Pollination, and soon thereafter fertilization, are fundamental processes for all different fruit species, among them cherry trees. The end of the pollination process is marked by the positioning of pollen in the receiving flower’s stigma. Then begins fertilization through the creation of the pollen tube, where pollen arrives at the stigma and the style before entering the ovary to finalize the process.
«Generally, pollination has two paths: one of them anemophilous, which is to say using wind naturally; and another entomophilous, which takes place through vectors such as insects. The latter mostly uses bees in our cases of scope; while in southern Chilean cherry growing under forced conditions (such as greenhouses and macro tunnels) the process is complemented using bumblebees (or Bombus sp.) that help to move this pollen. Even though there are other techniques that are far more modern and are being developed to aid pollination, such as electrostatic pollination or pollen transportation using artificial wind, these still fall into those two greater movement phases that are vectors and wind, regardless of them being natural or artificial. Therefore, this is the pollination process, considering that there must exist pollen that is compatible with the receiving flower«, explained Carlos Tapia, Specialist advisor in cherry production and Technical Director for Avium.
It is erroneous to think that self-fertile varieties needn’t the placement of bees; it must be done, just in a quite smaller proportion in comparison with self-incompatible varieties (which means that this variety’s pollen is not compatible with the flower of the same variety, therefore one that is genetically compatible must be searched for in order for pollination and fertilization to occur). It is worth remembering that the concept of self-fertility is defined as the case where pollen is compatible with the flower of the same variety, but this doesn’t mean that it doesn’t need to be pollinated with some type of system.
«In these cases, with the numbers that can be seen in Chile, for standard bee colonies used to pollinate self-fertile varieties we are talking about 4 to 6 colonies per hectare, and for self-incompatible varieties, depending on the amount of pollinizers present and a few other factors, you might need 10-12 colonies per hectare”, added Carlos Tapia.
In the case of cherry trees, stigmata are receptive after the flower blooms and they are covered in a viscous-looking, conspicuous secretion where the pollen grains “land”. This is where they become hydrated and germinate, thus producing a pollen tube for its later development. The aforementioned process takes place during the first day after proper pollination.
After different processes, the pollen tube enters the flower style to move through it until it reaches the ovary. Pollen tubes are made up of carbohydrates, necessary for their growth from the transmitting tissue cells that can be found in the style, which are filled with starch (a main source of carbohydrates), and which runs out once the pollen tubes grow. Inside the style, an intense competition between pollen tubes takes place – out of all the tubes, only two or three of them usually reach the ovary; this is conditioned by the complex structure inside the style, which leaves less space for the tubes to grow.
In the cherry tree the pollen tubes take approximately three days after pollination to reach the base of the style, where the ovary is. It must be considered that the environmental temperature plays a fundamental role in the development speed of pollen tubes. In order to have fertilization and posterior setting take place, only one of the two ovules that can be found in the ovary is needed.
In cherry blossoms, different complex processes can be distinguished that develop in the interior, between the pollen tube and the ovary. In these processes, the pollen tubes are “halted” and cannot achieve ovule fertilization.
“The fertilization process takes place once the pollen has positioned itself in the flower’s pistil’s stigma, and is produced through germination, which pollen generates in a channel called the pollen tube. That pollen tube, with its descending shape, has to reach the ovary in order to fertilize the ovules, and once the pollen tube arrives, what takes place is fertilization, which is independent from the pollination process «, said Tapia.
Is temperature relevant in the fertilization process? Of course it is, as it has a positive influence.
«The main factor is temperature. At optimal temperatures (between 15º and 22ºC) the pollen tube’s growth is quicker; therefore it can reach and fertilize the ovules quicker, considering that ovules have a short viability period. To ensure optimal fertilization, the pollen tube has to be fertilized in an amount of time that is shorter than ovule viability, in other words, if ovules have a three-day viability window, the pollen tube must germinate and develop before those three days pass, otherwise they won’t reach the ovules in time to fertilize them«, the Technical Director for Avium cleared up.
This process is known as fertilization and is physiologically defined as the pollination effective period (PEP).
«Once fertilization is successfully produced, we could say that the fruit setting process begins. This is when the pistil’s ovary is encompassed within the flower that will eventually become the final fruit, through all of the plant’s phenological and physiological processes” Tapia added.
Once the ovules are fertilized, the process known as fruit setting begins and with it the wait for the completion of all three of its development stages: cell division, pit hardening and cell elongation. These processes develop independently and demand certain needs within the plant, both nutritional and hormonal, considering that there are cultural tasks that are capable of fostering each of these processes, with the goal of achieving maximum productive potential.
– Carlos Tapia: personal communication.
– “Cherries, botany, production and uses”- Edited by: José Quero-García, Amy Lezonni, Joanna Pulawska and Gregory Lang.