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Hydric stress: what can be done?
Italy, Spain, California: several major regions have been hit this year
Field-grown tomatoes are a long-season crop and their cultivation needs water inputs in regular and large quantities. An average variety requires approximately 40 cm of water during the growing season, with humidity needs increasing progressively until the fruit load reaches its maximum development. The crucial stages at which the plant requires water are the blossom stage, fruit set and fruit development. Weather conditions in the regions where processing tomatoes are grown can sometimes be extreme, both in terms of temperatures and in terms of water availability. When this happens, plants are subjected to hydric stress.
Potentially devastating effects
According to the definition used by biologists, hydric stress, or osmotic stress, is "the stress that the plant undergoes in environmental conditions where the quantity of water transpired by the plant is higher than the quantity that it absorbs". This stress is characteristic of periods of drought, but it can also be the consequence of an increased salinity of the growing environment (leading to a drop in the osmotic potential of the environment). This can also occur in periods of low temperatures.According to a study carried out by the Omafra (Ontario) in 2007 (updated in 2013), the effects of hydric stress on tomato crops are numerous.
• The number of flowers per cluster, and therefore the number of fruits produced, decreases, leading to a de facto drop in agricultural yield.
• Conversely, the quantity of soluble solids increases in the fruit, which leads to a higher industrial yields at the processing stage and, usually, to an improved flavor for raw and processed tomatoes.
• The size of the fruit decreases, leading once again to a drop in agricultural yields and, in the processing plants, to a deterioration of industrial yields (largely linked to the increase in the proportion of dregs or pomace).
• Blossom-end rot is also more frequent and losses can be important if the varieties are particularly sensitive. Fruit that is affected in this way is unsuitable for processing, presenting black specks and mold particles in the processed product. When this problem is particularly pronounced, the percentage of rejected deliveries can be very high. Entire fields can be refused during grading and can be completely wasted, leading to heavy losses for growers.
• The temperature of the plant cover increases, which causes extra stress for the plant. This problem is linked to a drop in transpiration and photosynthesis, leading at the same time to a slowdown in growth. When the temperature of the fruit exceeds 30°C, the development of its color is affected.
Various natural strategies
Tomatoes tolerate hydric stress better than other crops like sweet peppers or cucumbers. They can alter their physiological processes in order to preserve water whilst pursuing their growth. Being exposed to this stress early in the season will make some plants more tolerant to further episodes of similar stress occurring later in the season. Although this adaptation allows tomatoes to survive were other crops would suffer irreparable damage, prolonged hydric stress nonetheless affects yields, as it draws heavily on the crop's energy resources.
More than other kinds of stress, hydric stress mainly impacts the growth and productivity of the plants, as well as the quality of the fruits that are produced, and its impact can be severe. In reaction to stress, the plant can develop resistance strategies in order to relaunch growth and increase productivity potential, for example by enhancing the development of new roots, often in the soil closest to the surface in order to absorb more water. Changes can also occur at the levels of the cell membranes, of the chloroplasts or of the plant's enzymatic activity. However, these changes can increase the plant's sensitivity to other forms of stress.
The common factor to most stress conditions is the limitation of water availability. The plant's responses to stress present a number of similarities, particularly in the common mechanisms that involve abscisic acid, used by the plant as a signal to trigger hydric stress responses. Abscisic acid is a natural growth regulator that is responsible for the breaking off of the stalk of the fruit when it reaches maturity. This acid is also a phytohormone that regulates the opening of the stomata, the above-ground organs that are most often situated on the underside of the leaves and that allow exchanges between the plant and the atmosphere (O2, CO2 and, most importantly, water vapor). When it is sensed by the specific receptors of the plasma membrane, abscisic acid induces the closure of the stomata thanks to a modification in the calcium content of the cells, thereby reducing plant transpiration and water loss.With this "avoidance" strategy, the plant reacts by protecting and reducing the transpiration surface – and therefore reducing the risk of water loss, in order to maintain its hydric potential at the highest possible level. Another mechanism, which is a "tolerance" strategy, allows the plans to function despite the lack of water availability. Ions and fluids gather in the vesicles – intracellular compartments containing water and various organic and inorganic molecules whose nature and contents vary depending on the requirements of the cell. Increasing the ion concentration in the vesicles leads to a heightening of the osmotic pressure: the hydric potential drops, with a consequential drop in the capacity of the cell to liberate water.
Understanding hydric stress: the most sensitive phase is about 20 days after the plant blossoms
Understanding the underlying mechanisms of fruit development is crucial in the field of agronomy. From the earliest stages of growth, when the cells of the fruit are undergoing an intense process of division and expansion, right up to the harvest, the impact of resource availability or, on the contrary, of a stress on the mass and composition of fruit, is a determining factor.
Map of the hydric potential of the plant. The lighter colors indicate a lower hydric potential, which is less favorable to fruit development.
© Inra, Gilles Vercambre
In a natural environment, the growth of fruit cells depends both on their age and on the availability of water and sugars, which vary according to the position of the fruit on the plant.
Researchers at the Avignon Plants and Horticultural Systems unit (France) have developed an advanced computer modelization that integrates different physiological, metabolic and physical mechanisms.
At the level of the tomato plant, this model allows scientists to calculate the production of sugars by photosynthesis and to simulate their storage and transfer within the plant and towards the fruit. Water loss by transpiration and water flow in the plant are also calculated in order to evaluate the hydric state of the plant's tissue in its different segments, which is then quantified by its hydric potential (see attached illustration: the hydric state is better in the mauve colored areas – in the lower parts of the plant – and less good in the white and yellow areas – in the plant's extremities.)
This new model enables scientists to predict and virtually analyze the effects of hydric stress at the plant-cell level, as well as at the level of the fruit, of the cluster of fruit, and of the plant as a whole. So when water shortage strongly reduces the mass of tomatoes, this model reveals that a limitation of the carbon input – thanks to the provision of shade – can lead to a temporary increase in the mass and water content, due to the response dynamics located within the fruit and within the leaves.
Thanks to the different results obtained by researchers, the age of cells and the transfer mechanism in the plant and the fruit have enabled the identification of the period when the impact of stress on the development of fruit will be most severe. According to this model, this phase of sensitivity is about 20 days after blossoming, when the fruit is in its strongest expansion period. During this phase, there is an intense production of new cells, whose growth is particularly affected by any deficit in water or carbon.
So researchers have drawn up a map of the hydric potentials of tomato plants and modelized the effect of water or light restrictions on the size and quality of the tomatoes depending on the age of the fruit and of its cells.
Further complementary initiatives
Results from this kind of research and the knowledge acquired regarding response modalities of the plants have led to the development of "antistress" products intended to promote the plant's natural balance and to protect it and the crop from damage caused by the different types of stress, particularly water deficits.
Therefore, in the region that has suffered the worst effects of the drought that is currently affecting the north of Italy (since last October), the CIO (Consorzio Interregionale Ortofrutticoli) has collaborated with the Thermoflora company to carry out its test trials on processing tomato crops in order to measure the beneficial effects of treatments intended to limit the abiotic stress (not caused by living organisms) caused by transplanting. The product tested, which is called EVEO (Équilibreur Végétal Organique), is a new formula based on complex substances that, within the plant, act as osmoprotectors specifically designed to improve the natural balance of the plant's development, particularly under difficult climatic conditions.
This product is different from formulas already available on the market (whose effect is often limited to nutritional inputs or a hormonal effect) in that it stimulates the natural metabolic processes of the plant (root development, heat resistance, etc.). ; It results in better balanced crops, which are better able to absorb water and nutrients and retain them in the parts of the plant where growth and development functions most need the help. The benefits of this kind of treatment are found in a wide range of growing conditions, but specifically in conditions that are stressful for the plants, particularly featuring stresses linked to weather conditions (very high temperatures, or very low temperatures, water shortage, low-quality soil, etc.).
A comparison between the fields that are treated and those that are not treated has demonstrated a capacity for recovering from the stress linked to replanting, and development dynamics that are much faster and stronger for plants that have been treated compared to control plants.
Parma, Emilia-Romagna, Consorzio Interregionale Ortofrutticoli, Processing tomatoes
Sources: Omafra, INRA, ThermoFlora
Some complementary data
To the left, tomato plants presenting delayed growth caused by hydric stress. To the right, plants treated with EVEO.
Appendices/Annexes
Consorzio Interregionale Ortofrutticoli (CIO)
