The stigmatic surface and style

The stigma is the organ in the flower on which pollen must germinate to achieve subsequent fertilisation.

  • The surface of the stigma consists of tiny papillae, which are turgid at anthesis (i.e. when the flower first opens).
  • A surface secretion develops as the flower opens and this is when the flower is considered to be receptive. Apple stigmas are considered to be ‘wet’ in type.
  • The papillae on the stigmatic surface desiccate rapidly and collapse soon afterwards.
  • This process is very rapid in apple flowers taking only 2 days from flower opening (Braun and Stösser, 1985).
  • However, experiments by the same authors in Germany have shown that apple pollen grains are able to germinate on stigmas after they have turned brown.
  • As late as 10 days after anthesis pollen grains can germinate on the stigma, even though the surface papillae are totally collapsed by this stage.
  • It would seem that the condition of the stigmatic surface is not a significant constraint on pollen germination or the effective pollination period (EPP).
  • Poor fruit set as a result of short effective pollination periods (EPPs) is more related to ovule longevity and possibly also growth of the pollen tube down the style, than limitations on pollen germination.

The style is the conducting stem, which connects the stigma with the ovary, within which are the female egg cells or ovules.

  • The style centre is made up of special conducting or transmitting tissues; this connects with the papillae on the stigma surface.
  • The cells of the transmitting tissue are elongated and have large spaces between them filled with pectic substances.
  • The pollen tubes grow down the style.

Pollen germination and pollen tube growth down the style

  • In favourable conditions, pollen grains deposited on the stigma become hydrated (take up water) and subsequently germinate.
  • The pollen tube emerges through one of the three germ pores and grows into the papilose epidermis on the stigma.
  • After it penetrates the stigmatic surface between the papillae, the pollen tube enters the connecting tissue within the style.
  • Starch in this transmitting tissue is used as a food source by the extending pollen tube.
  • The tubes usually take two to four days to grow to the base of the style, but this is very dependent upon ambient temperatures.
  • At temperatures of 5-10ºC pollen tube growth is very slow.
  • Not all of the germinating pollen grains reach the base of the style.
  • If approximately 50 germinate and start to grow down the style only five or ten pollen tubes make it to the base.
  • At the base of the style the pollen tube must penetrate the pericarp before entering the ovule.
  • Although a very short distance, this penetration can be quite slow.
  • For example whilst pollen grains may take only two to three days to germinate and reach the base of the style, they may take a further six to eight days to transverse the pericarp and effect a union with the ovule.
  • Although the distances within the pericarp are less than down the style, the conducting tissues located in the latter are much more favourable for rapid growth (Stösser et al., 1996).
  • Once in the locule of the ovary, pollen tubes frequently show very little directed growth towards the micropyle of the ovule and may ramify for more than a day before locating and penetrating the micropyle (Stösser, 1986).

Factors influencing fertilisation

Pollen germination, its growth down the style into the ovary and the achievement of fertilisation, by the pollen’s entry into the micropyle of the ovule, are influenced by a number of factors. The most important of these are:

  • The apple scion variety
  • Pollen quality
  • The quantity of pollen on the stigma
  • Pollen compatibility
  • Climatic conditions

Apple scion variety

Apple scion varieties differ in the proportion of viable pollen produced, the average germination percentages achieved (over a number of seasons) and the climatic conditions under which the pollen will germinate.

The self fertile clones of Cox’s Orange Pippin (Cox Self-fertile Clone 8) and Queen Cox (Queen Cox Self-Fertile Clone 18) both produce low proportions of viable pollen compared with the standard clones of Cox or Queen Cox.

  • Much of the pollen produced by these self-fertile clones is aborted and infertile.
  • Whilst this has little consequence when the self-fertile clones are planted as monocultures (i.e. solid orchard plantings of the same clone), their use as pollinators in other orchards of self-sterile clones or varieties cannot be recommended.
  • Insufficient viable pollen is produced by these self-fertile varieties to be collected and transported efficiently by insects between flowers on different trees.

Pollen of some scion varieties germinates and grows down the receptor styles of flowers at quite low temperatures.

  • Although temperatures of 15-25oC are required by most varieties for efficient pollen germination and growth, pollen of a few varieties, such as Falstaff and Redsleeves, is able to germinate and grow efficiently at much lower temperatures (Petrapoulu, 1985).
  • These varieties also exhibit what is known as high stylar receptivity.
  • These studies showed also that high stylar receptivity, high pollen germination at low temperatures, precocity and high yield efficiency seem to be associated in apples with a low chilling requirement.
  • In support of this, it has been shown that lack of winter chilling is associated with poor fertility of Cox flowers (Jackson et al., 1983).

Quantity of pollen on the stigma

There is evidence with many flowering plants that the quantity of pollen deposited on the stigma influences its subsequent germination (Brewbaker and Majumdar, 1961).

  • Increasing the number of grains deposited on the stigmatic surface is thought to have a mutually stimulatory effect on grain germination, possibly attributable to the increased calcium ions associated with increased pollen abundance (Brewbaker and Kwack, 1963).

Climatic conditions

  • The ideal climatic conditions for pollen germination and growth are temperatures between 15oC and 25oC, no rain, and minimal or no desiccating winds.
  • At lower temperatures pollen germination and growth will be slower.
  • Very little pollen germination occurs at temperatures less than 10oC.

Fertilisation and fruit set

  • Once the pollen tube has located and penetrated the micropyle and fused with the embryo sac, fertilisation has taken place.
  • Pollen grains of apples contain two nuclei, which move with the cytoplasm in the pollen tube as it moves down the style.
  • At some point one of these nuclei (known as the vegetative nucleus) degenerates and the second one (the generative nucleus) divides into two sperm nuclei.
  • When a single pollen tube enters the micropyle it discharges the sperm nuclei into the embryo sac.
  • One of these nuclei fertilises the egg to form the zygote (embryo).
  • The other unites with the two polar nuclei in the ovule to form the triploid endosperm tissue.