The olive tree bears fruit in branches formed the previous year, so the cultivation techniques are focused on modulating the harvest in quantity and quality so that it is optimize economic and environmental use of the resources used.

The olive tree is a vice versa species in which years of high production alternate with years of low or no production. The number of fruits per branch is the result of the vegetative and reproductive processes that occur throughout a biennial reproductive cycle.

Both shoot growth and fruit development are cyclical phenomena in the olive tree. Both occur annually, but while shoot growth is completed within the same year, The processes leading to fruition require two consecutive seasons.

Buds are formed in the first year in the axils of the leaves of the shoots. Their fate, vegetative or reproductive, It is determined by the level of current harvest, since this is the main factor responsible for the interannual variation in flowering. In the second year, after the winter rest, The potentially reproductive buds that have met their cold requirements differentiate into inflorescences or flowers. Pollination takes place during flowering., fertilization and fruit setting, producing a massive abscission of flowers and fruits developing in the 6–8 weeks following flowering. Fruits that remain attached to the branch continue to grow until ripening, except for drops caused by abiotic stresses, pests, and diseases (Rayo and Cuevas, 2017).

Buds are formed in the axils of the leaves. as the shoots grow and form new nodes, remaining dormant until the following spring, when they will begin to sprout after a period of winter rest where they will accumulate cold (Rubio-Valdés, 2009).

Floral induction and initiation

Floral induction, defined as the internal physiological change in the apical meristem of a bud that determines its floral nature, precedes any visible morphological change. The reproductive or vegetative destiny of each bud, that is, The acquisition or not of its reproductive potential must occur before the establishment of latency, since during this period no cell division is observed, a necessary condition for floral induction.

Different studies indicate that la floral induction It is related to the initial growth of the seed and fruit before the hardening of the stone. The inhibition of floral induction is directly related to the increase in endogenous gibberellins synthesized in the seed endosperm (Stutte and Martin, 1986). This is related to the alternate-bearing nature of the olive tree, so that, The reduction in flowering is associated with high production from the previous year, where the abundance of fruits implies a high synthesis of gibberellins in the seeds.

Most buds formed before mid-September in trees undergoing deforestation form inflorescences the following season. Those formed in October remain dormant or fall. Conversely, in trees undergoing deforestation, only the few buds formed before mid-July form inflorescences (Piedra et al., 2012). In summary, The presence of fruits during summer and autumn affects flowering the following year and suggests the inhibition of floral induction at this time.

The absence of fruit does not necessarily guarantee high flowering the following year. Only after the bud breaks dormancy does floral initiation occur, marking the beginning of reproductive morphological development. Floral initiation is, therefore, the process undergone by a bud that irreversibly gives rise to a flower or inflorescence. This process involves internal changes in the buds attributed to a hormonal balance that is affected by external factors.

The olive tree needs winter cold for the buds of a variety to overcome dormancy and sprout under conditions favorable to growth. The usual method used to characterize temperature requirements is evaluation of the number of hours with temperatures between 2,5 and 15°C, with a maximum effect for those around 12,5º C (Rallo et al., 1994).

Between 15 and 16°C, they stop accumulating cold, and higher temperatures cancel out some of the accumulated cold (De Melo-Abreu et al., 2004). Under conditions of cold nights and mild daytime temperatures, typical of the Mediterranean climate, cold requirements are naturally met during rest (Rubio-Valdés, 2009).

By contrastIn areas where there is not enough cold, flowering is very staggered., observing buds in very different phenological states or even all the buds may remain dormant or fall at the time of sprouting in extreme cases, thus not producing flowering.

En este sentido, Global warming and the foreseeable climate change may modify the behavior of the olive tree in its traditional Mediterranean regions. (Rallo and Cuevas, 2017).

Floral differentiation

The olive flowers They are grouped in paniclesThe number of flowers per panicle is very variable (12 – 20), with differences between varieties and even between years for the same variety.

The olive tree is generally a plant andromonoecious, with hermaphrodite (perfect) flowers and staminate (imperfect) flowers with non-functional rudimentary pistils.

Exceptionally, there are androecious plants, with only staminate flowers; and trimonoecious plants, with hermaphrodite, staminate, and pistillate flowers (Fernández-Escobar, 1977).

During the short period of time between floral initiation and anthesis in olive trees (2,5 to 3 months),  Certain environmental factors can influence the sexual expression of olive flowers. (Badr and Hartman, 1971):

  • The low temperatures During the period of development of the inflorescence they are favorable to the development of the pistil.
  • The high temperatures They cause rapid development of the inflorescences, tending to increase pistil abortion.
  • La water shortage or low soil moisture, increase ovarian abortion.
  • El nutritional and health status of the tree It also influences the relative proportion of the type of flowers, so that a loss of leaves before anthesis, caused by poor nutrition or by the attack of pathogens, reduces the number of perfect flowers and increases the number of imperfect ones, causing poor flower development.

Generally, when conditions are unfavorable, it is the pistil's development that fails. The reason for pistil abortion appears to lie in competition for assimilates between flowers and between flowers and actively growing shoots during the pistil's development period (Rosati et al., 2011).

The abortion pistillate It does not limit the productive capacity of the olive tree in most casesHowever, in situations where water and nutrient availability for the crop is limited and there is a high demand for assimilates (years of high flowering), the development of inflorescences, flowers, ovaries, and seed primordia (ovules) can be affected in ways that affect the fruit-bearing capacity of the olive tree. In general, cultivation techniques that promote flower growth and development (early irrigation and good nutrition) or limit competition between developing flowers (pruning or thinning), promote the fruit-bearing capacity of flowers that reach flowering (Rallo and Cuevas, 2017).

Pollination

Pollination takes place during flowering, that is, the transfer of pollen from the anther of the flower to the receptive stigma of the same or another flowerIn olive trees, this transport is generally carried out by the wind (anemophilous pollination) which can carry the pollen over distances of kilometres, although more than 95% of the pollen is deposited at a distance of less than 40 metres from the pollen source (Griggs et al., 1975). For the fertilization of the flower to take place, the pollen tube must travel the path that leads from the stigma to the interior of the embryonic sac, where fertilization will occur, giving rise to the future seed and with it, the conversion of the flower into a fruit..

Fruit set is usually the index used to measure the effectiveness of pollination. But From floral initiation to fruit set, a series of processes occur These are influenced by certain internal and external factors, whose activation will result in the final fruit set. These are as follows:

  1. Internal:

a) Causes of sterility:

It is varietal in nature. Ovarian abortion is determined by atrophy of the female sexual apparatus (see above) or by pollen-pistil self-incompatibility. That is, the pistil of a flower biochemically recognizes and rejects pollen that has the same genotype, that is, pollen from the same variety. At the same time, it selects and allows the growth of the pollen tube from those pollen grains that correspond to other varieties.

b) Influence of the variety:
  • Pollen productionIt occurs when mature anthers dehisce, a phenomenon directly influenced by climatic factors. However, the quantity of pollen produced appears to be a varietal characteristic.
  • Period of stigma receptivity. In general, it is greater than in entomophilous pollinated species (Fankel and Galun, 1977). Olive stigmas have been observed to remain receptive up to 3 or 4 days after anthesis (Bradley and Griggs, 1963). Possibly due to the flowering season of the olive tree (May-June), the period of receptivity is shortened by premature desiccation of the stigma.
  • Germination of the pollen grain. It is one of the internal factors that could most affect pollination. Pollen grains, along with seeds, are the two dispersal agents of plants. Both have a mobile phase until they find the appropriate medium for germination and growth. In the case of pollen grains, the germination medium is very specific: a receptive stigma of a compatible variety. Transport efficiency during the mobile phase can be increased by abundant production of the donor cultivar.
c) Nutritional:

El Studying the conditions of pollen germination "in vitro" is the method generally used to assess their germination capacity, since in vivo studies can be masked by the influence of other factors. Over the years, methods for studying pollen germination have been developed, starting with those based on agar and sucrose at different concentrations, for which low germination percentages were obtained. Sucrose added to the pollen culture medium is the nutrient generally used in germination studies. The addition of boron in the form of BO3H3 in a culture medium based on aqueous sucrose solutions appears to improve germination percentages.

Curdled

The tissues of the ovary, which grow regularly until anthesis, stop growing at this point if pollination has not occurred. In this case, the flower falls due to the appearance of an abscission layer. This phenomenon can be intensified by ethylene production due to flower senescence, although it may not be the only cause, since pollination itself triggers increased release of the gas (Nitsch, 1971).

On the contrary, ovary growth continues in the absence of abscission, as long as the pollen tube germinates on the stigma, without it being necessary for it to reach the embryo sac. This effect is due to the fact that pollen germination on the flower's stigma triggers a series of hormonal changes responsible for the initial growth of the fruit.

While pollination is the agent capable of causing the initial development of the fruit, the growth of the fruit itself is controlled by the developing seeds.The influence of seeds on fruit growth is regulated by hormones, mainly auxins, gibberellins and cytokinins, whose levels decrease as the seeds mature..

The Frost and fog in the early stages of development can become a limiting factor in fruit development. Fruit position, associated with nutritional conditions or the incidence of a pathogen such as Prays, also affect fruit development. Finally, differences in fruit set between varieties of the same species have been repeatedly verified, regardless of climatic factors, and two types of fruit can be found:

  • Normal. Those pollinated with pollen from another compatible variety. They are of normal size and characteristics. They contain one or two seeds.
  • Parthenocarpic: Commonly known as "zofairones", they are fruits developed without fertilization of the ovule or the result of an ovarian abortion due to pollen-stigma incompatibility. small spherical or ovoid fruits, which stop growing early. They usually fall before ripening, although some remain on the plant alongside the normal fruit.

Recommendation

From the Cultifort technical department and Cultifort R&D, we recommend carrying out pre-flowering treatments with Cultiboro Plus with the aim of improving pollen viability and pollen tube germination. This is an environmentally friendly liquid formulation of boron complexed with monoethanolamine, which contains reducing sugars, forming a complex to improve its assimilation and transport by the plant. It is also chlorine-free.

The application of Cultiboro Plus is necessary in olive trees at the beginning of vegetative activity, prior to flowering, as well as at the beginning of fruiting., since Boron is a multifunctional micronutrient that in addition to improve pollen quality and pollen tube growth, participate in the cell division and growth (hence its deficiency at the foliar and fruit level, is shown with malformations); along with Calcium is essential en the synthesis of cell walls, also synergistically improving their mobility in the plant (remember that both nutrients have reduced mobility); participates in the transport of carbohydrates and potassium, in the Nitrogen metabolism and in the formation of proteins; intervenes in the rregulation of hormone levels (necessary in the early stages of fruit development); etc.

Highlighted

Badr SA and Hartman HT, 1971. Effect of Diurnally Fluctuating vs. Constant temperatures on flower induction and sex expression in the olive (Olea europaea L.). Physiology Plant, 24: 40-45

Bradley MV and Griggs WH, 1963. Morphological evidence of compatibility in Olea europaea L. Phytomorphology, 13(2): 141-156.

De Melo-Abreu JP, Barranco D., Cordeiro AM, Tous J., Rogado BM, Villalobos F., 2004. Modelling olive flowering data using chilling for dormancy release and thermal time. Agricultural and Forest Meteorology, 125(1-2): 117-127

Fernández-Escobar R., 1977. Factors affecting pollination and fruit setting in olive trees (Olea europaea LL.).

Frankel R. and Galun E., 1977. Pollination Mechanisms, Reproduction and Plant Breeding. Springer-Publisher.

Griggs WH, Hartman HT, Bradley MV, Iwakiri BT, Whisler JE, 1975. Olives pollination in California. California Agricultural Experimental Station French Newsletter, 869.

NITSCH, JP 1.971. Perennation attraverso seeds and Other structures. Fruit Development. Plant physiology. Ed. FC Steward. Academic Press. London. 413-479 p

Piedra MA, González M., Cuevas J., 2012. Olives bud fate depends on urge training data implications for flower induction timings. Record Horticulture, 949: 237 242

Rallo L. and Cuevas J., 2017. Olive cultivation. Fruiting and production. Ed. Mundi Press. Madrid. 145-186p

Rallo L., Torreño P., Vargas JA, Alvarado J., 1994. Dormancy and alternate bearing in olive. Record Horticulture, 356: 127 136

Rosati A., Caporali S., Paoletti A., Damiani F., 2011. Pistil abortions is related to ovaries mass in olive (Olea europaea L.). Scientia Horticulture, 122(3): 399-403

Rubio-Valdés G., 2009. Growth and dormancy of olive reproductive buds (Olea europaea LL.).

Stutte G. and Martin HC, 1986. Effect of killing the seed on return Bloom of olives. Scietia Horticulture, 29: 107-113