MORPHOLOGY OF THE GRAPEVINE
The grapevine has an aerial part and a root system:
- The roots develop both horizontally and vertically, depending on the soil structure and type, the level of the water table (the first layer of ground water found when digging) and the planting density. The root’s main functions are to anchor the plant and to absorb water and mineral salts.
- The aerial part consists of a trunk, which may be taller or shorter, depending on the pruning and training system used.
. The trunk works as the vine’s support, to store reserves and to communicate the various parts of the plant.
. The trunk branches out into what are known as arms.
. From the arms, herbaceous green shoots grow which eventually lignify (become woody) and turn into canes.
. The vine is a climbing plant and it tends to anchor itself using tendrils.
. The buds, leaves and flowers grow from the green shoots.
. The flowers are grouped into clusters called inflorescences or flower heads.
. Each flower will form a berry: the grape.
. The buds are found along the length of the cane and will lead to new green shoots during the following year.
. At the point where the cane joins the arm, there is a crown of buds that do not usually sprout. Then there is a very short internode (the space between buds or nodes) and after that we find the first bud, which usually does not sprout and is of little importance.
. The leaves sprout on alternate sides at each node.
. Opposite these, in the third and / or fourth node, we find the inflorescences. In the Vitis Vinifera, the inflorescences, or future clusters, appear opposite two consecutive leaves.
Parts of the grapevine shoot:
- Growing Tip: end of the shoot.
- Tendril: Very thin branch used by the grapevine to anchor itself to a support.
- Node: The point on a plant from which the leaves or lateral branches grow.
- Internode: Space between two nodes.
- Grape Cluster: a number of grapes growing close together
- Leaf Petiole: Part of the leaf attached to the branch.
- Leaf Blade: Upper side of the leaf.
- Shoot: A young branch.
- Grape Cluster: A group of grapes.
GRAPE AND CLUSTER COMPOSITION.
Once the grape is ripe, the cluster composition is the following:
- Stems: Between 2% and 8% of weight.
- Berries: Between 92% and 98% of weight:
. Skin: between 7% and 20% of berry weight, depending on the grape variety. The skin holds all the components that differentiate the grape variety: aromas, phenolic compounds: anthocyanins and tannins.
- Pulp: between 65% and 91% of berry weight. The pulp contains the sugars (fructose and glucose) and acids (malic, tartaric, citric and succinic).
- Pips: between 2% and 6% of berry weight. The pips are rich in polyphenols that, in ripe grapes, make a significant contribution of these compounds to the wine.
DEVELOPMENT THROUGHOUT THE RIPENING PROCESS
The grape berry composition undergoes a development process as it ripens. The secret of a grape, regardless the grape variety, is in its ripening. From veraison until the grape is ready for harvesting (according to the oenologist) the composition of the berry develops, and while acid content decreases, sugar content increases. The so-called technical ripeness is achieved when the proportion of sugars exceeds a potential alcohol content of 11-12% in reducing sugars, and the acid content is sufficiently high to provide the wine with balance.
However, other things take place during ripening. The components that characterise a grape variety are not in the pulp, but in the skin. Here are the compounds that will give the wine its colour: tannins and anthocyanins.
To make quality wines it is therefore desirable to achieve skin ripeness (what is known as Phenolic Ripeness) which usually arrives at a later time than Technical Ripeness. Phenolic ripeness achieves the following:
- Accumulation of aromatic compounds and anthocyanins.
- Reduction of tannin astringency and dryness.
- Hydrolysis of cell walls: increased colour extraction.
THE YEARLY GROWTH CYCLE
During each year of its life, the grapevine goes through various stages: when temperatures drop below 12 °C, the vine goes into winter dormancy; the low temperatures prevent practically all plant activity.
a) Bleeding: this is the first external sign of activity after winter rest. As temperatures become milder (10 °C) in early spring, the underground part of the vine begins its activity, pumping sap to the above-ground part. Since the aerial part is still dormant, the sap bleeds out of the wounds left by pruning. This lasts a few days.
b) Budding: This is when bud burst takes place and buds swell as a result of their cellular development.
c) Growth and Development of green shoots: After bud burst, the vine develops miniature organs that go through various stages and eventually become shoots, leaves or tendrils, depending on their location on the trunk.
d) Flowering takes place in late spring or early summer, when average temperatures rise above 15 - 16 °C.
d) Setting takes place in those flowers that are fertilised and turn into berries.
e) Development and ripening. The berries begin to develop after setting.
The berry begins to ripen when veraison occurs. This is the time when the hard, green berry turns translucent in the case of white grapes or begins to turn to colour in the case of red grapes.
Throughout the ripening process there is an increase in the volume and weight of the berry which is accompanied by:
- Reduced acidity: During ripening, acids become diluted because the berry is absorbing a large amount of water and the main acids (malic and tartaric) are being consumed in the grapes’ respiration.
- Increased sugar content: Sugars migrate to the berries from other parts of the plant. Sugar content increases from 10 to 200 g/l at an average daily rate of 5 g/l.
- Increased concentration of colour-giving compounds: Polyphenols (flavonoids and anthocyanins) become increasingly concentrated.
- Increased amounts of aromatic substances: These are essential oils or terpenes located mainly in the skins and to a lesser extent in the pulp. Over 600 different compounds of this type have been identified in grapes.