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Blossom

fioritura (I)
flor (ES)
fleur (F)
flower (GB)
flor (PO)
bloem (N)
Blütezeit beim Rebstock (I)

This stage of the grapevine 's annual vegetation cycle begins around six to eight weeks after budding. The timing depends on the year-specific temperature development, the temperatures reached at the respective location and the typical growth of the variety under changing weather conditions. Consistently warm temperatures with a good water supply accelerate the speed of shoot growth and thus the formation of the inflorescence (Latin: inflorescence, also known as flower, umbel, dome or spurge depending on the country and region) on the third to seventh shoot node.

Hermaphrodite flower

The flower bud of the cultivated grapevine is bisexual. Around 99% of all vines in the world are so-called hermaphrodite flowers, whose two sexes are united in one organ. The male sexual organs are the stamens or stamens (stamina), which consist of the stalk-shaped stamen (filament) and the yellowish anthers at the tip. The entirety of the stamens of a flower is called the androecium. An anther contains four pollen sacs in which the pollen grains with the male gametes (pollen, pollen) are formed. In the flower, five of these free-standing stamens surround the female reproductive organ in a circle.

The female reproductive organ consists of two fused carpels, which contain two compartments with ovules inside. This is why the grapevine is an angiospermous flowering plant. The organ consists of the ovary with the female ovules and the stigma to be pollinated. This makes fertilisation much easier than with unisexual plants. In more than 98% of cases, self-pollination (self-fertilisation) occurs, so that no external help or influence (such as wind, bees or other insects) is necessary.

Blüte - Graphik

The picture on the left shows a closed vine flower, the picture in the middle shows the shedding of the cap before pollination and the picture on the right shows the flower bud in full bloom shortly before pollination or fertilisation. 1 = five-lobed corolla (cap), 2 = stigma, 3 = anther, 4 = filament, 5 = pistil, 6 = ovary, 7 = nectaries (honey glands), 8 = calyx.

Ovary, pistil and stigma

The ovary merges at its tip into the pistil, which ends in the stigma. The disc-shaped stigma is a spongy tissue that secretes sticky secretions which cause the pollen to stick during the fertilisation process. The pollen can come from its own flower, a flower on the same vine, a flower on a different vine or a flower on a different vine. There are five nectar glands (honey glands) at the base of the stamens and the ovary, which are fused together to form a ring (discus or calyx). Five weakly developed sepals and five green petals that have grown together to form a cap cover the stamens and the ovary in the form of an inverted calyx. This yellow-green perianth (cap, caplet, perianthium) is shed at the beginning of flowering, after which pollination by pollen and subsequent fertilisation can take place.

Blüte - Blüten vor und nach dem Abwurf der Käppchen

The picture top left shows the closed flower buds with the caps before and the picture top right after shedding. The picture on the bottom left shows the flower before shedding and the picture on the right shows a full flower with 50% fertilised buds.

Blüte - vor dem Abwurf und Vollblüte mit 50% befruchteten

Start of flowering

Depending on the zonal climate, microclimate and solar radiation, flowering in the northern hemisphere takes place from mid-May to late July (in Central Europe, according to the old winemaker's rule, it 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 in colour and rather inconspicuous due to the lack of petals and showy flowers. The individual grape berries form from the individual flowers as the fruit ripens.

Pre-flowering

In the pre-flowering stage, the long axis of the inflorescence first stretches, then the lateral branches with the still closely packed, closed individual flowers spread out. With increasing development, the individual flowers detach from the flower cluster, swell and change colour to 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 base of the flower in dry, warm weather. When the humidity is low, the flower caps are shed by means of a cleverly devised spring mechanism so that the honey-scented stigmas are exposed and the stamens can spread. At this stage, late frost would have an extremely negative effect.

Flowering period & flowering rate

The full flowering stage is reached when around half of the flower caps have been shed. However, the individual flowers on the shoots never bloom at the same time, but in a somewhat delayed sequence. Depending on the exposure (sunlight) and position on the fruiting shoot, the individual shoots of the vine also begin to flower with a time delay.

The total flowering period is normally around eight days, but can last up to three weeks in bad, cold and wet weather. With increasing global warming and climate change, there is a trend towards earlier flowering dates and shorter flowering periods. The number and size of the flower clusters as well as the number of individual flowers were already laid down a year earlier when the winter bud was formed (see under Initiation). There are usually one to three (up to a maximum of five) inflorescences on a fruiting shoot in the form of a so-called panicle. Strictly speaking, the term grape cluster is therefore incorrect or even wrong for the inflorescence of the vine.

After flowering, pollination and fertilisation, the ovary develops into a berry and the cluster into a grape. On average, each bunch of grapes has about 150 berries. However, whether a berry actually matures from the maternal tissue of the ovary depends on the success of fertilisation. A vine can never bring all the berries of all the grapes to full ripeness. If fertilisation does not take place or is inadequate, for example due to bad weather (rain, frost), berries will not form at all, as such flowers coulure and fall off. Depending on the grape variety, the flowering rate is only 30 to 60%, i.e. a maximum of two thirds.

Fertilisation

Fertilisation primarily takes place within the hermaphrodite flower (autogamous) or between two flowers of the same clone, which is understood as self-fertilisation in the narrower sense. However, self-fertilisation in a broader sense also applies between two flowers of the same vine (geitonogamous). However, hermaphrodite plants of a grape 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-fertilised (xenogamous) without affecting the development of the flower into a berry. However, the germination capacity of the self-fertilised seeds (grapevine seeds) may be impaired. As a rule, cross-fertilised varieties are more fertile and more vigorous than those from self-fertilisation. However, this is irrelevant because in viticulture the grapes are intended for pressing and their seeds are not intended for sowing. Any pollen is sufficient for berry formation.

As already mentioned, the wind (the seeds can fly several hundred metres) and insects attracted by the scent of flowers are involved to a very small extent in pollen transfer. In principle, however, the cultivated vine does not need to be cross-pollinated and is therefore not dependent on these conditions like many other plants and the dioecious wild vines. This is a great advantage and decisive for yield security.

Wherever the pollen grains come from on the sticky stigma (own flower, neighbouring flower in the clump, flower of another vine), they stick there, germinate and form a pollen tube that grows into the stigma and the pistil to fertilise the ovule in the ovary. This fertilised egg cell (zygote) later develops into up to five (rarely six) genetically different grape seeds in the berry through meiosis (reduction division and recombination division). The individual grape seeds have different genetic material in the chromosomes. This means that the individual grape seeds (even in the same berry) would produce different grape varieties when sown.

Fertilisation produces a single berry from each individual flower. For example, a bunch of grapes with 150 berries could (theoretically) have 150 different fathers (fathers are other vines). 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 can come from the grape's own flower, from a flower from the same vine, from a flower from another vine of the same variety, from a neighbouring vine of the same variety or from vines of other varieties from neighbouring vineyards.

Fruit set

For fruit set (the next step in the vegetation cycle) and subsequent berry development, it is largely irrelevant whether the seed was self-pollinated (self-fertilisation) or pollinated externally (cross-pollination). So even if, for example, a Riesling scar was pollinated by a Silvaner seed, a Riesling grape will develop. The natural crossing of two grape varieties in this case is therefore only potentially present in the embryo of the grape seed and would only become effective when this seed is sown and germinates into a seedling. Two to three days pass between pollination of the stigma and fertilisation of the ovule, depending on the temperature. The pollen begins to germinate on the stigma, the pollen tube grows through the pistil, penetrates to the maternal ovule in the ovary and fertilises it.

Growth of the berry

The maternal tissue of the ovary subsequently grows into a berry with five to six seeds. As mentioned above, these can produce different varieties when they grow into seedlings. Three different grape varieties (Baron, Cabernet Carbon, Souvignier Gris) were created in Fribourg in 1983 from crosses between Cabernet Sauvignon x Bronner. The three seeds could theoretically all come from the same berry. If bad weather conditions such as high humidity, cold, rain, wind or hail prevail during flowering, fertilisation is impaired. The jumping mechanism does not work or only works to a limited extent and the caps are not blown off. As a result, pollination does not take place.

Late frost or cold hinders the growth of the pollen tubes. Therefore, all ovules are never fully fertilised. However, as the embryo in the seed stimulates fruit formation, the insufficiently fertilised berries remain smaller. The result is parthenocarpy (or stenospermocarpy = apparent maiden fruit) or premature coulure (falling off) of the completely unfertilised flowers. The grape then has fewer and smaller clusters. However, this need not be a disadvantage, as a reduced yield can improve quality. This is because the substances produced by the grapevine are now concentrated in a smaller number of berries, which can result in more extract substances, a higher sugar content and better overall wine quality. The berries reach pea size just a few weeks after flowering.

Wild vines

Wild grapevines (Vitis vinifera sylvestris) are generally dioecious - each vine bears only male or only female sexual organs. Although both organs are present as plants, only one organ is functional and the other is hormonally suppressed (phytohormones). In dioecious plants, self-fertilisation is therefore not possible and female plants can only be fertilised by cross-pollination with pollen from another vine.

The forced recombination of the genetic material of paternal and maternal plants maintains genetic diversity in a wild population, resulting in more viable offspring and thus facilitating adaptation to environmental changes (heterozygosity ). In viticulture, however, this is not desirable due to uncertain yields. However, many of the varieties used as rootstocks are hybrids of wild vines and are therefore dioecious. When breeding rootstocks, varieties with male flowers are favoured, which cause less effort due to the lack of grapes.

A self-pollination (self-fertilisation) is generally avoided by nature, as only cross-fertilisation ensures continuity. In hermaphrodite flowers, the female stigmas are self-sterile for their own male pollen due to chemical, physiological and mechanical mechanisms. However, as mentioned, there are other mechanisms in nature. In wild vines, self-fertilisation is simply prevented by dioeciousness or the separate sexes. In cultivated plants, these self-sterility mechanisms have often been completely or partially bred out. During fruit and seed formation, the grape cluster, berry skin and berry flesh, as well as the seed coats 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 fertilisation, the grape is unaffected. Riesling remains Riesling, even when fertilised by Silvaner pollen.

Female flowers

There are also varieties with functionally female flowers, but in which the stamens have been reduced or are not fertile at all. 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 the 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, making them suitable for the production of seedless dried berries. The growth in size of these seedless berries can be artificially induced by growth hormones (gibberellins).

Natural crosses

All vine species with the same DNA structure can be crossed with each other. This includes all European (Vitis vinifera) varieties, but also those with most American and Asian varieties. This means that most varieties can not only fertilise each other (which a priori does not mean reproduction), but can also cross with each other, resulting in new varieties. If the seeds germinate, the resulting seedlings would be spontaneous crosses between two parent plants. A variety with new characteristics that differ from the parent variety is most likely to emerge if cross-fertilisation by another variety has taken place.

natürliche Kreuzungen - Cabernet Sauvignon, Riesling, Tempranillo, Grüner Veltliner

In this way, countless grape varieties have developed spontaneously in a natural way over many thousands of years without human intervention (see four examples in the picture). However, the genetic material of the mother and father plants are also recombined when fertilisation takes place within two vines of the same variety. However, their genetic differences are usually small because the vines in the vineyard are mostly clones through vegetative propagation.

open-pollinated (open pollinated)

The paternal pollen donor - i.e. the father variety - is not known a priori for vines that have grown from grapevine seeds without human intervention. Such spontaneously developed varieties are referred to as "open-pollinated" (open pollinated, abbreviation "O.P.") in contrast to deliberate new breeding by humans. Today, the parentage is usually clarified by DNA analysis; see under grape variety identification.

Further information

See also under chromosome, generative propagation, vegetative propagation, crossing, breeding and hermaphrodite flowering. All tools, work and measures in the vineyard during the annual vegetation cycle can be found under vineyard care. A complete list of keywords relevant to grape varieties can be found under Vine.

Graphic: Taken from Bauer/Regner/Schildberger, Viticulture,
ISBN: 978-3-70402284-4, Cadmos Verlag GmbH
Flower buds: By Bauer Karl - Own work, CC BY 3.0 at, Link
Grape varieties: Ursula Brühl, Doris Schneider, Julius Kühn-Institut (JKI)

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