In water stress situations, plants set up adaptation mechanisms to rebalance their water status, at the expense of a fraction of their metabolism. The widespread lack of water in France does not yet affect their potential, but a hot period could, however, cause damage to crops.

Moisture stress in straw cereals varies greatly from sector to sector


A spring drought can affect the number of grains/m2 and the weight of a thousand grains.

The impacts of water stress on yield components are multiple and highly dependent on the stage of the crop.

Firstly, a lack of precipitation in the early run can induce nitrogen deficiency, even if the crop is not really under water stress. This is what happened this spring, with nitrogen inputs in the second half of March regularly poorly valued. This will lead to slower growth, a regression in pruning and, if the deficiency is prolonged, to a reduction in ear fertility via the regression of the lower spikelets. In this case, it is the number of grains per square metre that is affected.

In a second step, when the plant actually goes into water stress (i.e. the evaporative demand is not satisfied by the water taken up by the roots), the water status of the plant drops.

Champ de vignes
Drone d'intervention viticulture
Champ vignes

In a second step, when the plant actually goes into water stress (i.e. evaporative demand is not met by water taken up by the roots), the water status of the plant drops.

This has two consequences: a reduction in foliar expansion, and a decrease in transpiration through the closure of stomata. The final foliar index can therefore be penalized if water stress is expressed during upwelling, even if the water supply is later restored.

The closure of the stomata, on the other hand, will progressively slow down photosynthesis, and possibly cause heating of the tissues if the radiation is strong and the temperatures high. In this case, the reduction of the flowering biomass and of the plant's photosynthesis capacity during grain filling will penalize the thousand grain weight (MGW).

Concern for shallow soils, but little concern for deep soils yet

Today, practically all cereals on topsoil are in a state of water stress, whereas all barley reaches flowering or the beginning of filling, and wheat is spiked south of the Loire, and will soon be spiked in the north.

It is generally considered that from a cumulative water deficit of 40 mm, the impact on yield is systematic and significant. The most superficial plots have therefore often exceeded this value a long time ago, and will not be able to catch up to their initial potential even if the rest of the season becomes favourable again (Figure 1). For the deepest soils, this situation is also found in Poitou-Charentes, on the Mediterranean rim, in the south of the Rhône corridor as well as in Alsace and Lorraine (figures 2 and 3).

Hydric stress (viticulture): reaction of the vine to lack of water in the soil. How does the vine react to periods of higher than normal temperatures? This symptom of scalding, which can alter the colouring of the grapes, results in a reduction in the activity of the leaves that are most exposed to the sun (beware of those who have carried out too severe leaf thinning!). Given the high water stress, some vines may indeed suffer natural defoliation (summer leaf fall leading to a slowing down of the feeding of the grapes themselves). The result is atypical vintages, with wines that are sometimes emaciated, and others that see an outright change in their typicity.

The consequences of water stress

If water stress can lead to a notorious harvest deficit (up to 40%, especially during the heat wave of 2003), it brings its share of difficulties for the winegrower:

An increase in potential alcoholic degrees;
Decrease in total acidity, a support that is nevertheless essential for the longevity of a vintage. It is then necessary to consider reacidifying with tartaric acid, at the risk of making the wine unbalanced or angular;
Increasing the pH* with the consequences, the development of a wider spectrum of micro-organisms. Thus, with an increase in potential alcoholic degrees, bretanomyces will be favoured to the detriment of saccharomyces, the natural yeast of alcoholic fermentations. The high pH also favours the development of bretanomyces to the detriment of lactic acid bacteria during malolactic fermentation. Finally, it reduces the active part of SO2 which guarantees the microbiological protection of wines;
Disruption of the synthesis of phenolic compounds depending on the year;
Staggered evolution of maturity criteria with decoupling of alcoholic, aromatic, phenolic and tannic maturities.

* It will soon be possible to lower the pH (acronym meaning hydrogen potential and which represents the measure of alkalinity) by subtractive technology using electromembrane processes (currently being tested) which allow the extraction of cationic charges from wines, the cause of excessively high pH. The oenological rules for these processes have yet to be established in Europe.