The beginning of this stage in the annual vegetative cycle of the vine. is about six to eight weeks after budding. The exact point in time depends on the year-specific temperature development, the temperature totals achieved at the respective location and the vine-specific growth under changing weather conditions. Constant warm temperatures with a good water supply accelerate the speed of shoot growth and thus the formation of the inflorescence (Latin: Inflorescence, also flowering, umbel, droplets or glow per country and region) on the third to seventh shoot node. Depending on the zonal climate, microclimate and solar radiation, flowering takes place from mid-May to late July in the northern hemisphere (in Central Europe, according to the old winemakers' rule, flowering begins on 24 June = St. John's Day, plus or minus eight days), in the southern hemisphere from November to mid-December. In contrast to many other flowering plants, the individual flowers of the vine are small, greenish-yellow and rather inconspicuous due to the absence of petals and show flowers. As the fruit ripens, the individual berries of the grape develop from the individual flowers.
In the pre-flowering stage, the longitudinal axis of the inflorescence first stretches out, then the lateral branches with the still closely spaced, closed individual flowers spread out. As they develop, the individual flowers detach from the flower cluster, swell and change colour to a greenish yellow shortly before the actual start of flowering. Flowering begins when the first petals (perianthium), which have grown together to form a flower cap, detach from the flower base in dry, warm weather. In low humidity the flower caps are shed by means of an ingenious jumping mechanism, so that the honey-scented stigmas are exposed and the stamens can spread.
The individual flowers of the cultivated vine are hermaphroditic or bi-sexual in most grape varieties. This means that in a single flower they contain both the stamens with the male gametes in the pollen grains (= pollen) and the ovary with the female eggs and the stigma to be pollinated (see also detailed information under flower bud). The stage of full flowering is reached when about half of the flower caps have been shed. However, the individual blooms on the flower heads do not flower simultaneously, but in a slightly delayed sequence, just as the individual flower heads of the vine do not start flowering simultaneously, but in a delayed sequence, depending on exposure (sunlight) and position on the fruit shoot.
The total flowering period is normally about eight days, but it can last up to three weeks in bad, cold and wet weather. With increasing global warming or climate change, a trend towards earlier flowering dates with shorter flowering periods can be observed. The number and size of the buds as well as the number of individual flowers were already created one year earlier when the winter bud was formed (see under initiation). On a fruit shoot there are usually one to three (up to a maximum of five) inflorescences in the form of a so-called panicle (actually the term grape is not correct for the inflorescence of the vine). After flowering, pollination and fertilization, the ovary develops into a berry and the appearance into a grape. Each grape has an average of about 150 berries. However, it depends on the fertilisation success whether a berry actually ripens from the maternal tissue of the ovary. In bad weather, the process of pollination suffers and the unfertilized flowers trickle (fall off).
There are also varieties with functional female flowers, but where the stamens have been reduced or are infertile. This is often the case with table grapes. However, these are inevitably dependent on cross-pollination, which is why their yield varies greatly depending on flowering conditions and fertilisation success. In seedless varieties such as Korinthiaki or Sultana, the female reproductive organs are infertile. Without the seed set that stimulates fruit growth, the berries remain small and wither, but they are suitable for the production of seedless dried berries. The size growth of these seedless berries can be artificially induced by growth hormones(gibberellins).
In contrast, wild vines (Vitis vinifera sylvestris) are usually dioecious, i.e. each vine carries either only male or only female sexual organs. In the vine, both organs are present as an attachment, but only one organ is functional, while the other is hormonally suppressed. In dioecious plants, self-pollination is therefore not possible and the female plants can only be fertilized by cross-pollination with pollen from a male vine. The forced recombination of paternal and maternal plant genetic make-up preserves the genetic diversity in a wild population and thus facilitates adaptation to environmental changes(heterozygosity). In viticulture, however, this is not desirable because of the uncertain yields. However, many of the varieties used as rootstocks are hybrids of wild vines and thus dioecious. In the breeding of rootstocks, varieties with male flowers are preferred, which cause less effort because of the missing grapes.
Self-insemination (self-pollination) is usually avoided by nature, since only cross-pollination ensures survival. In hermaphroditic flowers, the female scars are self-sterile for their own male pollen by chemical, physiological and mechanical mechanisms. However, as mentioned, there are other mechanisms in nature. For in wild vines, self-insemination is simply prevented by dioeciousness or the separated sexes. In cultivated plants, these self-sterility mechanisms have often been bred away completely or partially. During fruit and seed formation, the grape skeleton, berry skin and flesh, as well as the seed shells with the supply tissue for the embryo are formed by the mother vine. Only the embryo in the grape seed is the result of sexual fertilization, the grape is unaffected. Riesling remains Riesling, even when fertilized by Silvaner pollen.
All vine species with the same DNA structure can be crossed with each other. These are all European (Vitis vinifera), but also most American and Asian varieties. This means that most varieties are not only able to cross-pollinate with each other (which a priori does not mean reproduction), but can also be crossed with each other, thus creating new varieties. When the seeds reach germination, the resulting seedlings would be spontaneous crosses between two parent plants. A variety with new characteristics different from the parent variety is most likely to be created when cross-pollination by another variety has occurred. In this way, over thousands of years, countless varieties of grapevine have spontaneously developed naturally (see four examples in the picture). However, the genetic material of the mother and father plants are also recombined when fertilisation has taken place within two vines of the same variety. However, their genetic differences are small because the vines in the vineyard are mostly clones by vegetative propagation.
The cultivated vine is mostly self-pollinating, primarily within the hermaphrodite flower (autogamous) or between two flowers of the same clone, which is understood as self-pollination in the narrower sense. Self-pollination in a broader sense also applies between two flowers of the same vine (geitonogamous). However, hermaphroditic plants of one vine variety can be pollinated not only with the pollen grains of plants of the same variety, but also of other varieties, and thus successfully cross-pollinated (xenogamous) without impairing the development of the flower into a berry. However, the germination capacity of the self-pollinated seeds (vine seeds) may be impaired. As a rule, cross-pollinated varieties are more fertile and vital than those from self-pollination. This is irrelevant, however, because in viticulture the grapes are intended for pressing and their seeds are not intended for sowing. Any pollen is sufficient for berry formation.
The wind is also involved to a certain extent in pollen transfer, and the seeds can fly several hundred metres. Insects attracted by the scent of flowers, such as bees, also transfer pollen from one flower to another. In principle, however, the cultivated grapevine does not have to be cross-pollinated and is therefore not dependent on the presence of fertilizing insects, as is the case with many other plants and dioecious wild grapevines. This is a great advantage and decisive for yield security. No matter from where the pollen grains from the stamens reach the sticky scar, they remain there (pollination), germinate and form a pollen tube which grows into the scar and the stylus to fertilise the (only) egg cell in the ovary (fertilisation). From this fertilized egg cell (zygote), the so-called meiosis (reduction division and recombination division) to five (seldom also six) genetically different grape seeds are later produced in the berry. The genetic difference of the nuclei results from a new composition of the parental chromosomes.
In vines grown from vine seeds without human intervention, the paternal pollen donor is not known a priori. Such spontaneously developed varieties are called "open pollinated" (O.P.), in contrast to deliberate new breeding by humans. After all, fertilization ultimately results in a single berry from each individual flower. This means that a grape with, for example, 150 berries could (theoretically) have 150 different fathers (whereby a foreign vine is to be understood as the father). In this case, each berry of this grape would have the same mother, but the grape would have 150 different fathers (pollen donors). The pollen may come from the vine's own flower, from a flower of the same flower, from a flower of another flower of the same vine, from a neighbouring vine of the same variety or from vines of other varieties in neighbouring vineyards.
The maternal tissue of the ovary then grows into a berry with five to six seeds. As mentioned above, these could produce different varieties when growing into seedlings. From crosses between Cabernet Sauvignon x Bronner, three quite different grape varieties were created in 1983 in Freiburg(Baron, Cabernet Carbon, Souvignier Gris). The three underlying pips could theoretically all come from the same berry. If bad weather conditions such as cold, rain, wind or hail prevail during the blossoming of the vines, the fertilization process is impaired. Rain, wind and high humidity hinder pollen release and flight, but also have the effect that the bouncing mechanism of the caps does not function or functions only to a limited extent and they are not bounced off, but remain loosely attached to the scars and stick there. This means that pollination can no longer take place. Cold hinders the growth of the pollen tubes. Therefore, never all spermatozoa are completely fertilized.
However, since the embryo in the seed stimulates the formation of fruit, the insufficiently fertilized berries remain smaller. The consequence is parthenocarpia (or stenospermocarpia = apparent virginity) or early trickling (falling off) of the completely unfertilized flowers. The grape then has fewer and smaller berries. However, this need not be a disadvantage, because a reduced yield can increase the quality. This is because the ingredients produced by the grapevine are now concentrated on a smaller number of berries, and this can result in more extractives, higher sugar content and overall better wine quality. The berries reach pea size just a few weeks after blossoming. This is already part of the next stage of development, the fruit set.
Regarding the process of fertilization, see also under flower bud and hermaphrodite flowering. A list of keywords relevant to grape varieties is included under grapevine. All aids, work and measures in the vineyard during the vegetation cycle can be found under the keyword vineyard care.