Is temperature relevant in the fertilization process? At optimal temperatures (between 15° and 22 °C) the growth of the pollen tube is faster, therefore it can reach the ovules in less time to fertilize them, considering that the ovules have a short viability period.
Pollination and subsequent fertilization are essential for different fruit species, including cherry trees. The end of the pollination process is marked by the positioning of the pollen on the stigma of the recipient flower, after which fertilization begins through the creation of the pollen tube where the pollen reaches the stigma and the style before entering the ovary to complete the process.
«Pollination, in general, has two routes: anemophilous, that is, by wind in a natural way and entomophilous, which is through vectors based on insects; the latter is mainly done by bees in our case, while in southern Chile and in cherry crops under forced conditions such as greenhouses or macro tunnels, it is also complemented by the use of bumblebees (or Bombus sp.) that help the movement of this pollen. Although there are other, much more modern techniques that are being developed with respect to pollination, such as electrostatic application of pollen, movement of pollen with some type of wind in an artificial way, it is still framed in these two great phases of movement that are vectors and wind in a natural or artificial way. Therefore, that is the pollination process, considering that there must be pollen compatible with the recipient flower.", explained Carlos Tapia, Specialist Advisor in cherry production and Technical Director of Avium.
It is a mistake to think that self-fertile varieties should not be used with bees; they should be used, but in a much smaller proportion than those varieties that are self-incompatible, which means that the pollen of that variety is not compatible with the flower of the same variety, therefore, one that is genetically compatible must be sought in order to be pollinated and fertilized. It should be remembered that the concept of self-fertility is defined in that genetically the pollen is compatible with the flower of the same variety, but not necessarily that it should not be pollinated with some type of system.
«In these cases, the standard numbers of bee hives used in Chile to pollinate self-fertile varieties range from 4 to 6 hives per hectare, and for self-incompatible varieties, depending on the number of pollinators and some other factors, they could be between 10-12 hives per hectare." indicated Carlos Tapia.
In the case of the cherry tree, the stigmas are receptive after the flower has opened and are covered by a conspicuous, viscous-looking secretion where the pollen grains “land”. It is there that they hydrate and germinate, producing a pollen tube for their subsequent development. This process occurs during the first day after pollination itself.
After several processes, the pollen tube enters the style of the flower to move through it until it reaches the ovary. Pollen tubes are composed of carbohydrates necessary for their growth from the cells of the transmitting tissue found in the style, which are filled with starch (main source of carbohydrates), which are exhausted once the pollen tubes grow. Inside the style there is intense competition between pollen tubes, of which, generally, two or three of them manage to reach the ovary; this is conditioned by the complex structure inside the style that leaves less space for the growth of the tubes.
In cherry trees, pollen tubes take approximately three days after pollination to reach the base of the style, where the ovary is located, considering that ambient temperature plays a fundamental role in the speed of development of the pollen tubes. For fertilization and subsequent fruit setting to take place, only one of the two ovules found in the ovary is required.
In cherry blossoms, different complex processes can be distinguished that develop inside, between the pollen tube and the ovary, and in which the pollen tubes are “stopped” and cannot achieve fertilization of the ovule.
“The fertilization process occurs once the pollen is positioned on the stigma of the flower's pistil, and occurs through germination that the pollen generates in a duct, the so-called pollen tube. This pollen tube must descend to the ovary to fertilize the ovules and once the pollen tube arrives, the fertilization process occurs and is independent of the pollination process.", Tapia explained.
Is temperature relevant to the fertilization process? Of course, it has a positive influence.
«The main factor is temperature. At optimal temperatures (between 15° and 22 °C) the growth of the pollen tube is faster and therefore it can reach the ovules in less time to fertilize them, considering that the ovules have a short period of viability. For optimal fertilization, the pollen tube has to be fertilizing in less time than the viability of the ovule, that is, for example, if the ovules have a viability of three days, the pollen tube has to germinate and develop before three days, because otherwise they will not reach the ovules to fertilize them.", explains the Technical Director of Avium.
This process is known as fertilization and is physiologically defined as the effective pollination period (EP).
«Once fertilization has occurred successfully, we could say that the fruit setting process begins, which is when the ovary of the pistil of the flower is engulfed and transformed into the final fruit, through all the phenological and physiological processes of the plant. Tapia added.
Once the ovules have been fertilized, the process known as fruit setting begins and the plant waits to complete its three stages of development: cell division, hardening of the stone and cell elongation. These processes develop independently and require different nutritional and hormonal needs within the plant, considering that there are cultural tasks capable of enhancing each of these processes, with the aim of achieving maximum productive potential.
Literature
– Carlos Tapia: personal communication.
– “Cherries, botany, production and uses”- Edited by: José Quero-García, Amy Lezonni, Joanna Pulawska and Gregory Lang.
