To understand the mechanisms underlying viticultural practices, it is important to understand some basic vine physiological processes and how they can be manipulated.
Grapevines, like all plants, make sugar, which is used to supply energy for chemical processes and as a substrate for the synthesis of all components of the vine and fruit. Sugar is also converted over the growing season into starch, which is stored and invested in the protection of buds, roots and woody tissues over winter and used to support initial growth the following year.
Sugar is made in the leaves through photosynthesis, which is a multi-step chemical process that requires sunlight, CO2 and water. For CO2 to move into the leaves, the stomata (pores) on the leaf undersides must be open to allow for water vapour to be transpired. Nearly all of the water taken up by grapevines is transpired.
The process of transpiration cools leaves and creates a stream that carries minerals and other chemicals from the root system via xylem in the trunk and woody branches (cordons and spurs) to shoots, leaves and fruit. The remaining small portion of water taken up is used in photosynthesis and other chemical processes and as the solution solvent in cells and a medium for chemical reactions.
Water, under pressure in cells, is also needed for expansive growth. Therefore, water supplied to vines plays a critical role in sugar production, growth, the movement of minerals from roots to shoots and as a major component of plant tissues.
Respiration is the process that releases energy from the chemical bonds in sugars and compounds made from sugars. Malic acid is such a compound and is respired especially during the latter part of berry development.
All live plant tissues respire, and the process requires oxygen. For roots, oxygen is provided from air in the soil pore space, thus water-logged or highly compacted soils can be harmful or lethal to grapevines.
Nearly all chemical processes in plants, including the steps in photosynthesis and respiration, are catalyzed by special proteins called enzymes. Enzymes make up most of the protein in grapevines, and some of the most abundant enzymes are those involved in photosynthesis.
A major mineral constituent of all proteins is nitrogen. As enzymes are important for all physiological processes in grapevines, nitrogen supply is of critical importance to grapevine growth and development. Other mineral nutrients are also essential as constituents of enzymes and other plant compounds. Examples are:
When these and other essential minerals are deficient, plant growth and development is hindered. Most or all of the vine’s mineral nutrients are taken up from the soil by roots. Small amounts of mineral nutrients, including nitrogen in specific forms (i.e. as urea, but not as N2 gas), may be taken up by leaves.
Plant hormones are compounds produced in one part of the plant that trigger physiological changes or regulate physiological processes in another part. These signaling compounds are important in determining budbreak patterns, shoot growth, leaf function and many other processes in grapevines.
Auxins are plant hormones that are produced in shoot tips and are transported away from this site where they have many effects, depending on their concentration, from stimulating elongation of shoots and roots to inhibiting bud development. Gibberellins are produced in several tissues and cause shoot elongation and are involved in breaking dormancy. Cytokinins are also produced in several tissues and stimulate cell division and prevent senescence.
Ethylene is a gaseous hormone that is stimulated by auxin and may be involved in auxin-induced effects. Ethylene is produced in leaves, where it triggers senescence, and in fruit, where it may be required for fruit ripening.
Abscisic acid (ABA) is involved in inducing dormancy, in the abscision of leaves, and in regulating stomatal conductance. Recently, abscisic acid was shown to influence important aspects of berry development including production of anthocyanins.
