Thirty-five years ago, the National Agricultural Research Institute (INRA) of Avignon developed a sensor that is attached directly to the plant: the Pépista®. A number of studies confirmed the benefits of micro-morphometric measurements for detecting not only plant water stress but also excesses of water. Following on from the work carried out by INRA over the last 25 years with the farmers, the company Agro Ressources, in collaboration with different technical organisations, has developed a method of scheduling irrigations by taking into account climatic factors and measurements for soil and plant water stress.
MONITORING THE WATER STATUS OF THE PLANT WITH THE PEPISTA® SYSTEM
The work undertaken by the National Agricultural Research Institute (INRA) in Avignon has shown that the Pépista sensor is a very good indicator of the plant’s water status because it directly measures the water stored in the tissues and their growth rate. The principle of the sensor is to measure the diameter of a branch or stem using the micro-morphometric method. This method can be used on all species of plants. There is a small reserve of water in the bark. During photosynthesis the plant draws water from this secondary reserve and the stem deflates during the day and reinflates at night when the sun goes down. These swelling and shrinkage movement take place every day. The sensor measures the diameter of the bark, and by default the range of these movements mentioned above. This range increases in the spring and is at its maximum in the summer.
If there is not enough water or there is water stress, the plant is unable to rebuild its reserve. If the branch stops growing in diameter for one day, then this could be a climatic anomaly, but if this should happen two days in a row, then this would be a case of water stress.
Irrigation must then be started so that the stress does not last for a long period of time.
This method is essentially used in fruit-tree growing and in vineyards. In fruit-tree growing, the fruit size is important. It only needs one week of water stress for fruit size to be badly affected and for below-grade fruit to be produced. The same applies to grape vines, particularly those used for white and rosé wine. On the other hand, as far as red wine is concerned, a little water stress does no real harm
MEASURING THE WATER STATUS OF THE PLANT BY USING WATERMARK-TYPE ELECTRIC TENSIOMATERS
The climatic data mainly used are ETP (Evapotranspiration) and rainfall. For this climatic approach, the crop coefficient value of the plant needs to be known: Kc for ETP and Ko for the Evaporometer, which requires references or quite onerous experiments.
Measuring the soil water status with an electric tensiometer of the Watermark type is a technique that can quite easily be used in the field.
It requires six sensors to be placed on a reference plot and allows for a bona fide scheduling of the irrigation by taking into account changes in the soil moisture tension. The tensiometer often allows for a 30% saving on water compared with a system that has no scheduling tool.
Tensiometry = measurement and analysis of changes in the forces linking the water to the soil (“tensions”). They can be directly interpreted as the soil water available to the plant.
The values vary between 0 and 200 centibars: Tension = 0cb: saturated soil Tension [50-80cb] ≈ adequate and readily available soil water content (depending on the soil).
Tension = 200cb: dry soil
The apparatus used to measure the tensions includes: 6 Watermark® sensors + a Monitor data logger.
In the plot, 6 sensors are positioned on 3 vines and at two depths (40 and 80 cm for example) so that mean values can be used as a reference for scheduling.
Plant measurements taken with the Pépista can effectively supplement the soil measurements: the device measures the increase in the diameter of the stem, branch or fruit (hundredths of a mm) and allows the user to ascertain the plant’s water status. By linking the tensiometric measurements to the Pépista measurements, the method enables the farmer to detect the optimum soil moisture tension thresholds for a given variety and according to a given soil type.
This measurement made directly at the plant also allows for the precise crop coefficient value to be calculated and applied to that of the ETP. Agro-Ressources also provides meteorological data by installing meteorological stations on site.
Agro-Ressources also provides meteorological data by installing meteorological stations on site. All of the information obtained is crossreferenced, allowing the producers to be best advised as to achieving optimum irrigation of their plot of land.
Agro-Ressources monitors 100 plots in total, mainly in the South-east of France, in the fruit tree and grape vine sectors. The aim is to reduce the amounts of water applied and to see just how far the farmer can go in terms of saving on water.
In orchards: increasing the fruit size, while maintaining eating quality In vineyards: look for a stress threshold in order to obtain an optimum quality, while preserving the vine stocks. After planting: ensure that the plants have a good start with a well-established rooting system.
Each week, Agro-Ressources provides its member farmers with advice on the basis of information obtained: The farmer is sent technical information adapted to each plot and relating to the irrigation to be carried out:
• At the beginning of the season: when to start irrigation?
• During the season: how much water to apply? And how often?
• At the end of the season: when to stop applying water? And when there is a problem in the field, the farmer is advised.
With its experience gained over the years, AgroRessources has developed genuine expertise in the irrigation management of different crops