Chapter 6A

• Growth


  ■ Irreversible increase in size

  ■ Accomplished by cell division and enlargement

  ■ Cell division increases cell number without volume increase

  ■ Increases potential for growth by increasing cell number

  ■ Most plant growth is brought about by cell enlargement
• Growth is an irreversible increase in size accomplished by cell division and enlargement.
• Cell division increases cell number without volume increase.
• Growth increases the potential for growth by increasing cell number.
• Most plant growth is brought about by cell enlargement.
• Development is the sum total of events that progressively form an organism’s body.
• Development involves three overlapping processes.
• Development occurs in response to instructions contained in genetic information that an organism inherits from parents.
• Specific developmental pathway followed is largely determined by environmental factors such as day length, light quality and quantity, temperature, and gravity.
• Morphogenesis is the development of form and shape.
• Planes in which cells divide and subsequent expansion are key factors of the process.
• The primary event in morphogenesis is the expansion of tissue.
• Cell and tissue differentiation follows morphogenesis.
• Differentiation is the process of growth and morpho-physiological specialization of cells produced by meristems.
• Polarity in plants and their parts is determined by environmental signals such as light and gravity.
• Hormones do not act alone, but in conjunction or opposition to each other such that the final condition of growth and development represents the net effect of a hormonal balance.
• Abscisic acid inhibits growth, stimulates stomatal closure, maintains dormancy, and is produced in leaves, stems, and green fruit.
• Auxin is involved in xylem differentiation.
• Auxin stimulates cell elongation, is involved in phototropism, gravitropism, apical dominance, and vascular differentiation, and stimulates ethylene synthesis and induces adventitious roots in cuttings.
• Hormones fulfill their coordinating and integrating functions in two ways: they serve as transportable messenger substance for information exchange between organs and tissues of hormone synthesis, hormone transport, site(s) of hormone action (target or response tissue), and hormone breakdown.
• Auxin inhibits growth of lateral buds (apical dominance) and induces formation of lateral roots and adventitious roots in cuttings.
• Cytokinin stimulates cell division, reverses apical dominance, is involved in shoot growth, delays leaf senescence, and is synthesized in roots and transported to other organs.
• Auxin is synthesized naturally as Indole - 3 - acetic acid (IAA) and Indole - 3 - butyric acid (IBA), and synthetic auxin is Naphthalene acetic acid (NAA) and 2,4 - D.
• Ethylene stimulates fruit ripening, leaf and flower senescence, and abscission, and is produced in tissues of ripening fruits, nodes of stem, senescent leaves and flowers.
• Gibberellin stimulates cell elongation, bolting and flowering in biennials, regulates production of hydrolytic enzymes in grains, and is synthesized in meristems of apical buds and roots, young leaves, and seed embryo.
• Cytokinin, both natural (Zeatin) and synthetic (Kinetin), stimulates cell division and promotes growth of lateral buds, reversing apical dominance.
• Abscisic acid maintains dormancy, causes stomatal closure, and acts as a 'stress hormone'.
• Jasmonic acid inhibits seed/pollen germination, retards root growth, promotes curling of tendrils, induces fruit ripening and abscission of flowers, and is involved in insect and pathogen resistance.
• Cytokinin, both natural (Zeatin) and synthetic (Kinetin), promotes shoot formation and acts as an antagonist to auxin.
• Numerous hormonal interactions influence plant growth and development.
• Cytokinin, both natural (Zeatin) and synthetic (Kinetin), delays leaf senescence and abscission.
• Fertigation regulates fruit development (parthenocarpy) and delays leaf abscission.
• Gibberellin, both active (Gibberellic acids GA3, GA1) and inactive (GA3, GA1), affects fruit setting and growth and promotes maleness in flowers.
• Ethylene stimulates aerenchyma and leaf epinasty observed in waterlogged conditions.
• Brassinosteroids (brassinolide, castasterone) promote stem elongation, pollen tube growth, grass leaf bending at sheath, xylem differentiation, and enhance ethylene production.
• Ethylene stimulates leaf senescence and abscission and promotes femaleness in flowers.
• Ethylene stimulates fruit ripening and is involved in climacteric rise.
• Gibberellin, both active (Gibberellic acids GA3, GA1) and inactive (GA3, GA1), releases seeds and buds from dormancy.
• Gibberellin, both active (Gibberellic acids GA3, GA1) and inactive (GA3, GA1), stimulates stem elongation and induces bolting and flowering in biennials.
• These activities have a genetic basis, but are also influenced by environmental factors.