THeme 5 Hort

• Flowering


  Process by which flower buds complete their development through cell division and expansion
• Pollination


  Transfer of pollen from one flower to another

  
• Fruit-set


  Sufficient proportion of flowers setting fruit, normally after pollination and fertilization
• Cross-pollination


  • Transfer of pollen from another flower or plant
  • Necessary for most pome and stone fruits as they are self-incompatible

  
• Self-pollination


  • Transfer of pollen from the anthers to the stigma within the same flower

  
• Self-incompatibility


  Inability to produce zygotes after self-pollination in a fertile hermaphrodite plant
• Cross-incompatibility


  Inability to produce zygotes after cross-pollination between two fertile species
• Successful cross-pollination


  1. Adequate pollen transfer with pollen vectors (e.g., wind, insects)

  2. Enough well-distributed pollenizer trees in the orchard

  3. Adequate coincidence of bloom periods of pollenizer and main cultivars every year

  
• Pollen vectors


  Agents that transfer pollen, such as wind and insects

  
• Lack of insect vectors


  Inadequate pollen transfer

  
• Temperature during flowering


  • Important for pollen germination rate, pollen-tube growth speed, and ovule longevity
• Effective pollination period (EPP)


  Difference between the period for pollen tube growth and that of ovule longevity

  
• Pollen production


  • Depends on number of flowers, stamens, and pollen grains per anther

  
• Pollen production


  1. Pollen mother cells undergo meiosis to produce haploid microspores

  2. Microspores develop into pollen grains with a vegetative cell and a generative cell

  
• Embryo sac


  Develops from a megaspore mother cell, with eight nuclei and an egg apparatus
• Late maturation of embryo sacs


  Results in erratic cropping behaviour of some cultivars
• High temperatures during flowering


  May be detrimental to ovule and embryo sac longevity
• Beginning of flowering


  • Requires breaking of endo-dormancy by cold, then activation by temperature thresholds
  • Starts earlier at lower latitudes and on slopes with northern exposure or higher altitudes
• Flowering duration


  Varies from one week to several weeks, with warm weather shortening it and cold weather lengthening it
• Flowering overlap


  • Best chances for good fruit set when pollenizer and main cultivar flower simultaneously
• Flowering phenology


  Study of recurrent natural events like flowering associated with seasonal climatic changes
• Flower-bud sensitivity to frost


  • Increases with time and varies by developmental stage
• Pollen germination


  Starts the progamic phase, leading to fertilization (syngamic phase)

  
• Pollen germination requirements


  • Water absorption from stigma secretion, suitable temperatures

  
• Boron promotes pollen germination and is often applied to in-vitro germination media

  
• Pollen germination


  Germination leads to the appearance of a pollen tube from one of the germination pores

  
• High germination rate


  A high number of styles growing down the style to the ovary
• Key requirement for pollen germination


  Water absorption from the liquid secreted by the papillate surface cells of the stigma

  
• Pollen germination


  1. Water absorption

  2. Generative cell divides once

  3. Tube contains 2 male cells, the microgametes

  
• Stigma surface


  • Made up of turgid cells
  • Cells degenerate at maturity and produce the liquid secretion
  • A wet stigma is considered receptive
  • Receptivity of the stigma usually coincides with flowering

  
• Stigmatic liquid


  Contains elements that promote germination e.g., boron
• Boron is normally applied to media used for in-vitro germination
• Suitable temperatures are needed for pollen germination
• Self-pollination


  • If the anthers and stigmas are close, chances are high
  • The stamen length vs style length determines the chances
  • In almond, there is a reduced likelihood due to the stigmas being situated well above the anthers
  • In peach and nectarine, anthers dehisce before and during anthesis, and are near and in contact with receptive stigmas, facilitating self-pollination

  
• Pollen tube growth


  1. Penetrates the stigma

  2. Grows through the transmitting tissue outside the cells in the extra-cellular matrix

  3. On the stigma surface, pollen tube growth is autotrophic (depends on its reserves)

  4. After entering the styles, pollen tube growth becomes heterotrophic (sustained by uptake of materials from the extra-cellular matrix)

  5. Growth is restricted to the tip, with callose plugs cutting it off from the rest of the tube

  6. After entering the micropyle, the tube that will fertilize the egg apparatus grows through the nucellus to the embryo sac, then bursts to discharge the two male cells into one of the synergids

  
• Transmitting tissues of apple

  • Secrete proteins and CHOs to support pollen tube growth
  • The level of CHO reserves affects pollen tube growth
  • Differences in CHO reserves may lead to differences in fruit set

  
• Many pollen tubes may grow down the style, but only one enters the micropyle, a phenomenon termed pollen attrition

  
• Fertilization


  1. After fusing with the sperm cell, the egg cell becomes a diploid zygote (2n)

  2. The zygote later develops into an embryo, which develops into a new plant with a chromosome number specific to the species

  3. Sometimes, the egg cell is not haploid because meiosis did not occur, resulting in a triploid zygote and triploid endosperm
• Apomixis


  • Embryo sacs and embryos can be formed without meiosis or fertilization
  • Observed in apple and citrus species
  • The embryo is completely derived from cells in the maternal ovule tissue, not from fusion of male and female gametes
• Forms of apomixis


  • Diplospory
  • Apospory
  • Adventitious embryony