Morphology and Viability of Pollen

Normal pollen from hermaphrodite long-stamened flowers and functionally male flowers forms a fine, light, sulfur-yellow powder (in very rare cases, slightly pinkish) during opening, which is easily transported by air currents (Figs. 218, 223–226). Due to the high lightness of pollen grains, their dissemination occurs predominantly by wind, defining the grapevine as an anemophilous plant. This characteristic is especially pronounced in male flowers, which optimally position their anthers under the action of the wind. The frequent activity of numerous insect pollinators has also been observed; they visit the flowers during full bloom and further disseminate pollen from flower to flower and from plant to plant.

In the majority of cases, the pollen of the genus Vitis is spherical (Figs. 223, 224, 226) without ornamentation, featuring two or three poorly defined folds; its two membranes (exine and intine) are clearly visible. An exception is found in Muscat of Alexandria, where the pollen is slightly ovoid (Fig. 225). Its dimensions vary depending on the species or variety, ranging from 19 to 34 micrometers in diameter, and are around 25 micrometers for most varieties of Vitis vinifera. Below are the precise diameter dimensions (in micrometers) for a number of species and varieties:

  • Vitis rotundifolia — 24;
  • Alphonse Lavallée — 22 to 25;
  • Barbarossa — 25 to 31;
  • Bicane — 25 to 28;
  • Black Alicante — 25;
  • Muscat Cannon Hall — 31 to 34;
  • Chasselas — 24;
  • Foster's White Seedling — 22 to 25;
  • Gros Colman — 22 to 25;
  • Gros Maroc — 25 to 28;
  • Muscat of Alexandria — 22 to 24 by 31 to 34;
  • Muscat of Hamburgh — 22;
  • Ugni blanc — 25 to 31;
  • Aramon × Rupestris Ganzin No. 1 — 19 to 25.

The germinative capacity of pollen can be preserved for a very long time in a dry environment and at a low temperature (5°C to 15°C)—for several months, and sometimes up to a year.

Pollen Tube Germination

The pollen of long-stamened grapevines germinates easily and almost immediately in pure water and in water with the addition of 1% to 10% sugar. In pure water or water sugared to 4%, within twenty-four hours in the month of June, under normal temperatures, the pollen tubes of Chasselas reach a length of half a millimeter to one millimeter. In Aramon pollinated by its own pollen under a normal temperature, forty-eight hours after the pollen has been deposited on the stigma, pollen tubes are already found inside the ovary, in the vicinity of the micropyle. Eight hours later (that is, after fifty-six hours), some are seen to have penetrated into the endostome. However, as the nucellar papilla is thick, it takes a rather long time for the pollen tube to traverse it, and it is only on the fifth day after the deposition of pollen on the stigmas (in Chasselas) that fertilization of the oosphere can be observed. By the eighth day (Chasselas), the embryo is constituted by a small ball of about thirty cells.

The morphological phenomena of grapevine pollen germination are characterized by a strict sequence (Figs. 227, 228). Before its maturity, the pollen grain divides its content into two cells of unequal size, one small, called the generative cell, and the other, large, called the vegetative cell. The generative cell becomes free in the pollen grain at a more or less advanced period of development. While the vegetative nucleus never divides, the generative nucleus, on the contrary, presents a bipartition along with the protoplasm that accompanies it, which takes place here inside the pollen tube. In Figure 227, the generative cell (cg), with its nucleus and its very distinct atmosphere of protoplasm, is not yet divided or at least has only outlined its division; in Figure 228, which represents a more advanced stage, the generative cell has already formed its two generative nuclei (ng). Before this division, the nucleus of the generative cell is distinguished from the vegetative nucleus by the absence of nucleoli and by a vividly stainable chromatic network; the vegetative nucleus always possesses one or several rather large nucleoli, and its chromatic framework stains poorly.

The relative position of the vegetative nucleus and the generative elements in the pollen tube of Vitis is variable. In Figure 227, the vegetative nucleus is, in three of the pollen tubes represented, behind the generative cell, and in the fourth, in front. In Figure 228, the vegetative nucleus is sometimes in front, sometimes behind, sometimes to the side of the generative nuclei; occasionally, it is even encountered between the two generative nuclei.

The destiny of the pollen nuclei is quite different. While the vegetative nucleus disappears more or less rapidly in the protoplasm of the pollen tube and, ordinarily, at the moment when the latter penetrates into the ovule, the two new generative nuclei, which have continued to grow after their formation, are found with their own characteristics until the moment of fertilization. One of them goes to fertilize the oosphere, while the other is not inactive and fuses with the secondary nucleus of the embryonic sac; this process of double fertilization ensures the evolution and transformation of the secondary nucleus into albumen.

Development of the Flower

The initial formation of the grapevine flower is characterized by a strict sequence of organ initiation (Fig. 248). In young inflorescences, small swellings form on the sides of the axes, which are the first signs of flowers, equipped with protective bracts. On this mamillated mass of tissue, the structures of the calyx are the first to appear. Following the sepals, the petals differentiate successively, then the stamens, and lastly the ovules, before the separation of the ovary into two locules. At the early stages of development (Fig. 249), the individual sepals are united with each other. Their separation into a reduced crown occurs later, due to the arrest of calyx growth, which coincides with the rapid elongation of the petals (Fig. 250).

Conversely, the petals do not fuse until relatively late in development; they were free at the early stages of formation, and the fusion takes place by the union of the adjacent parenchyma of two neighboring petals (Fig. 251). In the stamen, the anther develops first; the filament does not elongate until later. At this moment, the erect and naked ovule begins to curve toward the receptacle and acquires its two integuments—the inner first, and the outer second. In parallel, the pollen grains already begin to differentiate and turn yellow.

Floraison

The successive opening of flowers in a cluster follows the order of their development. The complete flowering of a cluster lasts from three to five days on average if the weather is favorable. The mechanism of shedding the corolla-cap is similar in male flowers and in hermaphrodite flowers with long stamens. Male flowers, with very long stamen filaments folded under the corolla, exert pressure on it by their elongation and distension. The petals separate near their base of insertion on the receptacle by successive slits, and the stamens lift the corolla above the flower like springs.

When the filaments are visible for a third or half of their height from the base, the cap is rejected to one side by the stamens, which begin to spread from the center of the flower. The stamens are then erect and move away from the center of the flower; their anthers, which under the cap were almost in line with the filament, place themselves, by rotation on the connective, in a plane perpendicular to the filament, presenting maximum exposure to the wind. It is then that the rupture occurs, and the pollen dust, very fine and abundant for these male flowers, is carried by the slightest breeze onto the stigmas of female or hermaphrodite flowers.

Female flowers with rudimentary stamens, or with short stamens and non-fertile pollen, detach their corolla by the same process of stamen pressure, but the corolla is not lifted; it is immediately cast out of the flower. The stamens then curve backward very abruptly and place their infertile anthers under the flower, against the pedicel. These stamens often dry up immediately, but sometimes they persist, green or browned, until the berries are set. Flowers with short stamens must be fertilized by foreign pollen; that of male flowers in the wild is the main agent of fertilization. In cultivated grapevines with short stamens (Muscat of Alexandria, Bicane, etc.), it is also the pollen of male vines that gives the best results for artificial fertilization.

In hermaphrodite flowers with long stamens (as in Chasselas), at the moment the corolla falls, the anthers are still closed, and no traces of pollen are observed on the stigma. It is only during the outward movement of the stamens, two, three, five minutes or more after the fall of the corolla, that the anthers open. The pollen appears at the edge of the slits and detaches at the slightest shake or even falls by its own weight. Thus, pollen is not deposited on the stigma under the corolla, as some authors state, at least in cases of normal flowering. The movements of the floral elements prevent self-fertilization within the same flower and favor cross-fertilization.

Stigma Secretion and Fertilization Conditions

At the moment the corolla-cap falls, the stigma possesses its definitive development: it is fresh, not humid, and easily retains the pollen. Shortly after, it can be seen to become humid, and then generally a small pearl of liquid appears at its extremity, which often persists for several days, diminishing or disappearing during the hottest hours of the day to reappear in the evening or morning. Under hot and dry weather, this secretion does not occur, at least during the day. In general, the deposition of pollen on the stigma causes the complete disappearance of this liquid pearl after twelve to twenty-four hours, as the humidity of the stigma is favorable to the development of the pollen.

However, the production of this droplet on the stigma is often a defect. In short-stamened varieties (Muscat of Alexandria, Bicane) that require artificial fertilization, a large droplet always forms on the stigma, especially in humid soils. It detaches easily, carrying to the ground the pollen grains before their germination or before the pollen tube has penetrated the stylar canal. Therefore, before dusting these varieties with pollen, one must previously shake the vines to make the water pearls fall from the stigma. To avoid these over-abundant liquid emissions, the soil should be rather dry at the moment of flowering and throughout its duration. Atmospheric humidity (high humidity, fine rains, dews, fogs) has the same disadvantage as soil humidity; it causes the persistence of the corolla on the summit of the stamens, where it dries, preventing external pollen dust from impregnating the stigma. In artificial fertilizations, before spreading the pollen, one must pass the flowering clusters lightly between the fingers to remove the caps. Humidity, coinciding with a drop in temperature, causes the same phenomena of capped flowers and provokes the opening of flowers in a star shape before the maturity of the stamens; coulure results from all these bad conditions.

Forced Self-Pollination and Anomalies

Soil humidity in rich, argillaceous-calcareous lands, by pushing vigorous varieties to an excess of vegetation, determines coulure (abortion of flowers) by causing the premature opening of flowers or, for certain varieties (Clairette), by determining accidental chloranthy (transformation of circles into petaloid leaves). This excess of vigor frequently causes an abortion of a certain number of flowers within the same inflorescence without any deformation of the various circles. This process is a consequence of the anatomical underdevelopment of the internal structures of the ovule, style, or stigma, resulting in coulure or millerandage (French terms meaning coulure — shedding/dropping of flowers, millerandage — shot berries/hens and chickens). Pinching, topping, and annular incision allow for the mitigation of this flowering accident by regulating shoot vigor and reducing flower drop.

A case that falls into this category is that of Muscat Cannon Hall, whose flowers are united in bouquets at the summit of a terminal ramification. The cluster stems and peduncles are very large and fasciated; on the contrary, the small flowers are carried on very slender, filiform pedicels. Pollen and ovary are normally constituted, but at the moment of flowering, many flowers fall, and the bouquets suddenly lose all their grouped flowers, which detach abruptly as if under the influence of a shock, with their pedicels adhering. Even blowing strongly on the flower bouquets causes a large number to fall, always carrying the pedicel in their drop. This weak adherence of the pedicel on the stem is very peculiar.

The most perfect conditions for a good flowering and fertilization are a high temperature, dry weather, a luminous sky, and light breezes. It is especially during the hottest and most luminous hours of the day that flowering is most abundant, from 9 o'clock in the morning (in June and July) to 2 or 3 o'clock in the afternoon. The action of light has no great influence independently of temperature; clusters in the shade flower just as well, though a little later due to temperature. Temperature plays the most important role for the opening of flowers and for fertilization. Different grapevines do not all flower at the same temperature: certain very late varieties (Black Alicante) can open their flowers at 15°C or 16°C, while others (Muscat of Alexandria), which are earlier, only have full flowering at a constant temperature of 25°C. In general, the opening of grapevine flowers begins only when the temperature maintains a constant average above 15°C; full flowering for most grapevines takes place from 18°C to 25°C.

The duration of flowering for a single vine varies from 4 to 8 days at constant average temperatures of 20°C. A cluster begins by opening the flowers at its base, and flowering ends at the apex; in two or three days, or even in a single very hot day (25°C to 30°C), an entire cluster can flower. Considered across a whole vineyard, flowering lasts from 8 to 15 days; this period is more extended than that of ripening. Flowering generally arrives two months after budburst.