CROP SCI UNIT IV

• UNIT IV PLANT GROWTH, DEVELOPMENT AND REPRODUCTION
• UNIT OUTLINE


  • Concepts of Growth and Development
  • Phases of Plant Growth and Development
  • Other Concepts Related to Plant Growth
  • Plant Life Processes
  • Plant Reproduction
• GROWTH


  An irreversible increase in size of the organism which occurs throughout the plant's life cycle
• Examples of growth


  • Elongation of the roots and stem
  • Leaf enlargement
  • Increase in the size of flower and fruits
• DEVELOPMENT or MORPHOGENESIS


  A series of changes by which an individual plant or animal passes from lower to a higher state of being from embryonic condition to maturity
• Development is a change in the complexity of the organism (less complex to more complex being)
• Two aspects of plant growth

  • Primary Growth – takes place in young, herbaceous organs resulting to increase in length of shoots
  • Secondary growth – follows primary growth in some plants and results in an increase on girth as layers of woody tissue are laid down
• Monocots and herbaceous dicots typically exhibit only primary growth
• Woody dicots exhibit both primary and secondary kind of growth
• 1. S-shaped (sigmoid) growth curve


  An ideal growth curve exhibited by numerous annual plants and individual parts of both annual and perennial plants
• PHASES OF GROWTH


  • Logarithmic or exponential phase
  • Linear phase
  • Stationary phase
• Logarithmic or exponential phase

  Establishment and seedling growth, the size increases exponentially with time, slow at first but continuously increases, relates to the germination of the seeds
• Linear phase


  Called the grand phase of growth, rapid and often increase in dry matter production, vegetative growth in annuals, terminates with the onset of flowering
• Stationary phase


  Reduction in growth rate until growth stops at crop maturity, at the end of the growth period, water lost from aerial parts of the plants, photosynthesis stops, and the crop begin to senesce
• Law of the Minimum, also known as Liebig's Law, states that growth is dictated not by total resources available, but by the scarcest resource (limiting factors)
• Yield is proportional to the amount of the most limiting nutrient, whichever nutrient it may be
• If one of the essential plant nutrients is deficient, plant growth will be poor even when all other essential nutrients are abundant
• Justus von Liebig: '1803-1873'
• Law of the Limiting Factors, also known as Blackman's Law of Limiting Factors, stated that a number of factors regulate the biological processes but the factors in different amount affect the process as a the whole
• Photosynthesis will only proceed at the rate determined by the factor in shortest supply
• Frederick Frost Blackman: '1866-1947'
• If CO2 increases


  The rate of photosynthesis also increases
• After a limit in light intensity any further increase in CO2 has no effect on rate and the rate become constant


  Light intensity became the limiting factor
• Law of Diminishing Return


  The increase in any crop produced by a unit increment of a deficient factor is proportional to the decrement of that factor from the maximum
• Increase in yield of a crop per unit of available nutrient decreases as the level of available nutrient approaches sufficiency
• Eilhard Alfred Mitscherlich: '1874 -1956'
• Factors Affecting Plant Growth and Development

  • Hereditary or genetic factors
  • Environment
  • Nutrition
  • Light
  • Temperature
  • Water
  • Hormones
  • Growth hormones
• Growth Hormones


  Hormone is derived from Greek word hormon means to excite, plant hormone is an organic compound synthesized in one part of a plant and transported to another part (low concentration), causes a physiological response or affects particular target cells
• Plant growth regulators (PGR)


  Include synthetic compounds, which can be organic and/ or inorganic (plant hormones occur naturally), substances which at low concentration, promote, inhibit or modify any physiological response, effective in small amounts usually produced in one part and transported to the site of action
• Phytohormones


  • Auxin
  • Cytokinins
  • Gibberellins
  • Abscisic acid
  • Ethylene
• Auxin


  1st plant hormone discovered, Charles Darwin was the first to discuss how plants response to light (photoperiodism) which led to discovery of auxin, if a seedling is allowed to grow in the dark it will become tall, skinny and white (etiolated)
• Auxin synthesis and movement

  • Pollen
  • Actively growing tissues: Shoot meristems, Young leaves, Developing seeds, Developing fruits
• Auxin Function/Uses


  Auxin is the active ingredient in most rooting mixtures, helps the vegetative propagation of plants, stimulates the production of secondary growth by stimulating cambium cells to divide and secondary xylem to differentiate, inhibit the activation of buds lower on the stems (apical dominance), promotes production of ethylene, promotes flower initiation, prevents leaf abscission, can produce seedless fruits using synthetic auxin, synthetic auxin are used as herbicides in large concentration (2,4-D and 2,4-5-T : Agent orange), used as defoliant
• Gibberellin or Gibberellic Acid


  In 1962, EIICHI KUROSAWA a Japanese researcher studied a rice disease that was known as the "foolish seedling disease" in Japan, plants grow extremely fast, look spindly and pale and break off easily, KUROSAWA detected that the reason for this abnormal growth is a substance that is secreted by a parasitic fungi (Fusarium moniliforme = Gibberella fujikuroi), it was termed gibberellin, GA3 was the first commercially available gibberellin, GA3 has been studied much more other than other forms of GA (because of it availability), so its is commonly referred to as gibberellic acid, gibberellins are acidic compounds and are therefore also called gibberellic acids (GA)
• Gibberellin synthesis and movement

  • GA is synthesized in the roots, young leaves, and seeds, young leaves are thought to be the major site of gibberellin synthesis
• Roles and Effects of GA

  • Cell elongation and cell division, Rosette or dwarf plants lack of endogenous GA often contributes to decreased height, cause bolting, cell division, enzyme induction, Stimulates flowering in long-day plants, Fruit growth – some species required both auxin and gibberellin for fruit setting, Sex expression – favors staminate flower formation on monoecious plants, Germination and Dormancy – embryo produces GA that triggers release of α-amylase and other hydrolytic enzymes, GA has the ability to break dormancy in a variety of crops
• Cytokinins


  Discovery is related to work on tissue culture, Van Overbeek discovered that plant embryo grows faster when they are given coconut water (endosperm of coconut), Zeatin and several other related molecules were first found in coconut milk and corn, Kinetin was the first cytokinin not found in plant (synthetic), the most common form of naturally occurring cytokinin in plants today is called zeatin which was isolated from corn
• Synthetic cytokinin


  • Kinetin (6-furfurylaminopurine)
  • BA (benzyladenine)
  • PBA (pyranylbenzyaldenine)
• Roles of Cytokinin


  • Cell division (induction as well as promotion, interacts with auxins)
  • Organ formation (interacts with auxin)
  • Overcoming dormancy
  • Retards leaf senescence by stimulating RNA and protein synthesis and delaying degradation of chlorophyll
  • Delays senescence of cut flowers and vegetables
• Ethylene


  The ripening hormone H2C=CH2, this has been used since the ancient Egyptians, who would gas figs in order to stimulate ripening, ancient Chinese burned incense in closed rooms to enhance ripening of pears, Croker (1935) proposed ethylene was a plant hormone responsible for fruit ripening as well as inhibition of vegetable tissues