier cell walls are primary in nature thin and cellulosic with abundant plasmodesmatal connections
• cellx proximal to meristamatic zone represent the phase of elongation. increased vacuolation ,cell enlargment and new cell wall deposition are the characterisitcs in this phase
• proximal to phase of elongation is portion undergoing maturation cells here attain maximal size in terms of wall thickening and protoplasmic modifications
• growth rates show an increase that maybe arithmetic or geometrical
• in arithemetic growth only one daughter cell continues to divide while the other daughter cell differentiates and matures
• in geometric growth both daugter cells continue to grow so intial growth is slow(lag phase) and then rapidly increases (exponential phase) however with limited nutrient supply growth slows down and the graph against time will be sigmoid
• arithemetic growth rate formula

  Lt=L0+rt
• geometric growth rate formula W1=W0 e raised to rt
• quantitative comparison can be made in two ways

  1. Absolute growth rate- measurement and comparison of total growth per unit time
  2. Relative growth rate- growth of a given system per unit time expressed on a common basis
• Essentials for plant growth

  1. water - turgidity(enlarging cells) and provides medium for enzymatic activities needed for growth
  2. oxygen- helps release metabolic energy needed for growth activities
  3. nutrients-synthesis of protoplasm and source of energy
• plants also have a optimum temp range and light and gravity can also affect its growth
• example of differentiation

  to form tracheary element cells lose protoplasm develop strong elastic secondary cell walls to carry water under high tension
• 1. After dividing up to a certain phase, the plant cells undergo maturation by the process of differentiation and stop dividing any further.
  2. When these mature cells regain their property to divide, the process is called dedifferentiation (to regain their dividing capacity).
  3. After dedifferentiation, the mature cells become meristematic cells leading to the formation of callus.
  4. When these secondary meristematic cells again lose their property to divide, they are said to undergo redifferentiation.
• Plasticity refers to a phenomenon in which plants follows different pathways in response to environment or phases of life forming different kinds of structures. e.g., Heterophylly in cotton
• PGR

  1. plant growth promoters- auxins, gibberilins , cytokinins
  2. plant growth inhibitor-absicisic acid
  3. ethylene is in both groups
• Auxin was isolated by F.W Went from tip of coleoptiles of oat seedlings
• Auxin was FIRST isolated from human urine
• Auxin = indole-3-acetic acid (IAA)
• Auxin

  1. Promote flowering in plants eg.pineapple
  2. initiate rooting in stem cuttings
  3. prevents abscission in early stage but promotes in older and mature leaf and fruits
  4. Apical dominance (apical bud inhibits lateral buds)
  5. Induce parthenocarpy eg.tomatoes
  6. used as herbicides eg.2,4-D kills dicotyledonous weeds
  7. Controls xylem differentiation and helps in cell division
• Gibberellins

  1. All GA are acidic
  2. they cause an increase in length of axis eg. used in grape stalks
  3. cause fruits to elongate eg. apple
  4. delay senescence so market period extends
  5. GA3(Gibberlic acid first discovered) speeds up malting process in brewing industry
  6. increases length in sugarcane increasing market yield
  7. leads to early seed production eg. jvenile conifers
  8. promotes bolting(internode elongation prior to flowering) in plants with rosette habit
• Cytokinins

  1. discovered as kinetin(from herring sperm)
  2. search led to isolation of zeatin from corn kernels
  3. synthesises in regions where rapid cell devision occurs eg.root apices,developing shoot buds etc.
  4. helps produce new leaves and chloroplasts in leaves
  5. lateral shoot growth is promoted(overcome apical dominance)
  6. adventitious shoot formation
  7. delays leaf senescence
• Ethylene is a gaseous plant growth regulator synthesized in large amounts by tissues undergoing senescence and ripening fruits
• Ethylene induces horizontal growth of seedlings, swelling of axis, and apical formation in dicot seedlings
• It promotes senescence and abscission of plant organs, especially leaves and flowers
• Ethylene is effective in fruit ripening and enhances the respiration rate during the ripening of fruits (respiratory climactic)
• It breaks seed and bud dormancy, initiates germination, and promotes rapid internode elongation in water rice, promoting root and root hair growth to increase surface area
• Ethylene also initiates flowering
• The most widely used source of ethylene is ethephon
• Absiscissic acid

  1. Acts generally as a plant growth and plant metabolism inhibitor
  2. inhibits seed germination
  3. stimulates closure of stomata and increases tolerence towards all kinds of stresses(stress hormone)
  4. important role in seed development maturation and dormancy
  5. dormancy helps seeds withstand dessication and other unfavourable conditions
  6. ABA is antagonist to GAs
• IAA and IBA have been isolated from plants as natural auxins. where as NAA and 2,4-D are synthetic auxins