The annual development cycle of grapes: dormancy and growing season periods.

How grapevines endure winter: three phases of the dormancy period

The annual growth cycle of a grapevine consists of dormancy and growing season periods. During the cold season, life processes within the plant do not cease, but merely slow down. Respiration, moisture evaporation, and the hydrolysis of stored starch with its gradual conversion into sugars continue within the vine tissues.

The relative dormancy period consists of three successive phases. First, in autumn, conditional (preparatory) dormancy occurs: annual shoots ripen and acquire the color characteristic of the cultivar. In this phase, buds do not develop under normal weather conditions, but if cuttings are moved to a warm and humid environment with good lighting, they will quickly start to grow.

Then comes the physiological (organic) deep dormancy, which protects the vine from sprouting during autumn thaws. It begins even before the end of the growing season with the lower buds of the shoot and lasts for about 1.5–2 months in the southern and central viticulture zones (in September — October). The speed at which buds emerge from this state depends directly on the origin of the specific cultivar.

Group of cultivars (representatives)	Duration of organic dormancy
Western European and Black Sea basin (Rkatsiteli, Cabernet, Cabernet Sauvignon)	Longest
Eastern (Khusayne Belyi, sultanas)	Shortest

From November to March, a forced (winter) dormancy lasts, caused solely by low ambient temperatures. Shoot growth ceases during this time, leaves fall off, but the viability of the buds is maintained. If cuttings are cut during this period and placed in water in a warm room, the buds will quickly emerge from dormancy and begin to open.

Growing season: six key phases of vine development

The growing season activates all life functions of the bush: from photosynthesis to intense shoot growth and fruiting. The start of the spring growing season coincides with the average daily air temperature crossing the 10 °C mark. The active period ends in autumn when the temperature drops below this same threshold.

The growing season of grapes is traditionally divided into six phases. Each of them is characterized by specific morphological changes in the plant organs. The timing of their progression changes under the influence of weather and applied agricultural practices, but the biological sequence always remains unchanged.

• Biological zero — 10 °C
• Deep dormancy — 1.5–2 months
• Forced dormancy — from 3–4 months
• Number of growing season phases — 6 phases

1. Sap flow ("bleeding" of the vine) — lasts from the beginning of spring sap flow to bud break.
2. Growth of shoots and inflorescences — the period from the beginning of bud break to flowering.
3. Flowering — covers the process from the opening of the first flowers to complete fruit set.
4. Berry growth — continues from the formation of fruit set to the beginning of fruit ripening.
5. Berry ripening — lasts from the beginning of berry ripening to their full physiological maturity.
6. Leaf fall — the final phase from full harvest maturity to leaf drop and shoot ripening.

Spring sap flow and physiological "bleeding" of the vine

In spring, even before the buds swell, the active vital activity of the grapevine's root system begins. Old roots that survived the winter begin to intensively absorb water and nutrients, delivering them under pressure to the above-ground part of the bush. This process manifests as the abundant secretion of a transparent liquid — sap — from all fresh cuts and injuries to the vine. The movement of sap occurs primarily through the phloem.

The beginning and intensity of sap flow depend on temperature, soil moisture, as well as the cultivar and rootstock of the grape. In bushes with shallow root systems, this process begins earlier. Large, well-developed bushes excrete more sap than weak-growing ones due to the larger volume of the root system.

For different types of grapes, the soil temperature threshold at the depth of the roots for the start of sap flow differs:

• European-Asian cultivars — when the soil warms up to 7–9°C;
• Amur grapes and some American species (V. labrusca) — at 4.5–5.2°C;
• V. vulpina — at 7–8°C (sometimes sap flow begins at 4–6°C).

With a soil moisture deficit or a significant drop in temperature, sap flow slows down. If the winter was frosty and the roots are damaged, or if the spring turned out to be dry, there may be no sap bleeding at all. With artificial irrigation, the volume of fluid released increases noticeably.

• Sap density — 1.0007 g/cm³
• Dry matter in sap — up to 2%
• Proportion of organic substances (sugars, amino acids) — 2/3
• Proportion of mineral substances (potassium, calcium, phosphoric acid, magnesium) — 73
• Period of active sap flow — 10–15 days
• Total duration of the phase — from 2 to 66 days
• Sap acidity — pH 6.8

According to field observations, the volume of sap released depends on the timely renewal of cuts on the shoots. Without performing this operation, the vine releases significantly less moisture.

Cut maintenance option	Sap bleeding in region 1, l/day	Sap bleeding in region 2, l/day	Sap bleeding in region 3, l/day
With shoot cut renewal	1.4	3	5
Without cut renewal	0.2	1.4	2

During sap flow, shoots and canes become saturated with moisture, gaining flexibility and elasticity. This is the optimal period for carrying out the main spring work in the vineyard, when the risk of breaking the vine is minimal.

1. Uncover the bushes (in regions of covering viticulture) and continue pruning the vine (in non-covering regions).
2. Perform dry tying of canes and shoots to the trellis, and lay down layers.
3. Conduct spring cleft grafting.
4. Remove surface roots (cataracting) and rootstock shoots on young grafted vineyards.
5. Complete deep tillage, and repair or install trellises.

Sap flow occurs unevenly: at the beginning of the phase there is little of it, then a peak occurs, and by the end of the period, volumes decrease. Sap flow stops completely as soon as buds burst on the shoots and the first leaves develop, which begin to transpire water.

Bud burst and shoot growth

With the establishment of a stable air temperature above 10°C, the grapevine transitions to the second phase of the growing season. Water and nutrients entering from the roots activate the internal processes of the plant. Turgor increases in the cells, respiration intensifies, and enzymes are activated, converting accumulated starch into soluble sugars.

Nutrients rush to the growth points, causing intense cell division. Eyes swell, their covering scales open, and the apex of a young shoot with the first green leaves appears in the gaps of the woolly pubescence. Due to the phenomenon of polarity, the upper eyes on the vine always open first, and then the process continues downwards.

The timing of bud burst depends on the geographic location of the vineyard. In southern regions, the phase begins at the end of March or the first half of April, while in northern regions it shifts to the beginning of May. The faster heat accumulates in spring, the more uniformly the eyes burst.

To monitor the development of the vine during this period, refer to the optimal environmental indicators:

Indicator	Optimal value
Air temperature	20–30°C
Soil moisture	85% of field capacity

To trigger the bud burst process, the grapevine requires a cumulative sum of active temperatures (above 10°C) in the range of 145–258°C. When air temperatures rise above the optimal values, this period shortens. If the spring is cool, the opening of the eyes is delayed.

Active shoot growth and preparation for flowering

During this period, the aerial part of the bush and its root system develop synchronously. Simultaneously with shoot growth, the formation of axillary and wintering buds occurs, and the roots actively branch and form new root hairs. At the beginning of the phase, the plant consumes carbohydrates accumulated in the roots and stems since autumn, so a reserve of moisture and nutrition is critical for a good start. As leaves develop, the bush transitions to its own photosynthetic supply.

• Maximum daily shoot growth — 7–10 cm
• Proportion of forming leaves — up to 80%
• Shoot length by the start of flowering — 60% of total length
• Phase duration — about 55 days

The rate of shoot growth depends on air temperature, the structure of the bush, and its polarity. Shoots located closer to the root system grow most intensively, as well as those that have developed in the upper and lower parts of the fruit cane. On high-trunk forms, growth rates are lower, and on scions grafted onto vigorous rootstocks, shoots grow faster. Growth intensity during daylight hours (from 12:00 to 16:00) is initially higher, but towards the end of the phase, when nighttime temperatures increase, this ratio may shift in favor of the night period.

Axillary and dormant buds are initiated along the entire length of the vine. On the lower nodes, they develop more weakly because they form at the beginning of the season, when there are still few leaves on the shoot and their photosynthetic activity is low. The most developed buds form in the middle part of the shoot during its intensive growth, when the leaves reach full development. Lateral shoots grow most actively at the third node from the base of the shoot.

During this phase, the agronomist must carry out a complex of green operations and protective treatments. On rootstock mother plantations, during this period, topping, tying of shoots, and removal of inflorescences are carried out. Periodic loosening of the soil is performed to break the crust and eliminate weeds. Also, in this phase, it is already possible to make a preliminary estimate of the volume of the future harvest for the current year.

1. Removal of excess shoots that are not needed for fruiting and bush formation (carried out at the beginning of the phase, when the inflorescences are clearly visible).
2. Tying of shoots to the support once they reach a length of 25–30 cm.
3. Topping and preventive treatment of vineyards with pesticides against downy mildew, powdery mildew, and pests.
4. Pinching of shoots and top dressing of plants immediately before flowering.

Flowering phase: temperature regime and berry fruit set

Grape flowering begins with the shedding of the corolla (cap) from the bud. The central buds in the inflorescence and the flowers at the base of the shoots, which warm up better by the sun, open first. On bushes with a low trunk and in large inflorescences, flowering begins earlier than on a high trunk and in small inflorescences. The susceptibility of the pistil stigma to fertilization remains for four to six days after the cap is shed.

The main factor determining the course of flowering and the quality of pollination is air temperature. During cold snaps to 11–15 °C, prolonged rains, and fogs, the caps shed poorly, and pollen is washed away by water. The temperature of the previous day also has a significant influence on the intensity of the process.

Effect of temperature on flowering	Air temperature, °C
Minimum for the start of flowering	16
Optimum for flowering	20–30
Slow pollen germination	11–15
Cessation of pollen germination	below 11

• 6–8 a.m.: start of opening;
• 9–10 a.m.: increase in the number of opened buds;
• 11 a.m.: suspension of opening;
• 3–4 p.m.: second, less intense wave of flowering.

Under favorable weather conditions, the duration of flowering of one inflorescence is 4–8 days, and for a cultivar it is 14 days. The start of flowering is considered the calendar day when 5% of the flowers open, mass (peak) flowering is 70%, and the end of flowering is 30%.

Flowering and fertilization of grape flowers occur as follows. In the period of the flower's readiness for flowering, a separating layer of tissue forms at the junction of the corolla with the disc. On the day of flowering, due to an increase in temperature and a decrease in air humidity, as well as due to the difference in the importance of the outer and inner petals of the corolla, their gradual tearing begins, and the corolla falls off in the form of a cap.

The anthers on the stamens crack, and pollen spills out of them. Landing on the pistil stigma, where droplets of secretory liquid appear by this time, the pollen sticks to it and, under favorable conditions — optimum temperature of 25–30°C and air humidity of at least 75–80% — germinates into numerous pollen tubes that pass along the style layer of the pistil column, penetrate through the micropyle into the embryo sac, the ovule, and the sperm from the pollen tube fertilizes the egg cell.

One seed develops from each fertilized ovule, and an embryo develops from the fertilized egg cell. Depending on the number of fertilized ovules, from one to four seeds or more can develop in a berry. After the fertilization of at least one ovule, the growth of the fruit set begins. It increases in size, the style and stigma dry out, and a berry is formed.

Research has established that many pollen tubes penetrate into the embryo sac. However, due to the selective ability of the egg cell, the contents of only one pollen tube, most corresponding to the development of a stable organism, fuse with it. The contents of other pollen tubes serve to nourish the developed embryo.

For its growth, the pollen tube consumes more than 40% of the nutrients of the conducting tissues of collenchymal origin. The fertilization process lasts 24 hours. However, not all flowers in the inflorescences are fertilized, and even if they are fertilized, depending on the grape cultivar, natural shedding of 40–80% of flowers and fruit sets occurs, which is considered a normal phenomenon. Maximum shedding of fruit sets is observed on the 9th day after mass grape flowering. It has been revealed that after fertilization, no more than 15–20% of flowers in inflorescences develop into berries.

It is important that after the shedding of flowers and fruit set, the number of fruit set remaining in the inflorescence corresponds to obtaining a cluster with a normal berry density characteristic of a specific cultivar. However, there are cases where excessive shedding of fruit set is observed, resulting in loose clusters with a small number of berries, which sharply reduces the yield.

Low fruit set depends on a number of reasons:

• poor pollination and fertilization of flowers caused by adverse weather conditions;
• deficiency of nutrients and water;
• defects in flower structure;
• lack of an appropriate pollinator (especially for cultivars with a functionally female flower type);
• biological characteristic of some cultivars, their tendency to flower shedding (Muscat Hamburg, Muscat Hungarian, Muscat of Alexandria, Saperavi, Cinsaut).

In each specific case, the reasons for excessive shedding of fruit set are identified and measures are taken to eliminate them:

• supplementary and artificial pollination of cultivars having both bisexual and functionally female flower types;
• pinching of shoot tips before flowering and their ringing;
• high level of agricultural practices, etc.

Fig. 29. Fertilization of grapes: pollen tube (1) that has penetrated the embryo sac (2); ovule (3) (according to Baranov).

Grape flowering in all regions is observed in June, and in some regions of the republics of Central Asia — in the 2nd ten-day period of May. Flowering periods may not coincide for different grape cultivars. Therefore, the study of the flowering phase is of great importance when selecting pollinators. During the flowering period, tillage, dusting against oidium, and spraying against downy mildew, as well as the treatment of seedless cultivar plantings with gibberellin, continue in the vineyards. However, it is not recommended to carry out irrigation at this time, as it increases air humidity and lowers its temperature, which negatively affects flower fertilization.

4th phase — berry growth

After the fertilization of ovules in seeded grape cultivars, the embryo develops, and the endosperm and seed coat are formed. The fruit set increases in size and turns into a berry. The more ovules that have been fertilized in the fruit set, the faster the pericarp develops and the larger the berry.

There are 3 stages in berry growth:

1. The first stage — intensive growth — follows immediately after the processes of pollination and fertilization.
2. The second stage — growth slows down (before the beginning of berry ripening).
3. The third stage — a new wave of growth (during their ripening period).

These changes are caused by specific internal reasons characteristic of each stage.

Berry growth phase: three stages of yield formation

Berry growth begins immediately after the end of flowering and lasts from one to two months. During this period, the actual volume of the future harvest is determined, so the grapevine requires maximum provision with moisture and nutrients. For the successful completion of this phase, the vine needs consistently high temperatures and good lighting.

• Optimal growth temperature — 25–30 °C
• Active temperature sum (ATS) — 800–880 °C
• Duration of the phase — from 30 to 60 days or more
• Maximum berry acidity — 20–30 g/kg

Berry development proceeds in three successive stages. At the first stage, cells divide actively under the influence of physiologically active substances (PAS), due to which berries rapidly gain 60–70% of their mass. This process is synchronized with the development of seeds, which serve as centers for the production of auxins and gibberellins. In seedless cultivars, growth is activated by hormones formed during pollination and at the start of the development of seed rudiments.

The second stage occurs when the berry reaches the size of a pea. At this time, stomata on the skin turn into lenticels, carbon assimilation drops sharply, and the content of starch and chlorophyll decreases. In the third stage, berries grow due to the stretching of cell walls and filling them with juice. The fruits reach the size characteristic of the cultivar, while the sugar content in them is only 0.5–0.6%, and the acidity reaches peak values.

The growth intensity of seeded cultivars directly depends on the quality of pollination and fertilization. The structure of the fruit set is influenced by the cultivar: it forms from 1 to 4–5 locules and from 1 to 7 ovules. However, under field conditions, more than 95% of fruit sets are two-locular with two ovules. The influence of the number of developed seeds on the growth of the pulp is presented in the table:

Number of seeds in a berry	Pulp mass gain
One more seed	About 10%
Four seeds	35–40%

In parallel with berry growth, other important processes occur in the bush. The leaves of the lower and middle tiers finish growth and reach their peak of photosynthesis. Shoots slow down in length growth but actively thicken, and their ripening begins from the base to the apex. In the leaf axils, overwintering buds with primordia of inflorescences for next year's harvest are formed, and intensive root branching takes place in the soil.

At this stage, a complex of technological operations is mandatory in vineyards:

• irrigation and root top dressing;
• tying shoots to the trellis and lateral shoot removal;
• protective treatments against pests and diseases;
• cultivation of row spacings and soil loosening in the rows;
• shoot topping — removal of their tips in areas where vine growth has ceased.

Ripening phase: chemical restructuring and sugar accumulation

The transition to ripening is easy to determine visually. Berries change color due to the breakdown of chlorophyll: white cultivars become matte white and slightly translucent, while colored ones develop dark spots. The skin of the fruit softens, becomes elastic, and gets covered with a protective waxy coating (bloom), which protects the harvest from rotting.

During this period, the direction of metabolism changes drastically. While earlier all resources were directed towards tissue growth, now rapid accumulation of carbohydrates begins. In just one week, the concentration of sugars in the juice increases 6–7 times, and the intensity of sugar accumulation significantly outpaces the berry growth rate.

Sugars accumulate mainly in the form of glucose and fructose. At the start of ripening, glucose predominates; by the middle of the phase, their ratio levels out, and by the moment of full maturity, fructose begins to dominate. The acid profile of the juice also transforms: if malic acid predominates initially, by the end of ripening, tartaric acid accounts for 80–90% of total acidity. Starch, initially present in the bunches and pedicels, disappears completely, giving way to nitrogenous and coloring substances, while the content of tannins decreases.

Physiological and technical maturity: how to catch the harvest moment

It is important for a viticulturist to distinguish between physiological (full) and technical maturity of berries. Technical maturity is determined by the sugar and acidity conditions of the juice for specific intended uses of the harvest. At full physiological maturity, berries reach their maximum weight, acquire their varietal color, characteristic taste and aroma, and seeds darken and harden. At this moment, the flow of nutrients to the berries ceases, and the vine redirects the organics produced by the leaves to the perennial parts of the bush and the roots.

If harvesting is delayed, berries begin to over-ripen, raisin, and lose moisture. White-berried cultivars become covered with a golden-yellow tan with dark brown spots. The crop loses turgor, wrinkles, and decreases in volume, which leads to direct losses in total harvest mass. Rains and excess soil moisture at this stage are extremely dangerous, as they cause mass cracking of the skin and delay ripening.

During the ripening of the crop, the following works are carried out in the vineyard:

• regular monitoring of the chemical composition of the juice;
• complete cessation of irrigation;
• soil loosening after every rain;
• continuation of shoot topping;
• testing, mass and clonal selection, as well as preliminary estimation of the harvest volume.

• Optimal ripening temperature — 28–32 °C
• Slowing down of ripening — below 14–16 °C or above 40 °C
• Active temperature sum for the phase — 1048–1644 °C
• Ripening period for early cultivars — 20–30 days
• Ripening period for late cultivars — 60 days

Vine ripening and preparation for overwintering

The sixth phase of the growing season is leaf fall. During this period, shoot growth practically ceases, and all the plant's strength goes into their ripening and preparation for winter. This process is facilitated by an autumn decrease in air temperature to 10–15 °C, sharp fluctuations between day and night temperatures, a reduction in daylight hours, and the first frosts. The rate of vine lignification depends primarily on the vigor of shoot growth, not on the cultivar's belonging to a specific maturity group.

During the ripening process, profound physiological changes occur in the shoot tissues: starch turns into sugars, proteins change structure to protect against freezing, and the proportion of free water decreases. As a result of cork cambium formation, the outer layers of the bark dry out, turning into a dense brown crust. In the Amur grape and its hybrids, this process begins before berry ripening, whereas in a number of Central Asian cultivars, berries ripen long before the start of vine lignification.

Group of shoot internodes	Ripening period, days
Lower (from 1st to 7th)	10–15
Upper (from 8th to 14th)	4–5

The degree of shoot maturation directly determines their resistance to winter frosts. In southern regions, the vine manages to mature along almost the entire length of the annual growth. In zones of cover-based viticulture, due to early autumn frosts, shoots mature only by 65–70% of their length. Unmatured tips inevitably perish from the first frosts, which is why in such areas, bushes are covered for the winter even before the start of natural leaf fall.

Vine tissue type	Limit frost for matured tissues
Buds	down to 25 °C
Shoots	down to 27 °C

If there are no frosts, the subsequent cooling, characteristic of the autumn period, causes the cessation of photosynthesis in the leaves, leading to processes of hydrolysis and the outflow of nutrients. As a result of chloroplast destruction, coloring pigments form in the leaves, giving them their characteristic autumn coloration. In white cultivars, leaves turn yellow, while in dark-colored ones, they turn purple-red with various shades. A dense corky abscission layer forms at the base of the leaf petiole where it attaches to the shoot, and natural leaf fall begins. The period from harvesting until shoot maturation and leaf fall is usually called the reserve phase.

During this time, the following complex of works is carried out:

• completion of the grape harvest;
• deep loosening of the soil with simultaneous application of organic and mineral fertilizers;
• preliminary pruning of bushes and harvesting of cuttings;
• in cover-based viticulture zones, bushes are covered for the winter.

Shoot maturation, leaf fall, and the preparation of the plant for winter (hardening) conclude the final phase of the growing season, after which the plant enters a period of relative dormancy.

The growing season phases of grape cultivars are studied through phenological observations, which allow for the determination of calendar dates for the appearance of specific morphological changes in plants that characterize the beginning and end of each phase. For this purpose, on plots typical for the farm, a small area is designated for each cultivar, or individual bushes in quantities of at least 25–30 are marked with labels and paint, and observations are conducted on each of them or on the group as a whole. The beginning of a phase is defined as the calendar date when signs of the phase are noted in approximately 5% of the bushes; mass entry into the phase is noted when signs appear in 50–60% of the bushes; and the end of the phase is marked on the day when the signs of the phase are lost in approximately 25% of the plants. The data are recorded in the phenological observation log.

The ability of the grape plant to transition from the growing season to the dormancy period is a characteristic biological trait that is maintained during grape cultivation both in open ground and in protected ground (greenhouses, conservatories) and in various ecological conditions — tropics, subtropics, etc. In doing so, primarily only the calendar dates of the plants' progression through the growing season phases shift.

Morphological changes and physiological processes occurring in the annual development cycle of a plant are determined by the change of seasons. The duration of the growing season depends on the cultivar, the grape-growing region, and the meteorological conditions of the year. For very early ripening cultivars, the growing season is short, 120 days, while for late-ripening cultivars, it lasts more than 170 days.

As grape culture moves into northern regions, the duration of the growing season decreases, which is explained by the compensation of lighting intensity, increased day length, and sufficiently high temperatures in summer. In the case of moving grapes into mountainous areas, the growing season also shortens, which is determined by the decrease in air and soil temperatures.

In regions of non-cover viticulture, the duration of the growing season is determined by the number of days from the start of sap flow to natural leaf fall, and in cover-based viticulture — from the beginning of bud break to full berry maturity. When studying the growing season phases of grapes, the average daily air temperature and the amount of precipitation are recorded.

Results of phenological observations of Rkatsiteli cultivar plants in various regions of its cultivation (average data over 5–10 years)

B u d b r e a k a n d s h o o t g r o w t h M a i n p h a s e s o f t h e g r o w i n g s e a s o n

T2el 7av. and 0 (Georgian SSR) K1ir 7ov. ab 0 ad 4 (Azerbaijani SSR) D2er 6be.nt 0 (Dagestan ASSR): 2 Ya 5lt. a 0 (Crimean Oblast)

During the growing season, monitoring the dynamics of shoot growth, their maturation, and berry ripening allows for identifying patterns of growth and development, as well as the plant's reaction to changes in environmental conditions.

The dynamics of shoot growth are determined by measuring the length (cm) of the same normally developing shoots every 10–20 days; the dynamics of shoot maturation are determined directly on the vines by the external coloration of their bark. To do this, every 4–5 days, the same shoots are inspected and the length of their matured part is measured (or the number of matured internodes is counted) and expressed as a percentage of the total shoot length.

The degree of berry maturity in grapes is determined by the quantitative accumulation of sugars and acids in the juice during the ripening process. The frequency of monitoring depends on the current state of the harvest:

• initially — every 3–5 days;
• as the required condition approaches — daily.

Long-term (three to five-year) phenological observations, taking into account the sum of active temperatures for each phase of the growing season, allow for the correct selection of the cultivar assortment corresponding to specific soil and climatic regions for commercial grape cultivation. Based on the data from long-term phenological observations, farms compile technological charts for the maintenance of vineyards and harvesting.

Grapes, like any agricultural crop, are subject to the influence of a large number of various factors, which alter vegetative and generative processes, the productivity of plantations, and the quality of the produce.

Grapes belong to the category of plants that exhibit high responsiveness to changes in environmental factors and cultivation practices. In some cases, changes in product quality are so significant that they determine the choice of specialization in viticulture and serve as the basis for developing types and brands of processed products.