Growth Phase of Grapevine Shoots and Inflorescences

The 3rd phase of the grapevine vegetation period begins at the moment of bud burst and lasts until the beginning of flowering: for example, according to long-term phenological observations in the vineyards of the southern coast of the Crimea, this phase begins in the second half of April and continues until the first decade of June, spanning a time interval of 40–50 days. During this period, the vegetative organs of the grapevine—young shoots and leaves—principally manage to develop, although their growth does not terminate within this timeframe; in fruiting vines, moreover, the inflorescences reach their full development during this same period.

The circumstance that the period of the most intensive shoot growth in the grapevine coincides with the period of inflorescence development deserves special attention, since under unfavorable conditions (for instance, with an insufficient reserve of plastic substances, poor vine nutrition, etc.), acute competition can arise between the shoots and the inflorescences during the distribution of the organic substances necessary for tissue construction.

With the onset of spring, the overwintered buds first "swell" for a certain period of time, i.e., they gradually increase in volume, and then their covering scales separate under the pressure of the vigorously dividing cells of the apical meristem. Folded primordial leaves appear in the resulting gaps between the scales, gradually freeing themselves from the long brown hairs that envelop them within the bud. In phenological observations, this moment is conventionally accepted as the beginning of bud "burst."

Within the same ecological environment, bud burst in different grapevine species and varieties does not begin simultaneously: there are varieties with "early" bud burst, such as Lignan, Agostenga, and Khusaine, and those with "late" bud burst, such as Mourvedre, Morastel, etc., with the difference in bud burst timing between them reaching up to 15 days.

For northern viticulture regions, late bud burst is a positive trait, as young, tender shoots are less likely to suffer from spring frosts under these conditions. Unfortunately, late bud burst in most grape varieties correlates with late berry ripening, which in itself makes these varieties poorly suited for northern viticulture; conversely, varieties with early bud burst, as a rule, belong to early-ripening ones.

While the sequence in which grape varieties follow one another in terms of bud burst timing remains fairly stable from year to year, the calendar dates of their bud burst within the same vineyard fluctuate very strongly across different years, as demonstrated by the following example taken from phenological observations in the Magarach vineyards (southern coast of the Crimea):

A comparison of the data obtained from phenological observations with the course of meteorological elements demonstrates with complete clarity that the onset dates of bud burst in grape varieties are determined by the sum of meteorological factors over a rather long preceding period, during which "preparatory" processes for the start of vegetation take place within the grapevine organs.

In the example cited above, the earliest bud burst was recorded in 1927: this year is characterized by the highest mean temperature for the entirety of March and the beginning of April across the five compared years. A contrasting picture is observed in 1928: during the month of March, the temperature was significantly lower than in other years, and although the temperature leveled out in April, bud burst in this year was nonetheless later. Until now, it has not been possible to establish more precisely the influence of individual factors (air and soil temperature, air moisture, etc.) on the onset of bud burst in the grapevine; it is indubitable, however, that air temperature plays a decisive role in this case.

In the first days after bud burst, the intensity of shoot growth is determined to a significant extent by the mean daily temperature. It has long been observed that if prolonged cold and rainy weather sets in during this time, the growth of young grapevine shoots proceeds very sluggishly during the day and almost ceases at night (Barberon). Only when the shoots reach a length of 10–15 cm does the air temperature cease to exert such a sharp influence on their growth.

The dependence of shoot growth on the mean daily temperature is well illustrated by the following example (according to Barberon):

If "growth graphs" for various grape varieties are constructed on the basis of daily shoot measurements, it can be seen that the energy of shoot growth gradually intensifies as the flowering period approaches, and that during flowering, shoot growth by no means slackens (see the graphs for Pinot noir, Riesling, and Portugieser varieties in Fig. 50, compiled by Barberon in the Abrau-Durso vineyards).

Furthermore, these graphs clearly highlight the differences among grape varieties regarding the vigor of shoot development over the same timeframe (for instance, 95 cm for Pinot, 130 cm for Riesling, and 165 cm for Portugieser by the beginning of flowering).

Shoot growth in length is a total value consisting of the direct growth of their tips due to the continuous division of cells in the apical meristem, and the subsequent elongation of the formed cells until they reach their normal sizes. Consequently, young internodes continue to elongate for some time, whereby, according to Durand's research, the zone of maximum stretching covers only the 5–6 youngest internodes (i.e., those immediately adjacent to the "growth cone" of the shoot), and as the distance from the shoot tip increases, the stretching of the internodes becomes progressively weaker and, finally, ceases completely. Moreover, the 4–5 lowest internodes at the base of the shoot remain shortened compared to the middle internodes of the shoot. Here is an example borrowed from Durand's data:

The same researcher showed that the zones of greatest stretching are located at the ends of each internode, whereas the middle part of the internode stretches relatively weaker.

The amount of reserve plastic substances in the tissues of two-year-old grapevine shoots decreases sharply during the period of vigorous growth of young shoots, leaves, and inflorescences. The upper parts of the two-year-old shoots (i.e., of the preceding year) become almost completely depleted of starch and proteinaceous substances at this time; a certain reserve of starch still remains in the lower parts of these shoots (Alexandrov, Makarevskaya).

Young, herbaceous leafy shoots contain an average of 70–80% water. As for the organic and inorganic constituents of young shoots and leaves, numerous analytical data available in the viticulture literature show large fluctuations in these substances depending on the grape variety and the conditions of collecting the material for analysis. In general, young grape leaves contain a slightly higher amount of mineral substances than adult leaves during the period of intensive assimilation.

Thus, for instance, the total amount of ash elements in the leaves, expressed as a percentage of the fresh matter, constitutes: 3.64% on April 26, 2.03% on July 10, and 2.89% on October 20 (Hilger and Gross); the mineral substances of young leaves consist mainly of K₂O, CaO, P₂O₅, and MgO, with the herbaceous tips of the shoots being particularly rich in potassium, calcium, and phosphoric acid.

Young leaves and herbaceous shoots are rich in organic acids (in the form of various salts)—tartaric, oxalic, malic, succinic, citric, etc. The very pleasant sour taste of young shoots and tendrils depends on calcium oxalate. We present highly interesting data by Kling characterizing the nutritive value of young grapevine shoots, which this author considers to be a fully wholesome feed for livestock: