The contribution of water to vine growing, i.e. irrigation, is causing much debate both against and in favour of it. In our view, the opinions expressed on this subject show very often a certain degree of frivolousness, as if the aim of irrigation was merely focusing on producing quantity but not quality. We believe that these judgments are much too simple.

Let me tell you why we irrigate.  

The vine is formed by multiple cells. The interior of these cells is called intracellular space. Each cell has its intracellular space and the whole of all these cells is defined as internal medium.

It is in this internal medium where all the chemical reactions of the cells take place. We call this totality of chemical reactions cellular metabolism.

In this context, the presence of three basic elements is required so that a chemical reaction can be carried out:

• The reagents, i.e. A and B; (element A reacts with element B producing element C).
• The  catalyst, which induces the accomplishment of the mentioned reaction (the catalysts have their origin in the genetic code).
• The aqueous medium, where reagents and catalysts are dissolved.

It is in this last section where water comes in as solvent. Without its presence, these reactions could not take place and the plant would die.

The water used as solvent and support for cellular metabolism is not consumed, it does not disappear in time, it is not wasted. The plant consumes water only during transpiration.
 
The properties of the plant to adapt to the environmental conditions provide it with the capacity to live in this environment. However, this capacity does not guarantee, in any of the conditions, that the plant can produce fruit that meets the requirements for elaborating quality wine.

Our demand for the best features of the fruit produced by the vine corresponds to some very specific environmental conditions. If we leave those conditions to random chance, they may possibly appear and thus let us obtain the adequate fruit to elaborate the ideal wine.

However, man does not have the mission of a passive observer with regard to his work. Man is an intelligent being capable of understanding the phenomena of nature. His work of study and observation leads him to intervene in the cultivation process in order to improve the environmental conditions so that grapes of high quality can be produced every year. The intelligent actions of man, which correspond to his effort in increasing the knowledge about the environment, must be appreciated as collaborative actions in the evolution of the ecosystem. 

Another important function of water in the life of the vine is the body temperature control. When the temperatures of the environment fall to certain limits, on which the metabolic reactions of the plant depend, the plant starts to transpire. Through transpiration the plant cools down and its body can maintain a temperature equal or inferior to air temperature.

Let’s have a look at the results of a study we carried out at the beginning of the nineties with the "Scholander bomb", sensors of soil moisture and thermocouples in order to observe the behaviour of vine at different stress levels:

• We measured the hydric potential in the leaves with the “Scholander bomb" to be able to know their stress level according to the water availability at radicular level. We measured daily the soil moisture with humidity sensors and the leaf temperature with thermocouples.
• We selected a series of plants, with a clear hydric deficit. We took measures every day: in the morning, at midday and in the afternoon. Our goal was to observe the influence of water stress on the leaf temperature:
  • Morning (until 10 a.m.).- There is no difference between leaf and air temperature. We will probably find the reason in the nocturnal recovery, because the vine does not need to cool down and this way it recovers from diurnal dehydration. In this situation the leaf temperature remains equal to the air temperature.
  • Late morning – afternoon (between 11 a.m. and 4 p.m.). This is the moment when the leaf temperature can be higher than the air temperature. It is the hottest moment of the day showing the lowest relative humidity possible. It is precisely in these hours when the plant needs water most, in order to withstand the heat, to cool down and maintain the temperature at an adequate level preventing so negative effects on its metabolism.
  • Evening (from 5 p.m. until sunset).- At this time of the day, in El Priorat, the sea breeze comes in and temperatures drop, the relative humidity increases and there is no difference between leaf and air temperature. In those moments transpiration is reduced considerably and the consumption of water keeps going down until it comes to a complete stop.

From this experience and other observations carried out, we can define certain reactions of the plants in specific situations with regard to soil moisture:

• High and constant humidity.- The plant synthesises growth hormones, the vine shoots do not stop growing and the big size grapes ripen only slowly. Low quality.
• Adequate humidity to hydrate the plant, but without growing.- In these circumstances the vine shoots do not grow, the plant uses only the water needed for transpiration. The movement of sap, which circulates through the vessels, is used for the sugar transport from the reserve parenchymas towards the grapes.
• Low humidity around the roots.- The plant is in a situation where the environmental water supply to the roots is inferior to the leaves’ needs to regulate its temperature by transpiration. In such circumstances the plant will respond in the following way:
  • Photosynthetic blocking. Photosynthesis is inhibited from 36º to 38º C. (When the vines can not be cooled down and the climate is very hot the leaves show temperatures between 6º and 8 ºC higher than the air temperature).  The vines are exposed to the sun, but will not be able to carry out the synthesis of sugars. On the contrary, they consume them, taking them form their reserves, in respiration.
  • Elimination of the basal leaves of the vine shoots. These turn yellow, due to the constriction of the vessels that transport the sap and fall off, thus losing considerable leaf surface.
  • Dehydration of the grapes. Before coming to the polyphenolic ripening, during many summers, the grapes crease up, seem to be over-ripe, but this is not the case, they are dehydrated and lose their qualities necessary for elaborating a quality wine.

Let's remember again that the vine uses water for two main functions:

• Means - water acts as support and solvent for the vital chemical reactions.
• Transpiration - water is used for the adaptation of the plant to the heat.

It is clear that the plant needs water. But when? How much? Only by observing and studying will we be able to get to know this better.

Josep Lluis Perez