Pollination, fertilization and fruit production in fruit tree species

Pollination is a key event for the success of fertilization in fruit tree species, and it is determined by genetic and environmental factors. Due to the genetic barrier that self-incompatibility and cross-incompatibility confer in most cases, pollen must come from cross-compatible cultivars. In any case, the presence of pollinating insects as pollen transport vectors is crucial to ensure an effective pollination. The weather can also be a limiting factor, since it affects the activity of pollinating insects and determines the availability of pollen. Once the pollen grains are deposited on the stigma surface, rapid adhesion, rehydration and germination take place. The stigma exudate, which is rich in proteins, carbohydrates and lipids, constitutes the ideal medium for pollen hydration and germination. The pollen tube emerges through the openings of its outer covering and begins to grow along the style through the intercellular spaces of the transmitting tissue toward the ovule. The progamic phase, or period of growth of the pollen tube in the pistil, is a very important stage in which specific interactions take place between pollen and pistil. The pollen tube is guided in its growth by chemical signals emitted by the ovary. At this time the pollen is nourished in a heterotrophic way at the expenses of the nutrients present in the style. In addition, at this stage, pollen tube growth is controlled by the genetic interaction known as self-incompatibility, which represents an important limitation of production, what must be taken into account to obtain a commercial yield. Although the final production of fruit tree species is undoubtedly influenced by environmental factors, it is actually determined by many other factors such as flower density, pollination efficiency, pollen-pistil interactions, flower fertility and nutritional status of the tree.