At Agro Vivero del Mediterráneo, we have been dedicated body and soul to the fascinating world of pistachios for years. Our experience has taught us that the success of a plantation does not only depend on choosing the right pistachio plant or impeccable agronomic management. There are environmental factors, often invisible, that play a crucial role in the health and productivity of our trees. One of these factors, whose importance is increasingly recognized, is air quality and, specifically, the presence of tropospheric ozone (O₃). 💨

Unlike stratospheric ozone, which forms a vital protective layer in the upper atmosphere, tropospheric ozone, also known as “bad” ozone, is a secondary pollutant that forms at ground level. It originates from complex photochemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs), in the presence of intense solar radiation. These precursors are emitted mainly by road traffic, industry, and certain agricultural activities. The typical meteorological conditions of the Mediterranean, with long, hot, and sunny summers, are especially conducive to the formation of high ozone concentrations, unfortunately coinciding with the period of maximum growth and development of the pistachio tree.

As experts in the sector, we have observed firsthand how this “silent enemy” can reduce the productive potential of plantations. The pistachio tree, despite its legendary hardiness and adaptation to arid climates, is not immune to the harmful effects of ozone. Understanding its impact is the first step to being able to mitigate its consequences and ensure the profitability of the plantation. Throughout this article, we will delve into how air quality, and specifically tropospheric ozone, affects the physiology and production of the pistachio tree, sharing our knowledge and experience to help other farmers protect their investments.

Tropospheric Ozone: An Oxidizing Pollutant and its Entry Pathways into the Plant

To understand the damage that ozone can cause, we must first understand how it interacts with the tree. Ozone is a highly reactive gas and a powerful oxidizing agent. Its main entry pathway into the plant is through the stomata, small pores located mostly on the underside of the leaves, which are essential for gas exchange: the uptake of carbon dioxide (CO₂) for photosynthesis and the release of oxygen and water vapor. 🍃

Once ozone passes through the stomata and enters the intercellular space of the leaf (the mesophyll), it dissolves in the thin film of water that covers the plant cells. In this aqueous medium, ozone breaks down rapidly, generating a cascade of reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂), the hydroxyl radical (OH•), and the superoxide anion (O₂⁻). These molecules are extremely unstable and reactive, and are the true culprits of cell damage.

This phenomenon is known as oxidative stress. Under normal conditions, plants have antioxidant defense systems, both enzymatic (such as superoxide dismutase, catalase, or peroxidase) and non-enzymatic (such as ascorbic acid or vitamin C, glutathione, or tocopherols or vitamin E), which neutralize ROS and maintain cellular balance. However, when ozone exposure is high and persistent, the capacity of these defense systems is overwhelmed. The excess ROS begins to attack vital cellular components, such as membrane lipids, proteins, and nucleic acids (DNA and RNA), triggering a series of detrimental chain effects that affect the entire physiology of the tree.

It is crucial to note that the sensitivity of the pistachio tree to ozone is not constant. It varies depending on the variety, the phenological state of the tree, environmental conditions (light, temperature, humidity), and the water status of the plant. For example, under water stress conditions, pistachio trees tend to close their stomata to conserve water, which indirectly reduces ozone entry and limits damage. However, this is not a solution, as stomatal closure also limits photosynthesis. If you need personalized advice on how to manage these factors, do not hesitate to contact us.

Detailed Physiological Impacts of Ozone on the Pistachio Tree

The effects of ozone-induced oxidative stress manifest themselves at various levels in the physiology of the pistachio tree. Below, we break down the most important ones based on our experience and available scientific studies.

Visible Leaf Damage: Necrosis and Chlorosis

One of the most obvious symptoms of exposure to high ozone concentrations is foliar damage. These usually appear as small dark, bronzed, or blackish spots or stippling on the upper surface of older, sun-exposed leaves. These spots are actually small areas of dead tissue (necrosis) caused by the collapse of palisade mesophyll cells, the most active in photosynthesis. 🍂

With continued exposure, these spots can merge, forming larger lesions. In addition, it is common to observe premature yellowing of the leaves (chlorosis) and accelerated senescence. This means that the leaves age and fall prematurely, reducing the total leaf area of the tree and, consequently, its ability to photosynthesize throughout the growing season. A weakened canopy with fewer leaves is unable to produce the energy necessary to sustain a good harvest and prepare reserves for the following year.

Reduction in Photosynthesis and Water Use Efficiency

The most significant and economically relevant impact of ozone is undoubtedly its effect on photosynthesis. This vital process, by which the plant converts sunlight, water, and CO₂ into sugars (energy), is affected in several ways:

1. Damage to the photosynthetic apparatus: The ROS generated by ozone directly attack the chloroplasts, the cellular organelles where photosynthesis takes place. They degrade chlorophyll, the pigment that captures light, and damage key proteins of the photosynthetic complex, such as RuBisCO, the enzyme responsible for fixing CO₂. This translates into a lower carbon assimilation rate.

2. Alteration of stomatal conductance: Ozone can cause an abnormal response of the stomata. In some cases, it induces stomatal closure to limit its own entry, but this also restricts CO₂ uptake, affecting photosynthesis. In other cases, it can damage the stomatal control mechanisms, causing a “sluggish” response or even preventing their complete closure at night. The latter increases water loss through transpiration, reducing water use efficiency and making the tree more vulnerable to drought. 💧

On our experimental farms and through the services we offer to farmers, we have found that a sustained reduction in the photosynthetic rate, even if not visible to the naked eye, has direct and cumulative consequences on tree vigor and final production.

Alteration in Carbon Allocation and Growth

The reduction in sugar production through photosynthesis forces the tree to make difficult decisions about how to distribute its limited energy resources. The plant must prioritize. Faced with an ozone attack, the tree diverts a significant part of its photoassimilates (sugars) towards defense and cellular repair mechanisms. The synthesis of antioxidant compounds, the repair of damaged membranes, and the production of stress-related proteins are increased.

This diversion of resources has a cost. The energy invested in defending and repairing itself is energy that is not allocated to vegetative growth (production of new shoots, leaves, and roots) or reproductive growth (flower development and nut filling). As a result, we observe:

• Less growth of annual shoots: This compromises production in subsequent years, since the pistachio fruits on the previous year’s wood.

• Reduction in trunk and root growth: A less developed root system implies a lower capacity to explore the soil for water and nutrients, which further aggravates any type of stress.

• Lower accumulation of reserves: At the end of the cycle, the tree has fewer carbohydrate and nitrogen reserves in its storage tissues (roots and wood), which negatively affects budding and flowering the following spring, potentially accentuating the phenomenon of alternate bearing or production alternation.

Direct Consequences on Pistachio Production and Quality

The physiological effects described above inevitably translate into both a quantitative and qualitative decrease in the harvest. At Agro Vivero del Mediterráneo, we have dedicated a lot of effort to quantifying this impact in order to offer the best advice.

Reduction in Yield per Hectare

The most direct and worrying consequence for any farmer is the loss of kilos per hectare. This reduction in yield is due to a combination of factors:

• Fewer fruits per tree: Lower energy availability during flowering and fruit set can increase the drop of flowers and young fruits.

• Lower individual nut weight: This is perhaps the most determining factor. The reduction in photosynthesis during the filling phase (which occurs in mid-summer, coinciding with ozone peaks) means that the tree cannot supply enough sugars to the developing pistachios. This results in a lower kernel weight.

• Increase in the percentage of empty fruits: In cases of severe ozone stress, embryo development can stop completely, resulting in pistachios that have a shell but lack a kernel inside.

• Decrease in the percentage of split fruits: The shell splitting process is related to kernel growth. If the kernel does not reach a sufficient size due to a lack of assimilates, the internal pressure will not be enough to cause the natural dehiscence of the shell. This reduces the commercial value of the harvest, as closed pistachios have a significantly lower price. 📉

Scientific studies have quantified these losses. Depending on the ozone concentration, the pistachio variety, and the growing conditions, yield losses can range from 5% to more than 20%. If we consider an average production of 1,500 kg/ha of dry pistachios, a 10% loss means losing the value of 150 kg per hectare, a non-negligible economic impact. If you wish to carry out a feasibility study for your project, you can use our reservation and quote form.

Effect on Kernel Quality

In addition to quantity, ozone can also affect the intrinsic quality of the pistachio kernel. Poor filling not only implies lower weight but can also alter the biochemical composition of the kernel. It has been suggested that oxidative stress can affect the fatty acid profile, vitamins (especially vitamin E, a natural antioxidant), and other compounds that define the nutritional value and organoleptic characteristics (flavor and aroma) of the pistachio. A smaller caliber kernel, with a less intense color and an altered composition, will have less acceptance in the most demanding markets, which seek a high-quality product.

Mitigation Strategies and Agronomic Management

Although we cannot eliminate ozone from the air, we can adopt a series of management practices in our plantations to minimize its impact and help our trees be more resilient. At Agro Vivero del Mediterráneo, we implement and recommend the following strategies:

Selection of More Tolerant Rootstocks and Varieties

Research has shown that there is genetic variability in the pistachio tree’s response to ozone. Some varieties and rootstocks show greater tolerance than others. For example, it has been observed that vigorous rootstocks like UCB-1, by promoting a more powerful root system and greater vegetative development, can confer on the plant a greater capacity to compensate for and withstand oxidative stress. The choice of plant material is the first and one of the most important decisions. Investing in a quality, certified pistachio plant adapted to our conditions is fundamental.

Although information on the specific tolerance of commercial varieties (Kerman, Larnaka, Sirora, etc.) is still limited, it is an active field of research. In the future, it is likely that we will have specific selections that combine high productivity with greater resilience to ozone.

Optimal Irrigation and Nutrition Management

A well-nourished and hydrated tree is a stronger tree with a greater capacity to activate its defense mechanisms.

• Adequate irrigation: Maintaining an optimal water status in the plant is crucial. As we mentioned, severe water stress can reduce ozone damage by forcing stomatal closure, but at the cost of production. The ideal strategy is to apply regulated deficit irrigation, avoiding both severe water stress and waterlogging. Good irrigation management allows the stomata to be kept open as long as possible during the hours of lowest ozone concentration (early morning), maximizing photosynthesis and minimizing pollutant entry. The use of soil moisture sensors and monitoring the plant’s water status are very helpful tools.

• Balanced nutrition: Correct fertilization is key. Nitrogen is essential for growth, but an excess can cause overly exuberant and tender vegetative growth, which is more sensitive to ozone. Potassium, on the other hand, plays a fundamental role in stomatal regulation and overall stress tolerance. Calcium is vital for the integrity of cell membranes. Micronutrients such as zinc and manganese are cofactors of antioxidant enzymes like superoxide dismutase. A fertilization plan based on soil and foliar analyses is the best guarantee to provide the tree with everything it needs to defend itself. 💪

Application of Biostimulants and Antioxidants

A very promising line of work, which we are already applying, is the use of biostimulants and products with antioxidant capacity. The foliar application of products based on seaweed extracts, amino acids (such as glycine-betaine or proline, which act as osmoregulators and cell protectors), silicon, or antioxidants such as ascorbic acid (vitamin C) at times of greatest risk (early summer), can help reinforce the plant’s natural defenses. These treatments do not eliminate the root problem, but they act as a “shield” that helps the plant better manage oxidative stress, reducing cell damage and freeing up resources for growth and production.

Cultural Practices to Improve Ventilation

Plantation design and pruning practices can also have an influence. An adequate planting layout and formative and production pruning that favors canopy aeration can help disperse pollutants and reduce foliar humidity, creating a microclimate less favorable for ozone action. Very dense and poorly ventilated canopies can create areas where the pollutant concentration remains higher for longer.

The Future: Monitoring and Prediction Models

Climate change foresees an increase in temperatures and periods of drought and high radiation in the Mediterranean basin, conditions that, as we have seen, favor the formation of tropospheric ozone. Therefore, it is foreseeable that this problem will worsen in the future. ☀️

Precision agriculture will play a fundamental role. The installation of air quality sensor networks in pistachio producing areas, combined with meteorological models, will allow us to predict the days and hours of greatest risk of ozone peaks. This information will be invaluable to farmers. We will be able to, for example:

• Adjust irrigation schedules: Irrigate at night or early in the morning to ensure the plant is well hydrated before ozone levels begin to rise.

• Schedule phytosanitary treatments: Avoid carrying out treatments during hours of maximum ozone concentration so as not to add additional stress to the plant.

• Plan the application of biostimulants: Apply protective products preventively, just before a predicted high pollution episode.

In conclusion, from our position at Agro Vivero del Mediterráneo, we consider that tropospheric ozone is an abiotic stress factor that no pistachio producer can afford to ignore. Its impact, although subtle and often confused with other problems, is real and has direct economic consequences on the profitability of the plantation.

It affects the health of the tree at the cellular level, reduces its photosynthetic capacity, compromises its growth, and, ultimately, decreases the yield and quality of our valuable harvest. The good news is that we are not defenseless. Through a comprehensive approach that combines the choice of top-quality plant material, exquisite agronomic management focused on nutrition and irrigation, the use of new tools such as biostimulants, and an eye on monitoring technologies, we can build more resilient and productive plantations.

The path to excellence in pistachio cultivation requires knowledge, dedication, and a constant ability to adapt. Understanding and acting against challenges such as air quality is what differentiates a good plantation from an exceptional one. If you have any questions or would like us to help you implement these strategies on your farm, do not hesitate to contact our team of experts. Together, we can protect the future and profitability of this exciting crop. 🌳✨