The Leaf Fall and Shoot Maturation Phase of the Grapevine

The seventh phase of the grapevine vegetative period encompasses the time interval spanning from the onset of full (physiological) maturity of the berries to the completion of leaf fall. Based on the nature of the physiological processes occurring within both the epigeal and hypogeal organs of the grapevine, this phase is characterized as the preparation of all plant tissues and organs for the winter period.

According to Müller-Thurgau, the full maturity of the grape berry is determined by the accumulation of its maximum absolute quantity of sugar. From the moment the further influx of photosynthetic products to the berry ceases, the stage of its overripening initiates, progressively transitioning into withering (raisining) of the berry. In white grape cultivars, the state of berry overripening is characterized by the appearance of a golden-yellow coloration with more or less dark brown spots, which indicate the initiation of cell death within the berry skin (Kroemer). The processes of water transpiration and respiration proceed intensively; consequently, this induces a loss of turgor in the cells of the berry pulp on one hand, and a continuous decline in the acidity of the cell sap on the other hand, driven by the ongoing oxidation of organic acids into carbon dioxide and water.

Concurrently, the concentration of sugar within the cell sap increases continuously, since the water lost via transpiration is not compensated by an influx of fresh water during the overripening stage: the berry becomes flaccid and acquires a syrupy-sweet flavor. The high sugar concentration attained by grape berries during their natural overripening and withering on the vines (i.e., without the application of artificial techniques such as cluster stem twisting) is demonstrated by the analytical data obtained during harvest at the Magarach vineyards (southern coast of Crimea).

During the overripening of the grape berry, the chemical nature of the nitrogenous substances also undergoes modification; for instance, the quantity of proteins that coagulate with alcohol increases (Kroemer).

Concurrently with berry ripening, the maturation of the shoots takes place, so that by the initiation of leaf fall, the latter manage to lignify along almost their entire length. The exception remains the youngest apical internodes, which typically remain herbaceous and, following the first frosts, abscise at the nodes and fall.

The maturation of the annual grapevine shoot initiates from the lowest internodes and progressively advances to the higher ones. The initiation of maturation in each internode is determined by the moment of phellogen (cork cambium) formation, through the activity of which a continuous cork layer (periderm) arises, situated at the boundary between the primary and secondary cortex.

The primary cortex, severed by the periderm from communication with deeper-lying tissues, dies, desiccates, and transforms into the so-called "old bark," which exhibits a reddish-yellow coloration in the annual grapevine shoot (this coloration, however, varies substantially among different cultivars). Simultaneously with periderm formation, the death of pith cells occurs, so that a fully matured shoot possesses two dead tissue complexes—the primary cortex and the pith—and two living ones—the secondary cortex and the xylem.

From the moment of periderm formation, the accumulation of starch initiates within the tissues of the central cylinder. This can be easily verified by applying the so-called "iodine" reaction for starch on a transverse section through an internode where the old bark has managed to form on only one side. Only that portion of the central cylinder adjacent to the old bark will stain blue from the iodine; in the other portion of the central cylinder, still covered by the living primary cortex, no staining from iodine will be observed.

The old bark, together with the underlying periderm, plays a highly critical role in protecting the grapevine shoot not only from winter frosts but also from desiccation. In winter, the absorptive activity of the grapevine root system nearly ceases, as the rootlets become covered with a layer of suberized cells to their very tips. In connection with this latter circumstance, the water supply to the epigeal organs of the vine drops sharply, so that if the shoots were not clothed with a periderm, they would inevitably perish from desiccation.

This is confirmed by data from Schmitthenner: the weight loss of excised shoots (with sealed, coated cut surfaces) over 12 days at 7–10 °C reached:

• 4.6–9.9% for fully matured internodes clothed with a normally developed old bark;
• 34–36.4% for poorly matured internodes clothed with a weakly developed bark.

In autumn, the assimilatory activity of the leaves progressively declines, as the chloroplasts within their cells become vacuolated and break down. The leaves of the lower tier cease their functions first, followed somewhat later by the leaves of the middle tier, and latest of all by the apical leaves of the grapevine shoot. Concurrently, the grapevine leaves acquire their autumnal coloration: golden-yellow in white cultivars and red in black ones; the tissue cells in such leaves are dead and virtually empty.

At the base of the petioles, an abscission layer forms, consisting of several transverse rows of loosely connected parenchymal cells. By the time the leaf ceases its activity, the middle lamellae of these cells undergo chemical modifications, dissolve, and disappear, as a result of which the petiole easily detaches from the shoot upon the slightest mechanical disturbance. The site of its detachment becomes covered with a protective layer composed of cells with lignified and suberized walls (the leaf scar below the wintering bud).

The timing of leaf fall, which indicates that the grapevine has completed its preparation for the winter period, represents a fairly characteristic cultivar trait: there are cultivars with early leaf shedding and also cultivars with very late leaf fall. However, the calendar dates for the initiation of leaf fall vary strongly across different years for the same cultivar within the same vineyard. To illustrate this point, data from phenological observations at the Magarach vineyards (southern coast of Crimea) are presented below.

The duration of leaf fall also fluctuates sharply both among different cultivars and within the same cultivar across individual years.

The data from these tables indicate that in some cultivars, leaf fall terminates rapidly (for example, in Traminer), whereas in others, it lasts nearly twice as long (for example, in Mourvedre). In general, in cultivars with early berry ripening, leaf fall initiates earlier and terminates more rapidly than in late-ripening cultivars. It should be noted, however, that according to data by N. P. Buzin, this inter-cultivar variation is nearly obliterated during arid years.

At the end of the vegetative period, a so-called "negative pressure" is observed within the tissues of the epigeal organs of the grapevine, which manifests in the fact that the cut surfaces of the shoots begin to avidly absorb water. The factors inducing this phenomenon remain completely unclarified; certain authors attribute the emergence of negative pressure to the internal consumption of liquid within the tissues of the trunk and shoots when the supply of water by the roots has nearly ceased during this period.

The quantities of water that can be absorbed by a grapevine shoot are quite substantial, which allows the utilization of negative pressure for the purpose of treating grapevine bushes against chlorosis: when fresh cuts are coated with a solution of ferrous sulfate, its penetration into the shoot tissues to a depth of up to 10–12 cm is observed.

In conclusion, it must be mentioned that by the time leaf fall terminates, all tissues of the grapevine shoots and roots are found to be completely filled with reserve plastic substances, primarily starch and protein.