So what really happens inside the grape vine during photosynthesis and why it is so important for a grape vine to be exposed to enough sunlight?
Photosynthesis in grape vines is necessary for the production of sugar, and sugar based elements that give grapes their colour and flavour. Photosynthesis is a biochemical reaction which combines water and carbon dioxide using the energy of the sun to form sugars in the grape vines. Important to this process are the green chlorophyll pigments in leaves which capture the suns energy. Photosynthesis is the essential first step in the winemaking process, as the sugars formed, along with other chemical elements, are transported to the grape berries and eventually fermented into alcohol to produce wine.

 

Photosynthesis is one of the most important physiological processes that take place inside the vine. A complex process where sunlight energy, is absorbed by the chloroplast inside the mesophyl cells of the vine, and is assimilated into CO2 and carbohydrates (mainly sucrose). Water is needed for this whole process to activate and stay active. All of these carbohydrates are transferred to the endodermic cells of the vine and transported to all the growing shoots and roots, the buds and clusters and some is stored in the vines stem.
Several factors influence photosynthesis: First light intensity - as the optimal photosynthesis in a grape vine, takes place when the sunlight intensity is around 25,000 to 30,000 lux. When the grape vine canopy is too dense, the sunlight cannot penetrate for photosynthesis to take place. That is why you can find yellow leaves inside these types of canopies. Photosynthesis can take place from reflected sunlight, but the intensity of light is normally not high enough for optimum photosynthesis. Having a dense or compact grape vine canopy will reduce its productivity. Canopy control is of utmost importance when growing grape vines.
Temperature: The optimum temperature for photosynthesis is between 25 - 28°C. It has been found that when the temperature goes below 20°C and above 30°C, the rate of photosynthesis drops dramatically. Though many other factors; influence successful photosynthesis.
Moisture: A grape leaf must have at least 75% - 85% moisture to be biochemical active. During photosynthesis, water is needed for the whole process to take place. The optimal soil moisture, where the vine grows, is very important for photosynthesis. The humidity of the atmosphere around the vine influence the effects the rate of photosynthesis - a higher humidity at the same light intensity has a higher rate of photosynthesis.
CO2 and O2 concentration: Normal CO2 levels in the atmosphere are around 300ppm. The rate at which photosynthesis takes place, increase as the CO2 levels increases up to a point when it reaches 500ppm: where it slows down and stops at a rate of +- 1000ppm. Just as the opposite happens when the concentration of O2 (oxygen) increases above 21%.
Internal factors: The leaf of a grape vine starts to photosynthesize at an early stage, but only until it is two-thirds the size of a mature leaf, as it is self-supporting. The secret lies in the energy that was stored in the roots and stems during the previous growing season. Viticulturists must make sure that leaves of a young grape vine reach at least two-thirds of its mature size as quickly as possible, before stored energy inside the vine is replenished. The need for photosynthesis increases as the size of the crop increases; so there is a need to control the crop size according to the growth of the vine. A young vine with a small canopy and a large crop will not be able to produce enough carbohydrates to fully ripen the harvest.
Genetic factors: Different varietals react differently to the exposure of sunlight; resulting in different levels of photosynthesis. The opening and closing of the stomas of different grape vine cultivars are not the same, and this will influence how much moisture is available for photosynthesis - less moisture, less photosynthesis!
Cultivation: The more vine leaves are exposed to direct sunlight, the better the photosynthesis will be and hence the need to design an appropriate trellis system. Row direction, pruning, vine distances and irrigation or the vine (if permitted) all play an important role. During photosynthesis, a key focus for viticulturists - do not let the vine grow out of hand. A dense vine canopy is an unhappy vine; and it will not produce physiologically ripe grapes.