Pollination and fertilization in cherry trees: importance, factors and current regulations

Pollination and subsequent fertilization are essential for fruit production in cherry trees. The pollination process culminates when pollen is deposited on the stigma of the recipient flower, which initiates fertilization. During this stage, a pollen tube is formed that travels from the stigma and style to the ovary of the flower.

Pollination pathways

Pollination can occur in two ways:

1. Anemophilous: Wind pollination.
2. Entomophilous: Pollination by insects, mainly bees. In southern Chile and in greenhouse or macro tunnel crops, bumblebees (Bombus sp.) are also used to help in the transfer of pollen. In addition, modern techniques are being developed, such as the application of pollen via electrostatics and the movement of pollen by artificial wind, although these also fall within the categories of vectors and environmental intervention.

Need for Pollinators

Although some cherry varieties are self-fertile, the use of bees is recommended even for these varieties, but to a lesser extent than for self-incompatible varieties. Self-incompatible varieties require the presence of compatible pollen from other varieties for fertilization. In Chile, between 4 and 6 hives per hectare are used for self-fertile varieties and between 10 and 12 hives per hectare for self-incompatible varieties, depending on other factors.

Fertilization Process

Once the pollen is deposited on the stigma, it hydrates and germinates, forming a pollen tube. This tube grows through the style until it reaches the ovary. Inside the style, the pollen tube competes with other tubes to reach the ovary. Normally, only two or three tubes manage to reach it, due to the complex internal structure of the style.

Relevance of Temperature

Temperature is a critical factor in the rate of pollen tube growth. At optimal temperatures (between 15° and 22°C), the pollen tube grows more rapidly and can reach the ovary in a shorter time. Since ovules have a short viability period, the pollen tube must reach and fertilize the ovules before they lose viability. If, for example, ovules have a viability of three days, the pollen tube must germinate and develop within that period for successful fertilization.

Fertilization and Fruit Setting

Once the pollen tube has fertilized one of the ovules, the fruit setting process begins. This involves the transformation of the ovary into a final fruit through various phenological and physiological processes. The fruit then goes through three stages of development: cell division, hardening of the stone and cell elongation. These processes are independent and have different nutritional and hormonal needs.

Cherry Varieties and Pollination Compatibility

In cherry trees, there are two main types of varieties in relation to their pollination capacity:

1. Self-compatible or Self-fertile Varieties: These varieties, such as Lapins, Santina, Skeena, and Staccato, can be fertilized with their own pollen. In other words, the flowers of these varieties have the ability to self-fertilize.
2. Self-incompatible or self-infertile varieties: Examples of these varieties include Regina, Kordia, and BingAlthough pollen can reach the stigma of the flower and germination occurs on the stigma, fertilization does not occur due to an incompatibility in the style of the flower, which prevents the pollen from reaching the ovule.

Regardless of the type of variety, whether self-compatible or self-incompatible, the pollination process requires that the pollen moves entomophilously (via insects) or anemophilously (via the wind).

It is important to note that self-fertility does not always translate into high fruit set potential. There are self-compatible varieties with low, moderate or high fruit set potential, as well as self-incompatible varieties with different levels of fruit set potential. Therefore, it is essential to consider the flowering date and the specific compatibility of each variety to optimize pollination (see Table 1).

Table 1. Flowering and harvest window for the main cherry varieties planted in Chile.

Incompatibility Groups and Genotypes of Cherry Varieties

Currently, cherry varieties are grouped into different incompatibility groups and genotypes. These varieties are classified according to their flowering date into three main categories:

1. Early flowering
2. Medium flowering
3. Late flowering

This classification is essential for pollination planning, as it allows cherry varieties to be aligned in a way that maximizes compatibility and the effectiveness of the pollination process (see Table 2).

Pollinator Manual in the Chilean Fruit Industry

In Chile, the fruit industry has established medium- and long-term initiatives to train beekeepers and farmers in order to improve efficiency throughout the entire pollination process chain. As a result of these efforts, the Pollinator's Handbook, developed in collaboration between the National Association of Seed Producers (ANPROS), the Federation of Fruit Producers (FEDEFRUTA) and the National Beekeeping Network Federation (RAN FG).

1. Current Regulations

According to exempt resolution No. 8,196 of 2015, the obligations of beekeepers are as follows:

• Declaration of Apiaries:Every October, beekeepers must declare the existence of their apiaries in paper format before the Official Livestock Information System (SIPEC).
• Internal Register: Beekeepers must keep an internal record of the movement of hives, indicating the identification of the apiaries, their origin (region, commune, locality, start and end date of pollination).

2. Safety Considerations

Moving hives:

• Prefer moving during the night or early in the morning. If hives were moved during the day, make sure there is sufficient ventilation to prevent the bees from suffocating.
• The load must be well stowed and secured. It is recommended to cover it with a mesh to prevent the bees from escaping.
• Have a first aid kit equipped with antihistamines and some injectable antidote.

Setting up the hives on the property:

• Do not inspect the hives on the day of transfer, as they will be more aggressive.
• Inspect the hives in the company of the beekeeper.
• Wear safety equipment at all times (suit and gloves).

3. Quality Parameters in Beehives for Pollination

According to Chilean Standard 3255 – 2011, a hive suitable for pollination must meet the following requirements:

• Minimum 8 frames with bees.
• 3.5 frames with brood.
• 1.5 frames with open breeding.
• 2 frames with closed brood.
• Free of pests and diseases.
• Number of bees per minute entering through the entrance equal to or greater than 50.

4. Installation of the Beehives on the Property

Prior to Installation:

• The producer must inform the beekeeper about the possible date of entry of the hives to the property.
• Provide a map indicating the location and number of hives per point. Prefer open areas or roads for installation.
• Ensure that roads are in optimal condition for the passage of vehicles.
• Provide water sources close to the hives, as lack of water can cause bees to stop collecting nectar and pollen.

During Pollination:

• Check that the hives are not receiving water from irrigation or other nearby sources. If the area is humid, ask the beekeeper to change the location of the hive.
• Prevent agricultural personnel from passing near the beehives.
• Prefer the application of agrochemicals during the afternoons or nights.
• Do not handle bees without the beekeeper's permission.

5. Field Control of Pollination

• The distance between groups of hives should not exceed 200 meters.
• Avoid installing hives on roads with high pedestrian or vehicular traffic.
• Hives should not be in direct contact with the ground; they should be mounted on pallets, trestles or other structures that isolate them from the ground, weeds and attacks by ants or rodents.
• Prefer dry and sunny places to set up the hives, which encourages the bees to work better.

Targeted Pollen Application

In the absence of pollinators or in the presence of negative abiotic factors for pollination, directed pollen can be applied following these considerations:

• This technique does not replace the use of bees or bumblebees.
• Identify areas of the orchard with setting problems to increase the number of hives and pollen.
• Pollen must be kept in the cold chain, with temperatures between -15ºC and -10ºC.
• Ask the supplier to ensure that the pollen dose corresponds to the amount that will be used during the day to avoid unnecessarily defrosting pollen.
• Do not apply pollen directly after removing it from the freezer; it is better to gradually raise its temperature.
• Identify hives with increased activity; it is recommended to use only 50% from hives for application.

During Application:

• Begin application when the first bees begin to fly in the morning.
• Start application with a 20% of open flowers.
• Always use personal protective equipment such as a veil, gloves and white overalls.
• Divide the recommended dose into 2 or 3 daily applications.
• Avoid application during adverse weather conditions such as heavy fog, drizzle or rain.

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Literature

• Carlos Tapia: Personal communication.
• «Cherries, Botany, Production and Uses», edited by José Quero-García, Amy Lezonni, Joanna Pulawska and Gregory Lang.
• Gil, G. (2012). Fruit Growing: Fruit Production: Fruits from temperate and subtropical climates. UC Editions, Chile.
• De la Cuadra Infante, S. and Rodríguez, P. (2019). Manual on pollination of agricultural crops.
• Greenpeace (2013). Dangers for pollinators and agriculture in Europe.
• Rodrigo, J., Negueroles, J., & Wünsch, A. (2019). Choosing pollinating varieties in cherry trees. Fruit Growing Magazine, 70, 68-71.
• Avium Team. (November 13, 2020). Flowering and harvest window for the main cherry varieties planted in Chile. Smartecherry. (Table 2).