Spring Sap Flow Phase of the Grapevine

The 2nd phase of the grapevine vegetation period begins with the appearance of the first signs of spring sap flow and continues until the start of bud burst. As an example, for the vineyards of the southern coast of the Crimea, according to long-term phenological observations, this phase lasts from the end of March to the middle of April.

Spring sap flow in the grapevine bears the highly characteristic name of "bleeding" ("plach"), as a clear liquid (sap) is abundantly exuded from all wounds on the trunk and shoots at this time. Spring sap flow is also observed in many other plants (for instance, birch and maple), but it can only be detected if the trunk is wounded. In grapevine bushes, however, numerous fresh wounds always remain after each winter pruning, which is why the phenomenon of spring bleeding acquires an especially pronounced character in them.

The physiological essence of the bleeding phenomenon lies in the fact that in spring, upon the onset of certain meteorological conditions, root hairs begin to vigorously absorb soil water via osmosis. Due to the developing osmotic pressure, this water passes from the cells of the root epidermis into the parenchymal cells of the cortex, then into the cells of the central cylinder of the root, and thus enters the vessels through which it ascends to the trunk and shoots; if the vessels happen to be cut, the liquid is continuously exuded outward at that site in the form of droplets.

The pressure under which the sap moves through the xylem vessels can easily be measured by attaching a mercury manometer to the cut site using a rubber tube: in the grapevine, this pressure reaches up to 900–1100 mm Hg, i.e., it can balance a water column with a height of 15 m (Merzhanian).

At present, it can be considered firmly established (Buzin, Merzhanian, Barberon, and many others) that the onset of bleeding in the grapevine is completely independent of air temperature, but stands in direct dependence on soil temperature. As soon as the soil temperature in spring, at the depth of the distribution of the main root mass, rises to approximately 9–10°C, bleeding begins in most grape varieties belonging to V. vinifera.

In the vineyards of the southern coast of the Crimea, a short-term bleeding of the grapevine is not infrequently observed even in the winter months—January and February—if fine weather sets in and the soil manages to warm up to the temperature indicated above. The complete independence of grapevine bleeding from air temperature is proven by observations showing that during a sharp and sudden drop in temperature, the bleeding that has begun does not cease, and the exuded sap can even freeze on the cut ends of the shoots in the form of icicles (Merzhanian); only when the cooling of the air leads to the cooling of the soil at the root depth is the bleeding of the grape suspended.

The soil temperature at which bleeding begins is unequal not only for different species but even for different grape varieties: for example, it is lower for V. amurensis and V. riparia than for V. Berlandieri and V. vinifera. Naturally, under the same conditions in spring, bleeding in different species and varieties begins non-simultaneously. A certain role in this is also played by the rooting depth: the more superficially the roots spread in the soil, the earlier spring sap flow begins in such varieties (or bushes); the deeper the root system lies, the later bleeding occurs.

In various varieties grafted onto the same American rootstock, bleeding begins simultaneously with the bleeding of the vines of the given rootstock, meaning that the individual characteristics of the grafted varieties are no longer manifested.

A close dependence can also be observed between the soil temperature and the amount of exuded sap. If, during the entire period of sap flow, the sap exuded from a grapevine bush is collected and measured daily, and a "bleeding curve" is subsequently constructed, a comparison of this curve with the course of soil temperature over the same period reveals with complete clarity that every decrease and increase in soil temperature entails a corresponding decrease or increase in the amount of exuded sap. This pattern is manifested with particular clarity in the first half of the bleeding period, while the buds have not yet burst. After bud burst, under the influence of increasing water evaporation by the young leaves, a rapid decrease in the amount of sap exuded from the cuts occurs, and soon the bleeding ceases completely.

That the gradual subsiding of bleeding depends precisely on the increasing evaporation of water by the young leaves can be proven in the following way: if, shortly after the cessation of bleeding, the young shoots are broken off, the bleeding resumes with considerable force.

Furthermore, if during the initial period of shoot growth a prolonged period of rainy weather sets in, during which water evaporation by the leaves almost ceases, under these conditions a resumption of grapevine bleeding is also not infrequently observed, even if the latter had already ceased long before (Merzhanian).

The total duration of bleeding fluctuates extremely widely in the same varieties across different years. For example, over a five-year period in the Magarach vineyards (southern coast of the Crimea), according to data by N. P. Buzin, the number of bleeding days fluctuated as follows:

The same can be said about the total amount of sap exuded during bleeding: Canstein obtained up to 20 liters per bush, Meissner up to 1.5 liters, and Merzhanian only up to 1.1 liters. The explanation is that some researchers repeatedly refreshed the cuts, preventing the vessels from becoming clogged by the exuded gum-like substances, while others did not remove the natural clogging of the severed vessels. Similarly, during spring pruning of vineyards, a significantly stronger bleeding of the grapevine bushes is observed compared to autumn pruning, during which the cuts, by the time of spring sap flow, turn out to be closed by a thick layer of dead tissues.

By its chemical composition, grapevine sap represents an extremely dilute solution of organic and mineral substances, the total amount of which usually does not exceed 1–2 grams per liter. Here are samples of sap composition in two grape varieties (V. vinifera) according to analyses by Merzhanian in the Magarach vineyards (southern coast of the Crimea):

Thus, in the sap, organic substances prevail over mineral ones; sugar accounts for almost half of all organic substances. The main mass of the mineral substances in the sap consists of CaO, K₂O, MgO, and P₂O₅. The specific gravity of the sap at 15°C was generally equal to only 1.0007. Such a low extract content in the sap indicates that grapevine bushes do not lose any significant amounts of nutrients during bleeding: in special experiments with repeated refreshing of wounds inflicted during pruning, the grapevine bushes did not subsequently show signs of exhaustion or weakening, even though their bleeding proceeded extremely intensively; only a slight delay in bud burst could be noticed in these bushes.

During the time from the beginning of bleeding to its end, the amount of starch in the tissues of grapevine shoots does not change noticeably compared to the winter period: it is abundant in both the phloem and the wood. Furthermore, during spring sap flow, the phloem becomes filled with proteinaceous substances (especially the thick-walled phloem fibers). The callus that closes the sieve plates of the sieve tubes in winter begins to dissolve gradually. During the bleeding period, sugar is found in relatively small amounts in the phloem fibers, in the sieve tubes, and in the parenchymal cells constituting the "sheaths" of the vessels. In the roots at this time, a decrease in the amount of proteinaceous substances is observed, which are apparently transported into the cortical part of the shoots (Alexandrov, Makarevskaya).