Flowering and fertilization biology background and main ordering principles of cultivar association

10. Flowering and fertilization biology background and main ordering principles of cultivar association

Author: Zsuzsanna Békefi

10.1. The flower structure

Flowers of fruit species in Rosaceae family are radially symmetric, pentamerous. The perianth (petals and sepals) consist generally of five pieces. The number of stamens is generally a multiple of five, situated in more circles. The stamens encircle a nectary, whose important purpose is to attract pollinating insects. In pome fruit, the number of ovaries is five, with one style for each. In apple, styles are fused at their base to a stylar column, while in pear, they remain free down to their base. In stone fruit, there is only one ovary.

There are generally two ovules in each ovary at the beginning, one of this aborts in stone fruits. However, in apple both ovules can develop to a seed, which means even ten seeds per fruit. In advantageous circumstances, more than two seeds can develop per ovary. In stone fruit the ovule is atropous, while in pome fruit it is anatropous.

10.2. Flower quality

If the stigma encounters with the right pollen at the right phenological state, the flower develops into a fruitlet. The chance of this transformation is determined also by flower quality. In apple, it is proved that if flower bud differentiation starts late and flowers are relatively young, a worse fruit set has a higher chance. If an apple flower has less than 5 carpels and less than 10-15 stamens, it can be considered as qualitatively poor. These flowers are formed usually on sides of long shoots, and the longevity of the ovule is also shorter. The fruit set of young trees is weaker in this case. If only a few leaves develop from a mixed bud, or if there are few flowers in the cluster, these can indicate a lower flower quality as well.

Carbohydrate content of flowers has also a role in flower quality. It is especially important in stone fruits, where flowers bloom before leaves unfold, so leaves cannot contribute to the flowers’ nutrition. Starch content of flowers is the highest in the time of anthesis, and decreases paralelly with seed cell development. Flower quality is directly related to receptacle diameter. Starch content of pollen is also important, because this provides energy for pollen-tube growth, thus determining the ability of pollen for fertilization.

10.3. Pollen quantity and quality

A number of physiological processes take place before pollen maturation. Anther initials appear already at the end of summer, during bud differentiation. At the time of budburst, pollen mother cells appear in the pollen sacs. Pollen tetrads develop by meiotic division of pollen mother cells. Anomalies often occur during meiosis. In case of triploid cultivars, because of the triple chromosome set, pollen grains develop after meiosis with different chromosome numbers, which have different fertilizing ability. Therefore triploid cultivars are usually poor pollenizers. Because of the huge number of pollens, there are functional pollens among them as well which are able to fertilize, but the fruit set can be worse. Triploids can often be found among apple and pear cultivars, such as ‘Jonagold’, ‘Mutsu’, ‘Beurre Diel’, ‘Pap pear’. Anomalies of pollen development can occur at diploid cultivars as well, the extent depends on the susceptibility of the given cultivar and on environmental conditions. In case of plum cultivars, male sterility can occur. The reason of it, that the outer exine layer of pollen is missing and tapetal cells repress pollen grains.

The cross-section of pome- and stone-fruit pollen grains is triangular. Apricot pollen is relatively large among fruit species, while apple pollen is small.

As regards pollen quantity, fruit species and cultivars show considerable differences, measured by the number of pollens per anther. For example, pear cultivar ‘Bonne Louise d’Avranches’ produces less pollen, while ‘Williams’ produces multiple of this. The average number of stamens per flower determines the pollen productivity of a tree, depending on the year. Ornamental apple cultivars contain approximately 40 stamens per flower, that is why they are used as pollenizers, while flowers of cultivated varieties contain 20 stamens on average.

10.4. Flowering features

The flowering periods of a cultivar and its pollenizer (mostly the main flowering, when the rate of open flowers is more than 50 %, Figure 10.1.) must highly overlap, which means simultaneous flowering is important to provide chance for mutual pollination.

Mutual flowering of cultivars is examined in different ways (rhythm and dinamics of flowering). The results can be shown in a flowering phenogram (Figure 10.1.), where the joint area of the the two cultivars’ curves indicate the extent of simultaneous flowering. Ideally, the overlap is 60-80 %. If this is not possible, two pollenizers have to be chosen for the cultivar, which together overlap its flowering period. The best, if one flowers a little earlier, while the other a little later than the cultivar to be pollinated.

A more useful piece of information for growers, if the measured data are classified into flowering periods. There are different numbers of flowering periods according to species (3-5) in literature. As it was mentioned in the BSc course, the pollenizer of a given cultivar can only be another cultivar falling into the same or the adjacent flowering period. As long as the main flowering time of two cultivars suitably overlap each other, the viability of the stigma of the cultivar to be pollinated and the pollen diffusion of the pollenizer cultivar are highly probable to overlap as well. It is preferable, that the pollenizer start flowering a little before the main cultivar, so first opened flowers can surely be pollinated - these have the greatest chance of producing good quality fruit.

The progress and duration of flowering, as well as the factors influencing flowering were described during the BSc course.

Figure 10.1.: Flowering phenogram of apple cultivars, overlap of flowering periods

 10.5. Pollination

When the stigma is viable, its surface produces a bright, so-called stigmatic secretum which is easy for the pollen to stick on. When blooming ends, stigmas start to brown, there are no secretum drops any more. The time of pollen ripening is the anthesis, when anthers open and pollens start to scatter. Anthesis usually coincides with the beginning of flowering.

In case of pollination, the pollen sticks on the adhesive surface of the stigma. Then the pollen starts to germinate in a couple of hours, the pollen tube grows. Pollen germination is the appearance of a pollen tube through a pore of the pollen. Germination requires humidity and nutrient, which are provided by the secretum drop of the stigma. This secretum is a sugar solution containing boron. Pollen germination improves at increasing tempretures. The pollen of some cultivars germinate well also at lower temperatures, which is a beneficial characteristic ensuring the possibility of fertilization in case of a colder weather. The collected and frozen pollen keeps its ability to germinate for years.

The germinating pollen tube, after penetrating the stigma, goes towards the ovary through the style. The pollen tube end contains the sperm cells, while the other tube parts develop callose plugs. The energy required for pollen tube growth is provided by carbohydrates present in the style. In the same style, many pollen tubes may develop in the same time, but only some of them reach the ovule. The growth rate of pollen tubes increases at higher temperatures.

10.6. Effective pollination period

Ovules and embryio sacs have a limited life span, this is an important factor of successful fertilization. When pollen grains get to the stigma surface, the pollen tube starts to grow through the style towards the ovule. Pollen tube growth requires time, and the ovule must still be alive when the pollen tube reaches it. Williams introduced the concept of “effective pollination period” (EPP). This is defined as the time of ovule longevity minus the time pollen tubes need to grow from stigma to the ovule. A quicker pollen tube growth and a longer ovule longevity increases the time of EPP. Higher temperatures speed up ovule senescence, but fastens pollen tube growth as well, so the length of EPP is highly determined by weather. Rootstock, nitrogen supply and other environmental factors also effect ovule longevity. The time when pollen gets to the stigma is a highly important element of effective pollination. Flowers that are pollinated at flower opening or shortly after it, are highly probable to set fruit. Differences in EPP can occur between fruit cultivars in the same year.

Growers can ensure the adequate EPP length by suitable cultivation methods. In-time and ample thinning, adequate nutrient provision (especially nitrogen in the period of bud differentiation), improving the orchard microclimate with windbreaks can beneficially influence the time of EPP. To have the adequate pollen supply at flower opening, a suitable pollenizer has to be provided for.

10.7. Fertilization features

The majority of pome and stone fruit cultivars are self-sterile (self incompatible), cannot fertilize itself with its own pollen, this is nature’s protection against inbreeding. Self-sterility is the case, when a fruit cultivar (clone), planted alone, cannot produce a suitable yield with germinable seeds, or when self fertility is hindered because of dichogamy. Self-sterile cultivars can only produce fruit, if their flowers are pollinated by the pollen of another cultivar: they need a pollenizer.

Self-fertile cultivars can be fertilized by their own pollen. Self-fertile is a fruit cultivar (or clone), if, planted alone, it can produce a yield suitable for economical cultivation as a result of pollination with its own pollen, and fruits produce germinable seeds. This own pollen can originate from the same flower (autogamy), from the same tree (geitonogamy) or from another tree of the same cultivar (allogamy). Self-fertile cultivars need a pollenizer, if the quantity or quality of fruits from self-fertilization is not sufficient.

Among fruit species, the rate of self-sterile cultivars is variable. The majority of almond cultivars are self-sterile, while almost all peach cultivars are self-fertile. In plum, totally self-sterile (e.g. ‘Cacanska rana’), partial self-fertile (‘Cacanska lepotica’) and self-fertile cultivars (‘Besztercei’) all exist. In order to estimate the optimum number and rate of pollenizers for a cultivar, we have to be aware of the degree of its self compatibility.

Table 10.1. shows fruit species according to their compatibility needs. On the base of this it can be stated, that the majority of fruit cultivars needs pollenizer(s). The creation of pure cultivar orchards is recommended only in case of peach cultivars and some apricot and plum cultivars.

Table 10.1.: Classification of fruit species according to compatibility needs of their cultivated varieties (Tóth, 2009)

The reason of incompatibility (self and cross incompatibility) during self and cross pollination is genetically based: regulated by the genes of the so called S-locus. These genes have a number of alleles, the S-allele variants (S-genotype) of different cultivars are responsible for the incompatibility between cultivars.

10.8. Types of incompatibility

Sporophytic (self)incompatibility (SSI):

It occurs on the stigma to be pollinated: the pollen is not able to grow a tube or to penetrate the stigma. The interaction between stigma papillae and pollen (or pollen tube) is rapid, and as a consequence, callose is deposited, which inhibits pollen tube growth. Among fruit crops, hazelnut has such a SSI system, which means that there are no self-fertile cultivars, and mutual cross incompatibility is also common between varieties.

Gametophytic (self)-incompatibility (GSI):

A type of incompatibility common in fruit crops of the Rosaceae family. The inhibition occurs in the style: the pollen tube cannot grow through the style and reach the ovary. The cultivar’s own pollen or a pollen with the same S-allele as one of the stigma S-alleles, is not able to fertilize. This means that cultivars with the same S-genotype cannot mutually fertilize each other. The two components of the S-gene (stylar component and pollen component) produce proteins (RNases and F-box proteins), whose interaction generates the inhibition in the style. A callose deposition can be observed at the end of incompatible pollen tubes.

10.9. Self-fertility

In sweet cherry, self-fertile cultivars carry a mutant S-allele, which can fertilize its own flowers and the flowers of other cultivars as well (universal pollenizers). Mutual cross incompatibility is frequent between cultivars. There are self-compatible cultivars also in Japanese pear (e.g. ‘Osa-Nijisseiki’), based on a mutation of an S-allele.

In apple, there are only a few self-fertile cultivars, flowers pollinated by own pollen often develop parthenocarp fruits. A self-fertile variant of ‘Cox Orange Pippin’, the apple cultivar popular in England is ‘Queen Cox’, produced by mutation breeding.

The expression of S-alleles is influenced by flower age. In apple and in Japanese pear, the expression of alleles is weaker in very young and very old flowers, therefore if pollinated at these stages with own pollen, which is in principle incompatible, flowers set fruit though. In Japanese pear it was found, that the activity of RNases coded by some S-alleles differs with the alleles. Therefore it is possible, that the self-compatibility level of cultivars with different S-genotypes varies.

A major aim of breeding is to obtain more self-compatible cultivars. These cultivars have a safer fruit set than self-sterile varieties. They can be planted into single-cultivar blocks, but as a disadvantage, they can overset, which means they often require thinning. Therefore it is better to choose cultivars with a moderate self-fertility, or which are self-sterile but have a tendency to parthenocarpy. Own pollen has a larger chance in pollination compared to other pollen in those flowers, where anthers and stigma are situated closely to each other.

10.10. Cross compatibility of cultivars

If two cultivars are able to fertilize each other, the relationship is compatible, if not, the relationship is incompatible. Cross incompatibility occurs between two cultivars, if their S-alleles (S-genotype) are the same. When one of the S-alleles of two diploid fruit cultivars is the same, then the relationship is partly compatible. Fruit set is usually weaker in this case, than if the combination were totally compatible. In diploid species (e.g. sweet cherry), incompatibility between cultivars can be calculated. However, for example in hexaploid European plums, it is hard to find cultivars with totally the same S-genotype, therefore cross incompatibility is rare among them. Unilateral incompatibility has been reported in apple, pear and sweet cherry cultivars, this phenomenon may be caused by the different expression of each S-alleles.

10.11. Fruit set

Not all of pollinated flowers will set fruit. Fruit set is characterized by the percentage of matured fruits compared to pollinated flowers. In case of some fruit species or even cultivars, adequate yield requires different rates of fruit set (Table 10.2.).

Table 10.2.: Average yield of fruit species and required optimal fruit set values (according to Soltész, 1996)

Generally, in case of species (e.g. apple, pear) and cultivars (e.g. ‘Mutsu’) with larger fruits, a smaller setting rate is sufficient for the suitable yield, while at species (e.g. sour cherry) and cultivars (e.g. ‘Jonathan’) with smaller fruits, a larger rate of flowers have to set. Cultivars with a higher flower density have a sufficient yield even with a smaller setting rate. There are cultivars with a high fruit setting rate (fructification tendency), excessive fruit set is harmful in this case: it leads to the production of small fruits (e.g. ‘Alex’ sweet cherry cultivar).

Some fruit species have a tendency to set fruit without regular fertilization: by parthenocarpy or apomixis. Parthenocarpy is common in some pear and apple cultivars. Parthenocarpic fruit development can be induced in pome fruits by applying gibberellin at the beginning of flowering. Gibberellin promotes cell division and growth, therefore fruit size will be bigger, their form will be elongated. It is frequently used in modern apple orchards because of its effects. Gibberellin preparations induce flower initiation, so they are suitable for replacing frost damaged flowers and can induce a second flowering as well.

Apomixis is common for example in walnut and in citrus fruits. Types of apomixis are apospory and adventitious embryony. The phenomenon of apospory can be observed in apple, where the embryo develops from a cell of the embryo sac. In adventitious embryony, seeds develop from individual cells of the nucellus or the integuments, this is common in citrus fruits.

10.12. Pollination features

It was mentioned during the BSc course, that in Rosaceous fruit species pollen transfer from flowers to flowers is carried out by insects. Walnut and hazelnut are wind pollinated. Pollen from the male flowers of chestnut transfers to the air by the activity of nectar collecting insects, then it gets to female flowers by wind.

The flower-visits of insects depends on the nectar and pollen production of the flower. Nectar is produced in so-called nectaries, which are situated between anther base and ovary in Rosaceous fruits. The attractivity of different fruit species to honeybees depends on the quantity and nutrition values (mainly sugar) of the nectar produced by flowers, as well as on the efficiency of collection. Flowers’ nectar production is wrong in cold or dry weather. The sugar content of nectar is the most important factor to attract bees. Among temperate zone fruits, sour cherry nectar has the highest sugar content, then apple, plum and peach follows. Sugar content of apricot nectar is low, while pear nectaries produce the less sugar. It is not surprising, that pear flowers are not attractive for honeybees.

The major important pollinator species of fruits is honeybee, but among other Apoidea species, bumblebee is a frequent nectar collector as well. Nectar collecting bees stand on the stamens and extend their tongue towards the nectar, during this pollen gets to the stigma from their body. Other bees land on the petals forcing their tongue between petals and anthers towards the nectar, these are called side workers. They do not contribute to pollination, because do not have a direct contact to the stigma. Side workers are frequent in cultivars with long filaments (e.g. ‘Red Delicious’). Pollen gatherers have the best results in pollination, because they work standing on stamens and regularly touch the stigma. A flower attracts pollen gatherer bees first in its life, then nectar gatherers follow.

Bees often show high fidelity to a fruit cultivar which is attractive for them. Therefore cultivars to be pollinated by each other have to be equally attractive for bees. It can happen, that bees fly more likely to other plants. These competitor plants (such as dandelion, rape) have to be avoided in and near the orchard. If pollenizer and the cultivar to be pollinated are inside the foraging range of the hives, pollen exchange can take place in the hive as well.

Cold weather, rain, strong wind reduces bee visits, thus hindering the success of pollination. The number of bee colonies per hectare suitable for successful pollination depends on cultivar and habitat. Generally, 2-5 bee colonies per ha is needed for pome fruits and even 8 colonies for stone fruits.

10.13. Aspects of pollenizer selection

Beside the fact, that the flowering time of a suitable pollenizer overlaps with that of the main cultivar and its pollen is compatible, it has other beneficial characteristics as well. The suitable pollenizer has a long flowering period. Its flowering time is fixed in the cultivars’ flowering order - has only slight fluctuation between years. It has a large flower density, the flowers contain a large amount of viable pollen.

Its habitat need should be the same as that of the main cultivar. It should have a sufficient commercial value. Agro- and phytotechnical works, plant protection, harvest needed for the pollenizer should be coordinated with works required for other cultivars in the orchard.

10.14. Number and rate of cultivars

It was observed in fruit cultivars requiring cross pollination, that a bigger distance from pollenizer involves a worse fruit set. Pollenizer rate is influenced by the yield of the main cultivar and the commercial value of the pollenizer as well. If there are to many pollenizers, overset can result in too small fruits and alternancy can occur in fruit production of trees. It was observed in apple orchards, that fruit of trees near pollenizers become small, while fruit of trees situated far away remain small because less seed develops in them.

It is advisable to create orchards where pollenizer is also a market variety. This is possible in case of most fruit species owing to the wide cultivar assortment. Thus the rate of cultivars depends on the grower’s intention. In case of insect pollinated fruit species, the rate of pollenizer should be at least 15-20 %, while at wind pollinated species (walnut, hazelnut, chestnut) it should be 2-15 % depending on the species. If we intend to plant one main self-sterile cultivar, pollenizers can be cultivars with less commercial value, with a rate of 5-10 %.

In commercial fruit production, generally a small number of cultivars is preferred, planted in one block if possible, thus facilitating agro- and phytotechnical works. However, the less cultivars are planted, with a bigger distance between them, the successful pollination is less ensured. Due to reasons of orchard management, it is advisable to plant 2-3 cultivars with the same ripening time, or 3 or more cultivars constituting a ripening order.

The recommended number of fruit cultivars to be planted in orchards is shown in Table 10.3. Triploid cultivars are not suitable pollenizers because a triploid cultivar needs two diploid cultivars.

Table 10.3.: Recommended number of cultivars for joint planting

10.15. Arrangement of rows and trees

Ideally, the rows of an orchard lie in a north-south direction. These rows have a better light distribution compared to east-west rows, therefore fruit quality is more consistent. Larger yields were observed in apple and pear orchards with north-south orientation. In east-west rows, southern side of trunks heats up better, causing bigger frost cracks because of the high temperature difference. In rows exposed like this, flower buds on the southern side are more developed, have a larger chance of frost damage.

A higher plant density per hectare increases the yield until a certain limit. However, it is not all the same, what arrangement do we use to reach a high plant density. A strong decrease of distance between trees can lead to shading of trees, thus to less and weaker quality yield. This way the rate of the distances between rows and between trees is 3:1. If the distance between rows is decreased and the distance between trees is increased (rate 2:1), light distribution will be better and no yield loss has to be calculated.

There are examples of arrangements, where rows are close to each other, there is only a 0.5 m lane between neighbouring trees (counted from canopy border) for management works. In these orchards, plant management is hard to carry out, cannot be solved with today’s rootstocks and machines.

Safe pollination is another important aspect, thus cultivar arrangement is subordinated mostly to it. The distance from pollenizer is important. The ideal distance is influenced by tree size and cultivar productivity as well.

After determining the distance between rows, and being aware of the maximal distance from the pollenizer, we can conclude the maximal number of rows in blocks of the same cultivar at different fruit species. Arrangement of cultivars in orchards is shown in Table 10.4.

Table 10.4.: Most common cultivar arrangement versions (according to Soltész, 2002)

Literature:

 Brózik, S., Kállay, T-né (2000): Csonthéjas gyümölcsfajták. (Stonefruit cultivars) Mezőgazda Kiadó, Budapest.

Nyéki J. (1980): Gyümölcsfajták virágzásbiológiája és termékenyülése. (Floral biology and fertilization of fruit species) Mezőgazdasági Kiadó, Budapest.

Nyéki, J., Soltész, M. (1996): Floral biology of temperate zone fruit trees and small fruits. Akadémiai Kiadó, Budapest.

Nyéki, J., Soltész, M., Szabó, Z. (2002): Fajtatársítás a gyümölcsültetvényekben. (Cultivar assiciation in fruit orchards) Mezőgazda Kiadó, Budapest.

Soltész, M. (1997): Integrált gyümölcstermesztés. (Integrated fruit production) Mezőgazda Kiadó, Budapest.

Tóth, M. (1997): Gyümölcsészet. (Pomology) Primom Kiadó, Nyíregyháza.

Tóth, M. (2009): Gyümölcsfaj- és fajtaismeret. (Fruit species and cultivars) Egyetemi jegyzet. BCE Gyümölcstermő Növények Tanszék, Budapest.

Wertheim, S.J., Schmidt, H. (2005): Flowering, pollination and fruit set. In: Tromp, J., Webster, A.D., Wertheim, S.J. (szerk.): Fundamentals of Temperate Zone Tree Fruit Production. Backhuys Publishers, Leiden, p 216-239.

Control questions:

1. What is the difference between pome and stone fruit species regarding the number, location and viability of ovules?

2. What flower characteristics indicate good quality?

3. What do you know about pollen producing ability of triploid fruit cultivars? Indicate some triploid cultivars!

4. How can we determine the overlap of two cultivar’s flowering time? What degree of overlap is required for a safe pollination?

5. Define the concept of effective pollination period!

6. How can the length of effective pollination period be influenced?

7. What forms of incompatibility do you know in fruit species? Describe the genetic background of their formation!

8. Describe beneficial and disadvantageous characteristics of self-fertile cultivars!

9. Why cultivars with a tendency to bear parthenocarpic fruits have significance? What fruit species have a tendency for natural parthenocarpy?

10. How artificial parthenocarpy is induced in apple? What other effects does the applied substance have?

11. What is the role of nectary? Where is it situated in the flower of fruit species?

12. What flower characteristics are attractive for bees?

13. What are the criteria of an ideal pollenizer cultivar?

14. What are the main principles of row and tree arrangement in fruit orchards?

15. What are the most common cultivar arrangement versions which facilitate pollination?