Manual of techniques for sustainable mountain viticulture Josep Lluís Pérez Verdú

Therefore, a shortage of transpiration water (hydric stress) has two harmful effects: plant dehydration,

with the loss of leaf area and grape berries and a reduction in photosynthesis activity with, as a result,

less available sugar production for ripening. 

However, when there is excess extracellular water in relation to that required for transpiration (more

supply than demand), the plant synthesises growth hormones and uses the sugar from the reserves to

grow. This must take place during the spring months when the plant must develop all its growth and

form ELA, although it must be avoided from early July until the grape is harvested, as described in the

following section. 

3.3.2 Need for irrigation water throughout the vegetative cycle

For the plant to correctly develop its vegetative cycle and for all the grape berries to reach the appro-

priate ripeness, the amount of water available to the plant is essential and must be controlled at all

times: 

•

During the spring months, the plant needs water to grow. If it does not rain, water must be provided

through irrigation. Too much water during this period has no significant negative repercussions,

except for the wasting of a scarce resource. A lack of water would prevent the plant from expressing

all its vigour and reaching the required architecture. If some shoots do not grow enough, the ELA

will not be entirely formed and, as a result, will not attract the necessary solar energy to ripen all

the target grape production correctly. Furthermore, it will be more vulnerable to an episode of

heavy rain during ripening (late August and September). 

•

During the month of June, the plant must reduce its growth rate to stop completely early July.

Growth is no longer necessary because the required architecture has already been formed and

because it is not foreseen in the means of controlling the plant. Were it to continue growing, the

shoots would double in size and the ELA would not increase. The sugars synthesised by the chlo-

rophyll function must be accumulated in the reserves to await mobilisation for grape ripening

during August and September. Irrigation must be reduced until the soil moisture is sufficiently dis-

tant from the field capacity, so that he plant can transpire to cool itself and continue with the chlo-

rophyll function to accumulate sugar reserves, although without synthesising growth hormones

that would lead to the sugar being used. 

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•

This humidity must be maintained during the hotter months (July and August) and, therefore, the

amount of irrigation required becomes more important: The plant needs the right amount of water

- no more (growth in detriment to sugar reserves for ripening), no less (hydric stress and plant

dehydration, with the loss of ELA when the humidity of the root soil drops to levels close to wilting

point). However, excessive specific rainfall in July or early August does not necessary affect the

grape quality, as the plant still has time to redirect its metabolism and accumulate enough sugar. 

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Soil moisture, field capacity and wilting point

Soil moisture is defined as the weight of water present in the soil per unit weight of dry soil.

Where:

Msw: moist soil weight

Dsw: dry soil weight (dried in oven to 110ºC)

Therefore, moisture M (in %) = (Msw- Dsw) /Dsw * 100

After heavy rain, soil may be saturated with water (all its pores are full of water) to a certain depth.

When the rain stops, part of this water (that contained in the larger pores) drains by gravity to

lower layers at a speed that depends on the permeability of the soil. Once this infiltration has

occurred, the moisture level remaining is known as the field capacity. Therefore, the field capa-

city is the maximum moisture the soil can retain once all the gravitational water has migrated to

the lower layer, which is why it is also known as the water retention capacity.

Where no new water is soaked up, the soil continues to lose water by evaporation and by plant

transpiration (in this case, vines). Water absorption by plants becomes more and more difficult as

the soil moisture decreases, until the plant can no longer absorb any more water because the

force of the particles and soil salts is greater than that the roots can apply. The soil moisture at

this time is known as wilting point, i.e. the level of moisture when the plants can no longer absorb

any more water. 

In the Priorat “Licorella” slate soil, the field capacity and wilting point are around 17% and 7%,

respectively.

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•

The situation becomes critical when the plant is concentrating on ripening (late August and

September), given that there is no enough time to redirect a change in metabolism. The plant must

direct the sugar towards berry ripening. Too much water would be counterproductive, although not

because the accumulation of sugar would be stopped or because of a grape compacting effect

(morphology is determined with the distribution of vigour and there is no longer cellular multiplica-

tion in the grape), but because the change in plant metabolism towards growth would leave ripe-

ning unattended (the activity of the hypodermic cells of the grape berry responsible for using solar

radiation to transform the sugar in polyphenols, tannins, aromas and colour, etc.) would be reduced,

with negative effects on the quality of the wine. It is then that control of the situation becomes more

important. This is achieved using two basic mechanisms: 

- The soil moisture, which remains sufficiently distant from the field capacity, must be concentra-

ted as much as possible within a section of 15 cm around the roots of the stock (which is achie-

ved by using underground irrigation, as described in Section 3.3.4). Hence, the soil can absorb

any heavy rain at its lower layers and the soil moisture prior to the episode of rain is re-establis-

hed in a few days. Irrigation during this period must be as precise as possible. 

- The high ratio between the effective leaf area and the root soil volume of the stock (ELA/RSV) in Mas

Martinet architecture is of the utmost importance here (Figure 3.5). In fact, the need for transpiration

water (demand) per unit of supply in the roots is large enough to absorb a specific increase in soil

moisture following heavy rainfall. Under these conditions, the risk that excess water be generated

and the plant’s metabolism change towards growth is much lower than in a plantation with a com-

paratively weaker demand. In general, greater demand per unit of supply provides greater control

over the plant at all times, i.e. with greater dependence on the fate of the weather. 

In synthesis, the plant must have the water it needs: If it has too little water, then it must be supplied

(irrigation) and if it has too much them it must be possible to effectively dissipate it. Managing the

water available to the plant means that its vital functions can be controlled: 

•

Growth control (the plant expresses all its vigour and forms the foreseen architecture). 

•

Hydric stress control: the necessary sugars are accumulated for ripening and the plant remains

hydrated to keep its ELA in good condition and ensure the grape berries continue to develop.

•

Ripeness control: Changes in metabolism due to too much water are avoided and the grapes

obtain the appropriate quality and quantity of polyphenols, aromas and likely alcohol content.

It can be said that the time of irrigation is just as important or even more so than the amount of water

provided.

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