Vegetative recovery and mobilization of reserves
With the arrival of spring and the overcoming of winter dormancy, the olive tree resumes its annual cycle. When growth resumes, the plant mobilizes the energy reserves accumulated in its woody organs (branches, trunk, and roots) in the form of starch, derived from the conversion of solar energy into chemical energy through photosynthesis the previous year.
As temperatures and metabolic demand increase, starch reserves are hydrolyzed by alpha- and beta-amylase enzymes, releasing soluble sugars consisting primarily of sucrose and mannitol. These compounds, through the phloem, are translocated to the emerging buds, supporting the formation and development of shoots until the new canopy reaches full photosynthetic autonomy.
During the initial phenological phase, vegetative buds generally open before floral buds. This phenomenon, known as heterochrony between budding and bud break, reflects the plant's metabolic priority in restoring its foliage. Vegetative buds, being structurally simpler and physiologically less demanding, require fewer energy resources and hormonal stimuli than floral buds, favoring their earlier emergence. In this way, the plant effectively mobilizes energy reserves, preparing for the subsequent, more costly flowering phase.
Flower morphology
The pinking marks the conclusion of the processes of floral induction (biochemical) and differentiation (anatomical), not visible to the naked eye, and corresponds to the development of the structures that will give rise to the flowers, gathered in paniculate inflorescences (panicles), commonly called pinkies.
The olive tree (Olea europaea) is considered an andromonoecious species, as it produces both hermaphroditic (perfect) flowers and staminiferous (male) flowers on the same plant, in which the pistil aborts at different stages of development (Cuevas & Polito, 2004).
Hermaphroditic flowers possess both sexual organs: the pistil (gynoecium), which constitutes the female organ, and two stamens, which represent the male organ. The pistil comprises a broad, rounded basal portion called the ovary, the style (a short extension of the ovary), and the stigma, located at the apex. It is bifid and rich in papillae, which increase the contact surface to accommodate pollen and promote its germination.
The stamen (androecium) is made up of the filament that supports the anther, which is responsible for producing pollen grains and releasing them when it opens (longitudinal dehiscence).
Pollen is formed in pollen sacs (usually four), located inside the two thecae (or lobes) that make up the typically bilobed anther. The pollen sacs (microsporangia) are lined internally by the tapetum, a specialized tissue essential for nourishing the developing pollen grains and for the synthesis of the pollen wall.
Pollen formation (microsporogenesis)
About a month before the flower opens, the pollen mother cells (microsporocytes) are found inside the anthers. These are diploid, with a chromosomal set characteristic of the species: 2n = 46 chromosomes (Cruz et al., 2016), organised into 23 homologous pairs, each composed of one chromosome of paternal origin and one of maternal origin.
Each microsporocyte undergoes meiosis, a particular reductional cell division typical of germ line cells.
Through this process, which begins approximately 20–25 days before the opening of the flower (anthesis), four haploid daughter cells, called microspores, are generated, each with n = 23 single chromosomes, i.e. half the number of chromosomes of the mother cell.
Initially, these four cells remain united, forming a structure called a microspore tetrad, surrounded by a protective wall of callose.
Meiosis, essential for sexual reproduction, allows the number of chromosomes to remain constant across generations. Indeed, if the chromosome complement were not halved during this meiotic process, the number of chromosomes would double at each fertilization, with the fusion of male and female gametes.
Furthermore, during prophase I of meiosis, a genetic mixing called crossing-over occurs, which increases genetic variability and contributes to the adaptation and survival of the species.
For this reason, the olive tree prefers cross-pollination between different specimens of the same species, which is considered predominantly allogamous.
The callose wall that surrounds the tetrad performs a temporary protective function: it isolates the four newly formed microspores and prevents them from fusion.
Subsequently, thanks to the action of callase enzymes produced by the tapetum, the callose is degraded, allowing the release of individual microspores into the pollen sac.
Microgametogenesi
Each haploid microspore undergoes a first division through asymmetric mitosis, generating two cells: a larger one, called the vegetative cell, and a smaller one, called the generative cell, enclosed in the pollen wall (sporoderm), made up of intine on the inside and exine on the outside.
During this maturation phase, the tapetum undergoes programmed cell death (PCD), releasing the “pollen coat” onto the surface of the pollen grains, a yellowish, viscous substance consisting mainly of lipids, carotenoids, phenolic compounds, proteins and carbohydrates.
The pollen coat, which covers the exine, performs several fundamental biological functions: it protects the pollen grain from dehydration, UV radiation and pathogens; it also promotes the processes of mutual recognition between pollen and stigma, facilitates the hydration of the grain and the initiation of pollen tube germination (Pacini & Hesse, 2005).
The exine, composed of sporopollenin, one of the most resistant biological substances in nature, presents germinal openings (coli and pores) through which the exine of the vegetative cell extrudes giving rise to the pollen tube.
During pollen germination on the stigma and the elongation of the pollen tube towards the ovule, the generative cell undergoes a second mitotic division, giving rise to the two haploid male gametes.
Olive pollen is tricolporate, characterized by three longitudinal grooves (ectoapertures), called colpi, arranged symmetrically along the equatorial axis of the pollen grain, each with an internal pore (endoaperture). The grains, spheroidal to subprolate in shape, remain medium-small in size (approximately 20–22 µm), and may vary slightly between cultivars.
The tricolporate structure, associated with a reticulated exine, makes the granules light and resistant, protecting them from dehydration and facilitating their anemophilous transport.
Germination, fertilization and incompatibility
Once it reaches the stigma, compatible pollen germinates thanks to the hydration and nutrients provided by the stigmatic exudates (Rejón et al., 2016).
The pollen tube, penetrating along the style, transports the two sperm cells to the ovule. Through the micropyle it reaches the embryo sac, where double fertilization occurs: one sperm cell fuses with the egg cell forming the diploid zygote (2n), while the other unites with the two polar nuclei (n+n) of the central cell, giving rise to the triploid endosperm (3n), a nutritive tissue that will support the development of the embryo.
In many Olea europaea cultivars, fertilization is regulated by diallelic self-incompatibility (DSI) mechanisms, which inhibit germination or pollen tube development on stigmas of the same genotype or genotypes belonging to the same incompatibility group. Recent studies have highlighted a diallelic system according to which cultivars are mainly classified into two genetic groups, G1 and G2, in which cultivars belonging to the same group generally do not fertilize each other (Mariotti et al., 2020, 2021).
Although some cultivars exhibit partial self-fertility (pseudo-self-compatibility; Alagna et al., 2019), the olive tree is predominantly allogamous. The presence of compatible pollinating cultivars, belonging to different groups (G1 and G2), is essential for optimal fruit set. However, for pollination to be effective, cultivars must flower simultaneously and the pollen must be viable and capable of germinating, essential conditions for ensuring regular fruit set.
Final note: Recent studies indicate that the olive tree's DSI system may be more
complex of the simple diallelic model, showing some similar characteristics to a
unconventional homomorphic sporophytic diallelic system
system”) (Cuevas et al., 2024).
Bibliography:
- Cuevas, J., Polito, V. S. (2004). Self- and cross-incompatibility in olives (Olea europaea L.):
Pollen–pistil interactions and fruit set. Annals of Botany, 93(2), 257–264. - Cuevas, J., Chiamolera, FM, Pinillos, V., Rodríguez, F., Salinas, I., Cabello, D., Arbeiter,
A. B., Bandelj, D., Božiković, M. R., & Selak, G. V. (2024). Backcrossing failure between
'Sikitita' olive and its male parent 'Arbequina': Implications for the self-incompatibility system
and pollination designs of olive orchards. Plants, 13, 2872. - Mariotti, R., Fontanella, F., Baldoni, L. (2020). Diallelic self-incompatibility in olive: Group
assignment and implications for orchard management. Plants, 9(8), 1010. - Mariotti, R., Fontanella, F., Baldoni, L. (2021). Pollen–pistil interactions and compatibility
groups in olive cultivars: Advances in understanding DSI. Horticulturae, 7(4), 93. - Pacini, E., Hesse, M. (2005). Pollenkitt – Its composition, forms and functions. Plant
Systematics and Evolution, 250(1-4), 1-14. - Rejón, J.D., Delalande, F., Schaeffer-Reiss, C., Alché, J.D., Rodríguez-García, M.I., Van
Dorsselaer, A., Castro, A. J. (2016). The pollen coat proteome: At the cutting edge of plant
reproduction. Proteomes, 4(1), 5.
Photo credits:
Photos 2 and 3: Dr. Matteo Zucchini, present as Figure 4 in his PhD Thesis “Growth control and productive aptitude of Olive tree (Olea europaea L.)”, Polytechnic University of Marche, Academic year 2022-2023
Photo 4: Olea – Treatise on olive growing”, edited by Piero Fiorino, Edagricole, 2020, Fig. 3.7)
Photo 5: Olea – Treatise on olive growing, edited by Piero Fiorino, Edagricole, 2020, Fig. 3.7.
Photo 6: J. Cuevas in “Why Olive Produces Many More Flowers Than Fruit?”, Posted Date: 19 November 2024, doi: 10.
