20-30

• Morphological adaptation


  Exemplified by the presence of metamorphosed or specialized organs which performs non-typical functions
• Physiological adaptation


  Exemplified by the closing of stomates of many bromeliads during the day to help conserve water as well as the abscission of leaves in deciduous plants to reduce the evaporative surface area thereby conserving moisture and lowering of compensation point
• Biochemical adaptation


  Biochemical changes with some bearing on certain survival mechanisms such as the increase in proline and abscisic acid and osmolytes in plants during period of moisture stress to regulate increased water-holding capacity of tissues for moisture as well as stomatal closure to conserve water
• Phytohormones


  Organic substances other than vitamins and nutrients which are active in very minute (often <2μM) amounts
• Phytohormones


  Formed in certain parts of the plant and which are usually are translocated to other sites where they promote, inhibit, or otherwise modify physiological, biochemical and/or morphological processes
• Promotive plant growth regulators (PGRs)


  At relatively low concentrations but become inhibitory at relatively higher concentrations
• Auxin


  Generic term applied to growth regulators with the special capacity to promote cell elongation
• Naturally occurring auxins


  The indole-auxins represented by indole-3-acetic acid (IAA)
• IAA


  Synthesized from the amino acid tryptophan primarily in actively-growing tissues. It is also produced in mature leaves and root tips, although at much lower concentrations
• IAA transport


  Cell to cell and is polar in nature. The basipetal transport to the root and acropetal transport to the upper organs involves vascular and non-vascular tissues
• Effects of auxin


  • Promotes cell enlargement and cell division in the cambium in tissue culture
  • Stimulates root initiation in cuttings
  • Stimulates differentiation of phloem and xylem
  • Induces ethylene biosynthesis at supra-optimal concentration
  • Mediates the tropic bending responses of shoots and roots to gravity, light and touch
  • Promotes apical dominance
  • Delays leaf senescence and leaf and fruit abscission
  • Promotes fruit setting and fruit development in some plants
  • Can also delay fruit ripening but may promote flowering in some plants (Ex. bromeliads)
  • Induces femaleness in dioecious flowers (via ethylene)
  • Induction of parthenocarpic (seedless) fruit development (Ex. tomato)
  • Popularly used as herbicides (Ex. 2,4-D, 2,3,5-T [agent orange], dicamba etc)
• Gibberellic acid (GA)


  Belong to a family of compounds based on the ent-gibberellane structure
• GA synthesis

  From mevalonic acid in 1) elongating shoots, 2) young leaves of developing apical buds, 3) developing seeds and fruits, and 4) apical regions of the roots
• GA transport

  The major conduit for the non-polar transport of GAs is the phloem
• Effects of GA

  • Stimulates stem elongation (may reverse physiological and genetic dwarfism in plants)
  • Promotes balting (rapid elongation of floral stem) in long day plants
  • Induces germination of seeds that normally require a cold treatment (stratification) or light (positively photoblastic seeds)
  • Stimulates de novo synthesis of a-amylase in germinating cereal grains
  • Promotes fruit set and fruit growth in some fruits (Ex. grapes)
  • Induces maleness in dioecious flowers of some species
• Cytokinin (CK)


  Adenine derivatives which have the capacity to induce cell division in tissue culture
• Zeatin


  The most common CK base in plants, the first natural CK which was isolated from corn endosperm
• CK synthesis

  Synthesized through the biochemical modification of adenine. The major site of CK biosynthesis is at the root apical meristem although seeds (embryo) and developing leaves have been shown to produce significant amounts of CK as well
• CK transport


  CK produced in the roots is transported to the upper organs via xylem
• Effects of CK

  • Regulates morphogenesis in cultured tissues (in synergy with auxin)
  • Releases lateral buds from apical dominance
  • Delays leaf senescence
  • Promotes cotyledon and leaf expansion
  • Promotes nutrient mobilization
  • Enhances stomatal opening in some species
  • Enhances accumulation of chlorophyll as it promotes the conversion of etioplasts into chloroplast
• Ethylene


  Only phytohormone occurring in gas state
• Ethylene synthesis


  Synthesized from the amino acid methionine (primary precursor) in many tissues in response to stress
• Ethylene movement


  Being a gas, ethylene moves by diffusion from the site of biosynthesis
• Effects of ethylene

  • Promotes ripening of climacteric fruits
  • Induces epinasty
  • Induces lateral cell expansion
  • Formation of adventitious roots
  • Induces flowering in pineapple and other bromeliads
  • Enhances flower, fruits and leaf senescence
  • Induces femaleness in dioecious flowers of some species
  • Promotes shoot and root growth differentiation
  • Releases tissues/organs from dormancy
  • Promotes leaf and fruit abscission
  • Enhances flower opening in some species
• Abscisic acid (ABA)


  Synthesized from mevalonic acid in mature leaves particularly in response to water stress. Seeds are also rich in ABA which may be imported from the leaves or synthesized in situ
• ABA transport


  ABA is exported from leaves in the phloem. There are some evidences that ABA may circulate to the roots in the phloem and then return to the shoots in the xylem
• Effects of ABA

  • Counteracts the effect of gibberellins on a-amylase synthesis in germinating cereal grains
  • Enhances stomatal closure (eg. during water stress)
  • Promotes leaf senescence
  • Promotes storage protein synthesis in seeds
  • Induces transport of photosynthates towards developing seeds and its subsequent uptake by growing embryos
  • Induces and/or maintains dormancy in seeds and buds
• Liebig's Law of Minimum


  The growth in lowest supply (climatic, edaphic, biological or genetic) sets the capacity for yield
• Blackman's Theory of Optima and Limiting Factors


  When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the slowest factor
• Mitscherlich Law of Diminishing Return


  When plants had adequate amounts of all but one limiting element, the growth response was proportional to the limitation element. Plant growth increased with additional increments of a limiting factor but not in direct proportion. The response is curvilinear contrary to Blackman's concept of linear response
• Sexual propagation


  Most common method by which plants reproduce in nature. Most efficient and widely used method for cultivated crops. Seeds arise from the fusion of male and female gametes to form a single cell (zygote) within the ovule of a flower
• Seed germination


  Series of events which take place when dry quiescent seeds imbibe water resulting in an increase in metabolic activity and the initiation of a seedling from the embryo
• Epigenous germination


  Cotyledons above ground, hypocotyl elongates and brings epicotyl emerges and the
• Hypogenous germination


  Cotyledons remain below soil surface
• Seed dormancy


  Physiological or physical condition of a viable seed that prevents germination even in the presence of otherwise favorable germination conditions
• Seed quiescence


  Condition in which seed cannot germinate because of unfavorable condition
• Types of dormancy

  • Primary
  • Exogenous or coat-imposed dormancy - essential germination components not available
  • Endogenous dormancy - caused by environment during seed development and maturation
  • Secondary - Imposed by: temp, light/darkness, abnormal amount of water; chemical and gases
• Types of dormancy

  • Ecodormancy- due to one or more unsuitable factors in the environment with non specific effect
  • Paradormancy- due to physical factors or biochemical signals originating externally to affected structure
  • Endodormancy - regulated by physiological factors inside affected structure
• Hastening seed germination/ breaking dormancy

  • Scarification - Any treatment that removes the seed coat or alters it, making it more permeable to water and air. This can be done mechanically by rubbing into a rough surface or the use of chemicals such as hydrogen peroxide and muriatic acid
  • Physical scarification - soaking in water (tap, hot or boiling water) for a specific period of time
  • Mechanical scarification - piercing, rubbing on sand paper, filing, grinding with abrasives
  • Chemical scarification - treatment with sulfuric acid and organic solvents
  • Stratification- Placement of seeds between layers of moist sand, soil, or sawdust at high or low temperature so the action of water and high and low temperature will soften the seed coat
  • Vernalization - Seed treatment to cold temperature prior to germination
  • Embryo culture - Aseptically removing the embryo from the seed and placing it in a sterilized culture medium to germinate
• Asexual propagation


  Involves reproduction from vegetative parts of plants and is possible because the vegetative organs of many plants have the capacity for regeneration