Fructification

In a botanical sense, the fruits that are harvested and that we consume are fertilized ovaries (from the flower in angiosperms). The fruits give rise to and protect the seeds until the moment of maturity and dispersion, which is when the seed, by some means, leaves the fruit and is viable to perpetuate the species through a new generation.

Gymnosperm plants (plants without flowers) are characterized by having “naked” seminal primordia (this is the case of conifers, for example); in angiosperms (flowering plants), however, the seed primordia are always enclosed in the ovary (receptacle formed by the carpelar leaves), from which they will not emerge until they reach maturity. Once the seeds are formed, the ovary (alone or together with other parts of the flower) develops and becomes a fruit.

Properly, in order for the fruit as such, and the seeds, to exist, there must be a previous step, which is pollination; which is the movement and arrival of pollen to the female organs of the flower. In a large percentage of species, this occurs successfully through the wind, known as wind pollination. With the evolution of the co-evolution of species, a pollination aided by the activity of animals, mainly insects, known as zoophily, has developed, although there is also that caused by drops of water, called hydrophily.

In angiosperms, when the pollen grain is deposited on the stigma of the pistils, its germination begins. The exine (outer layer of the pollen grain) opens, and the vegetative cell of the pollen tube begins its formation by progressively lengthening the intine (inner layer). The pollen tube grows along the style, through its tissues, from which it is nourished, until it reaches the micropyle of the seminal primordia. It then opens and releases its two sperm cells; one penetrates to the egg cell, merging its protoplasts (plasmogamy) and its nuclei (karyogamy), and the other fuses with the secondary nucleus of the embryo sac. This gives rise to a diploid zygote in the egg cell and a triploid endospermic nucleus in the embryo sac (Azcón-Bieto and Talón, 2008).

Pollination is affected by environmental characteristics, especially temperature and relative humidity. The effect of temperature can be indirect, by altering the activity of bees (the main pollinators of fruit trees), or direct, by affecting the growth of the pollen tube. The efficiency of bees in transporting pollen is maximum with an average temperature of 20-22 °C, and practically null when it is lower than 12 °C (Azcón-Bieto and Talón, 2008).

After fertilization or parthenocarpic stimulation of the ovary, it begins its development until it becomes a mature fruit. This transition takes place in successive phases, with well-defined characteristics, but variable in duration according to environmental conditions, species and varieties.

The accumulated growth of a fruit follows a sigmoid curve or, in some cases, double sigmoid. Three phases are distinguished in it: an initial period characterized by cell division (phase I), followed by a period of cell elongation (phase II), and a final period in which the fruit practically stops growing and matures (phase I). III). In some fruits, those with a hard endocarp (bone), the transition from phase I to phase II is characterized by presenting an intermediate state in which the growth of the fruit stops, at the same time that the endocarp lignifies; this phase without growth is what gives rise to a double sigmoid curve (Azcón-Bieto and Talón, 2008).

The growth of a fruit is mainly influenced, in most cases, by the growth of its edible part. During the 2-10 weeks following anthesis, depending on species and varieties, its growth is a consequence of cell division and intensifies over time, giving rise to an exponential curve. After reaching a maximum of activity, mitosis gradually ceases, at the same time that cell elongation and thickening become important. This new period is one of linear growth, and in it the fruit acquires, on average, up to 80% of its final size, culminating in a progressive slowdown in growth, which ceases, and the fruit changes color and matures.

The fruits of the angiosperms are those of greatest commercial interest, in this group are found the vast majority (all) of the cultivable and usable species, such as grains, fruit vegetables (and also leaf vegetables, although the marketable organ is not be the flower) such as tomato, eggplant, pumpkin, etc., and fruit trees. Under this premise, knowledge of the phenological phase of fruiting is of vital importance to understand how to give the plant the best conditions (in agriculture) to be able to carry out optimal production in terms of quantity and quality.

The fruits can be simple, when they are unique (eg orange), or compound, when they are made up of the grouping of several of them (eg strawberry). When auxiliary, extracarpellar organs play a decisive role in their formation, they are called false fruits (eg, apple). Some species are capable of forming more than one kind of fruit on the same individual (eg, some crucifers); this phenomenon is called heterocarpy (Azcón-Bieto and Talón, 2008).

The final size reached by the fruit is regulated by a set of factors of variable nature and incidence, among the internal factors of the plant that determine the final size of the fruit, genetic factors stand out, the position of the fruit in the bud and the competition between developing bodies.

A factor of great importance in determining the final size reached by the fruit is the competition between developing organs. The greater the number of these, whether flowers or fruits, the greater the competition between them, both for mineral elements and for photosynthesis products, which limits their growth possibilities and, consequently, their final size (Goldschmidt and Monselise, 1977).

In addition to the endogenous factors, other factors, external to the plant, have a marked influence in determining the final size of the fruit. These include climatic and soil conditions, as well as cultural practices, mainly irrigation and fertilization.

The accumulation of metabolites in the fruit and, therefore, its growth, are directly associated with temperature (Sinclair, 1984). The fruit becomes highly susceptible to high temperatures in some phases of its initial development (cell division period), during the ripening period the absence of fruit growth is associated with low temperatures. Temperatures below 3ºC have a significantly depressing effect on fruit growth.

Nitrogen and phosphorus whose excess causes reduction in size and loss of quality of the fruit (Chapman and Rayner, 1951; Jones et al., 1957). Potassium, however, is an exception since foliar concentrations higher than those considered optimal improve fruit size without negatively affecting its quality (Guardiola, 1980).