|Annual Cycle of Growth
Like most other plants, the grapevine has a fairly predictable cycle of growth. This chapter discusses the various growth stages beginning with the dormant season.
This season begins in autumn in temperate regions when the vine sheds its leaves and enters the dormant period. There are some grape growing areas in the world, such as the subtropical climate of southern India, where grapes do not shed their leaves naturally. In such areas it is necessary to induce a type of dormancy by stopping growth for a time to get new shoots and good crops. Usually all leaves are removed by hand and often heavy root pruning and withholding of water are practiced. In some regions this must be done twice each year.
In the winter temperate regions, much of the starch is converted into sugars that protect the vine against low temperature injury. Near the end of winter or the beginning of spring the vine may exhibit the phenomenon of bleeding. If a cane is cut, liquid flows from the xylem tissues. More than a gallon can be collected from one cut cane. If the cane tips of a vine are cut off every other day, from 5 to 7 gallons (18.9 to 26.5 liters) can be collected (page 32) (winkler et al., 1974). Bleeding has no harmful effect on the vine.
Dormancy can be divided into periods of quiescence and rest. The first type of dormancy is under exogenous control, in which buds fail to grow because of unfavourable external conditions. Rest is under endogenous control, in which internal factors prevent growth despite favourable environmental conditions (Weaver, 1972). During the period of rest the inhibitor-promoter balance of hormones is weighed in
|favor of the inhibitor, but at termination of rest the balance is shifted in favor of the promoter. Bud rest can be broken by cytokinins, heat cold or desciccation. Rest is prolonged by gibberellin.
Grape seeds are in a resting condition at fruit maturity. The usual method of terminating rest is to stratify the seeds at about 400F (40C) for 3 months. Gibberellin can terminate rest.
Bud Break to Bloom
In the spring, when the mean daily temperature reaches about 500F (100C), the buds begin to swell and the green shoots emerge from them. This is commonly known as bud break. The shoots grow rapidly in length and thickness, and leaves, tendrils, clusters, and new buds in the leaf axils develop rapidly. As the daily temperature rises, shoots often attain a growth rate of an inch per day. Blooming usually occurs around 8 weeks after bud break, but the interval depends upon the weather. Bright warm weather brings on blooming earlier than cool rainy weather and reduces its duration. Rapid shoot growth in length usually begins to slow down by bloom time (Fig. 3-1).
The process of flower initiation for the following year?s crop begins before bloom, and the development continues until about harvest time (Fig. 3-2).
|Bloom and Fruit-set
Bloom is the period when the caps (calyptras) fall from the flowers. Since bloom may progress for several days over a vine and an individual cluster, one must estimate the percentage of cap fall to designate the stage of development. Many grape growers consider full bloom that time when an average of 50 percent of the calyptras have fallen from the flowers.
When the calyptras fall from the flower a cloud of pollen is released from the anthers of the stamens which move away from the pistil. Pollen grains fall on the stigma and germinate if conditions are favourable. A pollen tube grows down the style to the embryo sac and serves as a pathway by which two sperm reach the embryo sac. One sperm then unites with the egg cell to form the zygote from which the embryo plant develops.
In cold rainy weather calyptras may not fall from the flowers. These persistent calyptras often reduce the amount of fruit setting.
Several days after the bloom period, pistils and impotent berries often 50 to 60 percent or more shatter from the cluster. Many of these pistils have not been fertilized. This shatter of berries is important because it keeps clusters from becoming too compact. On the other hand, some varieties of clusters may set poorly and have many small, seedless berries that fail to enlarge (shot berries), a condition known as millerandage. Frost or rainy weather at bloom time may cause shot berry formation in clusters that would otherwise set well. The amount of set varies from season to season. A normal setting cluster of a seeded variety often has a mixture of berries containing from 0 to 4 seeds. A direct relationship exists between number of seeds and berry size: the greater the number of seeds, the larger the berry. This is because seeds produce gibberellins and other hormones that diffuse into the flesh of the berry and stimulate growth.
Some varieties set fruit without fertilization, a process known as parthenocarpy. Black Corinth usually require only the stimulus of pollination. In other seedless varieties (Perlette, Thompson Seedless, Black Monukka) fertilization occurs but the embryo subsequently aborts, a process termed stenospermocarpy. Other varieties, such as Chaouch, produce hard seeds that are hollow or empty.
The berries that do not fall from the cluster after bloom are said to have set. This referred to as the fruit set stage. Fruit set and development are probably controlled by an interaction of naturally occurring hormones including auxins, gibberllins, cytokinins, ethylene, and inhibitors (Weaver, 1972).
|The explanation for the slow growth period (stage II) is unknown at present. Perhaps the osmotic pressure resulting from accumulation of sugars in the berry effects the beginning of Stage III, since the movement of water into the fruit causes cell enlargement and growth (Coombe, 1960). Ethylene may initiate the growth that occurs in stage III (Maxine and Crane, 1986).
Seedles (stenospermocarpic of parthenocarpic) berries usually show less distinctive growth periods than do seeded berries. Generally they exhibit little or none of the double sigmoid growth curve.
|Vine Growth Processes
The growing vine increases in bulk and complexity. Vine growth can be defined as an irreversible increases in sizes as measured by an increase in its dry weight. It reflects a net increase in protoplasm, the living substance in the vine. Vine development refers to differentiation and anatomical and physiological organization and specialization.
This process involves the use of light energy by the green leaves to convert carbon dioxide and water into energy rich compounds. All organic matter in the vine is ultimately provided by photosynthesis, one of the most significant of all life processes. Photosynthetic reactions occur in the chlorophyll containing plastids found in green leaves, and the reaction can be summarized as follows :
673,000 calories of flight energy
6 CO2 + 12 H2O + in presence of chlorophyll, enzymes, --> and cofactors
C6H2O6 + 6 O2 + 6 H2O
This chemical equation shows that 6 moles of carbon dioxide, 12 moles of water , and 673,000 calories of energy yield 1 mole of glucose, 6 moles of molecular oxygen, and 6 moles of water. The process is , of course, far more complex than this equation indicates. From the sugar produced, other compounds found in the vine are metabolised.
Carbon dioxide is normally found in the air at a concentration of 0.03 percent, and it enters the leaves through small holes called stomata (pp.17). In confined areas it is possible to increase vine production by CO2 enrichment of the air (Kriedemann, private communication). The rate of photosynthesis increases with light only to a certain intensity. At this point the leaf is said to be light saturated. In grapes this figure ranges from about 2500 to 5000 footcandles (ft-c) when grown under favourable conditions (Kriedemann and Smart, 1971). As the amount of light is reduced to about 125 ft-c the compensation point is reached, where the amount of food manufactured in photosynthesis is just equal to that lost by respiration (Kriedemann, 1968). With further reduction in light, the leaves can be said to be parasitic on the rest of the vine. Usually the outer tier of leaves absorbs around 90 percent of the light used for photosynthesis, leaving only 10 percent for use by the other leaves.
On a clear day, when leaves in the sun are at light saturation, there can be 12,000 ft-c of light. Since leaves shaded by other leaves will be below light saturation and may thus not be working at maximum capacity, as many leaves as possible should be in direct sunlight for greatest efficiency. This can best be attained by using proper training and trellising to give the most leaf exposure.
A high light intensity is also of value because more fruitful buds are produced. Vines grown in shade often produce little fruit in the subsequent year (May, 1965).
Grape leaves reach maximum photosynthetic activity when they attain full size (Kriedemann et al., 1970), about 30 to 40 days after unfolding; afterward the photosynthetic rate gradually declines. High temperatures inhibit photosynthetic. Above 860F (300C) the rate decreases and practically ceases at 1130F (450C) (Buttrose, 1968; Kliewer et al., 1972). Optimum leaf temperature for maximum photosynthesis ranges from 770F (250C) to 860F (300C).
The stomata open in the morning when light strikes the leaf. The brings about an increase in sugar concentration that causes an increase in the osmotic pressure within the guard cells, and water then flows into the guard cells causing them to swell and become turgid. The thin peripheral wall bulges outward, pulling the thicker elastic wall along with it. Stomatal closure can result from lack of water in the soil, a high rate of transpiration, or low light intensities, or darkness. The growth inhibitor, abscisic acid, may play an important role in stomatal closure. Vines that will close their stomata; as a result CO2 uptake is reduced, causing a reduction in photosynthesis. Leaf water potentials less than about 15 bars decrease the photosynthetic rate (Kriedemann and Smart, 1971).
Carbohydrate nutrition is the study of metabolism of sugars and starch, and mineral nutrition is the study metabolism of nutrients absorbed from the soil. The study of metabolism of nutrients concerned with soil chemistry and biology.
Water is the most abundant compound in an active grapevine cell, and in actively growing tissues water comprises from 85 to 95 percent of mainly of organic matter derived from photosynthesis, and from inorganic nitrogen such as that found in ammonium or nitrates. The amount of mineral matter taken up from the soil is relatively small but are not lost in large quantities except at leaf fall in autumn. Water loss occurs constantly form from a living vine is called transpiration. Although most diffuses out through the stomata, some may be lost directly through the leaf cuticle. As water molecules evaporate from the wet cell walls inside the leaf, the wall imbibes more water from inside the cell, and the cell then gains water by osmosis from adjacent water saturated cells. These cells in turn gain water from adjacent cells and, finally from the water filled tracheid of a vein ending. Thus a water potential gradient is established from the vein to the outside air. The rate of transpiration is affected by several factors including air movements, relative humidity of the air, air temperature, light intensity, and soil conditions.
Water moves upward from the roots through the trunk and shoots, and then to the leaves from which vapor escapes to the atmosphere. A plausible theory about the processes involved in the ascent of water is known as the transpiration pull and water cohesion theory. The columns of water in the xylem are pulled by the force of water evaporation from the leaf cells and imbibitional forces that develop there. The columns of water maintain their continuity because of the strong cohesive forces between water molecules and adhesive forces between the cell walls and water molecules.
|The usual mechanism of water absorption by roots results from osmotic movement of water into the roots. When soil solutes accumulate in the cell sap, water movies into the cells from the water in the soil solution. Under a high transpiration rate there is a water deficit in the plant tissues, and water moves into the root passively. Given these conditions, even dead roots can absorb water. When vines are subjected to conditions that cause rapid transpiration, the vine are subjected to conditions that cause rapid transpiration, the vine may show some temporary signs of water deficit and wilting since water absorption lags behind water loss despite available water in the soil.
Absorption of Nutrients
Nutrients or solutes may enter the root cells from the soil by diffusion, which is movement from a location of high solute activity to one of lower activity. Root cells can however, accumulate nutrients in their vacuoles in concentrations far higher than that in the soil solution. This movement of nutrients from a region of low concentration to one of high concentration is termed active solute absorption or accumulation. Energy supplied by respiration is necessary for the movement of nutrients against a concentration gradient.
Movement of most mineral nutrients occurs in the vessels of the xylem and it is usually upward. Salts may also be conducted in the phloem. The main path of food movement is through phloem. The main path of food movement is through the phloem. The movement of assimilates is usually from a source (region of manufacture) to various sinks (regions of utilization). In the grapevine mature leaves are the main source for assimilates is usually from a source (region of manufacture) to various sinks (regions of utilization). In the grapevine mature leaves are the main source for assimilates, and shoot tips, roots and clusters are the major sinks (Hale and Weaver, 1962).
The source and sinks and their power change during the growth season (Fig. 3-4). A young leaf is a sink since it only imports assimilates, but when the leaf is about half its full size it begins to export food materials to both the tip of the shoot and also downward. After fruit set, berries become a powerful sink.