California Fruit and Vegetable

Category: Tomatoes

  • Extensive Heat Waves Influence Drop in 2024 California Processing Tomato Forecast

    Contracted production for California processing tomatoes is forecast at 11.3 million tons, averaging 50.0 tons per acre. The current production forecast is 12 percent below last year’s contracted production of 12.8 million tons, and 2 percent below the May forecast. The projected harvested acreage of tomatoes grown under contract is 226,000 acres, down 11 percent from 2023.

    Mild spring temperatures and adequate water provided tomato growers with favorable planting conditions and a good start to the developing crop. However, extremely high temperatures throughout the summer and into harvest has negatively affected yields. There were concerns regarding curly top virus detected in the San Joaquin Valley, but no significant damage was reported.

    The processing tomato harvest began the first week of July in the southern Central Valley and progressed to the northern counties by the middle of August. Harvest is expected to finish in October, significantly earlier than last year. Quality is reported to be fairly good for the early crop, but the impact of the heat waves on late season plantings is uncertain.

    The Processing Tomato Advisory Board published shipments through August 24, 2024, showing a 29% increase compared to this time last year. However, shipments were significantly behind schedule in 2023 because the crop was delayed. Current shipments compared to the halfway point of last year indicate a decrease of 12%.

    This processing tomato estimate is funded by the California League of Food Producers.

  • Over Half of Mexican Tomato Production is Destined for the U.S.

    Mexico’s 2024 tomato production is forecast at 3.30 million metric tons (MMT), a two percent increase over 2023, driven by export demand and investment in protected agriculture systems. Production in 2023 is estimated at 3.22 MMT, down eight percent compared to the previous year due to water constraints from drought conditions in some production areas. Drought and adverse weather impacts remain the main challenges for Mexico’s tomato production. While some producers in Mexico have invested in protected production, other regions continue to produce in open fields, with lower yields and more exposure to climatic factors. Sinaloa remains the leading state in tomato production, contributing 22 percent of national production.

    The United States is Mexico’s top tomato export market, with 1.82 MMT in exports valued at USD 2.7 billion in 2023, up by one percent in terms of volume and ten percent in value compared to 2022. Investments in greenhouse technology in recent years has enabled producers to deliver better quality products.

    Production

    Mexico’s tomato production is forecast to increase two percent year-on-year in 2024 to 3.30 MMT, due to strong export demand, mainly from the United States. Production in 2023 showed an 8 percent decline compared with 2022, falling to 3.22 MMT. The top five tomato production states deliver over 51 percent of national production. Sinaloa remains the leading tomato producer with 22 percent of national production, followed by San Luis Potosí (13 percent) and Michoacán (7 percent). Baja California Sur, Jalisco, and Sonora each contribute 5 percent to total production.

    Adverse climate conditions have contributed to a shift towards protected production during the last decade, mainly production in greenhouses, shade houses, and high tunnels instead of open field cultivation. This shift decreased production in the short-term, while resulting in increased production over time as yields and produce quality rose. In the areas where open field cultivation predominates, adverse weather conditions such as drought, excessive rain, or abrupt temperature changes usually cause significant production losses. The shift towards protected tomato production in Mexico began in the early 2000s but accelerated during the beginning of the 2010s. The trend has been steadily increasing since then, as producers look to increase their yields.

    Mexico’s tomato production occurs throughout the year across several geographic regions. From December to April, the state of Sinaloa, Mexico`s largest open-field and shade house tomato producer, dominates the domestic market and exports over 80 percent of its crop to the United States. From May to November, the major producing states include San Luis Potosi, Michoacan, Baja California Sur, Jalisco, and Sonora.

    Mexico produces a wide variety of tomatoes, with the most common being the cherry, bola, and saladette varieties, which are popular for their sweet flavor and versatility. Other high end or gourmet tomatoes varieties, including grape and cocktail, are grown almost exclusively for export to the United States. A wide variety of tomatoes are used in the processing industry due to their intense flavor and high yield.

    Production varies seasonally, with peak production in October and the lowest production period in March. Local producers report efforts to extend their growing and harvesting season from 28 to 40 weeks, partly through the use of substrates growing material. For example, production of saladette tomatoes in Jalisco state begins in July and stretches into January, at which point the plants are transplanted to a new substrate to promote continued production.

    On average, growers can harvest around 30 kilograms per square meter over the course of the season, which equates to about 1 kilogram per square meter per week. Large producers enjoy year-round production due to improvements in water management, greenhouse production and other technology improvements.

    Profits have been constrained in recent years by Mexico’s strong “super peso” and the rising cost of substrates, labor, pesticides, and other inputs. Most producers obtain substrate from suppliers in Europe and Asia, while some partner with research institutions to develop their own substrate. In Mexico, greenhouse structures are made of steel, which has seen high prices in recent years but typically lasts around three years.

    Tomato prices tend to be highest from October to December, when production is lower outside of the top producing state of Sinaloa.

    Growing Conditions

    Mexico’s tomato production is year-round, with the October to December period accounting for 42 percent of production. In recent years Mexican tomato growers have been implementing and adapting protected growing methods to improve tomato yields. According to Government of Mexico data, approximately 66 percent of tomato production takes place in greenhouses while the remaining 34 percent is carried out in open fields. Protected production allows for better pest and disease management, resulting in better yields.

    Tomatoes require well-drained loamy or sandy loam soils, with moisture holding capacity. In Mexico, the best temperatures for tomato production are between 60°F and 85°F, with temperatures above or below this range tending to slow growth and fruit maturation. Ideal conditions also include constant sunlight, humidity of 65-85%, and between one and two inches of water per week.

    Consumption

    Graph 4: Monthly Share of Tomato Production

    According to the Government of Mexico, the annual per capita consumption of tomatoes is 12.7 kgs per person, making the tomato the number one vegetable consumed by Mexicans. Tomatoes are the foundation of Mexican cuisine, an integral part of the basic food basket, and essential to every Mexican household. Tomatoes are predominately used for sauces (both spicy and sweet), tacos, and stews. Tomatoes offer flavors that balance out the spicy elements of chili peppers, providing versatility to recipes used in both traditional and modern Mexican cooking.

    Trade

    Aside from distilled spirits and beer, tomatoes are the number two agricultural product exported to the United States at USD 2.71 billion, just behind avocados at USD 2.724 billion and ahead of berries at USD 1.971 billion. Mexico fulfills 25 percent of the worldwide demand, making it the eight largest producer worldwide.

    According to Trade Data Monitor, 2023 Mexican tomato exports totaled USD 3.02 billion and 2 million metric tons (MMT). The main export market is the United States with 99.8 percent of exports, followed by Canada and Japan. 2023 represented a record-breaking year for Mexican tomatoes being exported to the northern neighbor.

    Approximately 90% of saladette production is exported to the United States, with cherry, grape and other high end niche varieties exported almost entirely. According to producers, transport time from Mexican fields to U.S. supermarket shelves is about two days, via border ports in Texas and Arizona. Mexico’s 2023 tomato imports totaled less than USD 1 million, primarily from the United States.

    Policy

    The Tomato Suspension Agreement (TSA) between the U.S. Department of Commerce and signatory producers/exporters of fresh tomatoes grown in Mexico ensures that signatory producers and exporters sell Mexican tomatoes at or above the TSA reference price to eliminate the injurious effects of exports of fresh tomatoes to the United States. The Agreement, which was updated effective September 19, 2019, applies to all fresh and chilled tomatoes except tomatoes for processing. — By Manuel Mandujano, USDA Foreign Ag Service

  • Fresno County Leads Nation in Processing Tomato Production (May California Forecast)

    As of May 15, California’s tomato processors reported they have or will have contracts for 11.5 million tons of processing tomatoes for 2024. This production estimate is 1 percent below the January intentions forecast of 11.6 million tons and 10 percent below the final 2023 contracted production. The May contracted acreage of 228,000 is 2 percent below the January intentions forecast of 232,000 acres and 40,000 acres less than last year’s final contracted acreage.

    Fresno County remains the top California county and leading the national in contracted planted acreage for 2024 with 53,000 acres. Yolo, Kings, Merced, and San Joaquin make up the remaining top five counties, accounting for 65 percent of the 2024 total contracted planted acreage for California.

    Mild spring temperatures and adequate water have aided the development of the processing tomato crop. Planting began in late February in warmer areas of the state and has progressed with little to no weather delays. There were reports that the beet leafhopper has been detected in some areas, warranting close monitoring and increased pest control. At this point, yields are projected to be a little higher than last year. Harvest is expected to start in early July.

    Tomato supplies were replenished after growers in 2023 produced the largest crop in years. As a result, contracted acreage has decreased in most counties. Planted acres in Kings County increased significantly, as cropland that was previously under water has since been recovered. Processors anticipate that this year’s tomato harvest will be on schedule and will not extend beyond normal timelines, reducing the need for late- season deliveries.

    This early processing tomato estimate is funded by the California League of Food Producers.

  • How Might Tomatoes Provide Health Benefits?

    (USDA Photo by Lance Cheung.)

    Scientists at U.S. the Department of Agriculture’s Agricultural Research Service (ARS) and The Ohio State University (OSU) have been working to investigate how tomatoes may be imparting health benefits in a recently published study.

    Studies in animals have shown that incorporating tomatoes into the diet can reduce the prevalence of chronic illnesses like prostate and liver cancer. It has been thought that compounds naturally produced by tomatoes are responsible for these effects. After absorption from a meal, many of the chemical compounds found in tomato fruits travel to the liver, where they are metabolized. Some compounds remain for some time, while others are quickly removed from the body.

    Meanwhile, in the liver or other tissues, some of these compounds can alter gene expression in ways associated with positive health benefits. Researchers in the past have largely focused on lycopene, a pigment that gives tomatoes their red color. However, tomatoes produce thousands of compounds, and it has been shown that tomato consumption offers more benefits than lycopene alone. A “big picture” view was missing.

    “We know that eating tomatoes is associated with a number of health benefits, and our study intended to dive deeper into what happens when you eat tomatoes from the standpoint of what is absorbed and how gene expression is altered,” said Michael Dzakovich Ph.D., a scientist with USDA-ARS Children’s Nutrition Research Center. “Rather than focusing only on one compound, we utilized a technology called metabolomics to broadly profile how hundreds of chemical compounds were changing in the liver as a result of tomato consumption. We also used transcriptomics to measure how all the detectable genes in the liver were changing at the same time. This approach gave us valuable insight into the potential mechanisms by which tomato consumption affects the liver and potentially the whole body.”

    Scientists tested liver tissue from mice that were fed control diets or control diets enriched with tomato to determine what tomato compounds were found in the liver and how gene expression changed. Tomato-fed mice were given one of two diets with the addition of commercial orange and red tomato varieties. Since not all tomato varieties are chemically identical, using multiple varieties allowed for a more comprehensive examination of how tomato consumption affects the liver in general.

    “We discovered a series of metabolites [molecules produced by metabolism] that have never been reported in the liver. Several of these compounds have been found in blood, skin, and urine, but our data show that these molecules are more extensively metabolized than we realized,” stated Dzakovich. The metabolites are from steroidal alkaloids uniquely produced in tomatoes.

    “Steroidal alkaloids have been shown in both in vitro [in the laboratory] and animal studies to lower the absorption of cholesterol, reduce cancer cell proliferation, and reduce muscle atrophy. They also resemble many important signaling molecules made by the body. It seems reasonable to hypothesize they might be a part of a suite of compounds found in tomatoes that benefit human health.”

    In addition, scientists observed that regardless of the tomato variety, there was an increase in the activation of genes related to xenobiotic metabolism, a series of biological processes that help our body detoxify itself. This led the authors to hypothesize that one way in which tomatoes may be benefiting human health is in their ability to promote production of the enzymes that allow excretion of potentially toxic compounds. Similar gene expression profiles have been associated with the prevention of cancer development because of consuming vegetables like brassicas (for example, broccoli).

    Michael Dzakovich initiated this study during his Ph.D. program in the laboratory of Jessica Cooperstone Ph.D. at The Ohio State University in collaboration with Mallory Goggans MS, Jennifer Thomas-Ahner Ph.D., Nancy Moran Ph.D., Steven Clinton MD Ph.D., and David Francis Ph.D. More details about this study can be found in Molecular Nutrition and Food Research.

    The Agricultural Research Service is the U.S. Department of Agriculture’s chief scientific in-house research agency. Daily, ARS focuses on solutions to agricultural problems affecting America. Each dollar invested in U.S. agricultural research results in $20 of economic impact.

  • Video Released on Nitrogen Management in California Processing Tomatoes

    Efficient nitrogen fertilization management for all irrigated crops is gaining much attention as the Irrigated Lands Programs in the Central Valley and Central Coast begin efforts to minimize nitrate movement past the root zone of fruits, nuts and vegetables. The first in a series of seven educational videos on nitrogen fertilizer management is now available online. The video series is funded through a grant to CURES by the Fertilizer Research and Education Program (FREP) at the California Department of Food and Agriculture.

    “The 4Rs of Nitrogen Management in Processing Tomatoes” features Zheng Wang, Vegetable Crops Advisor with the University of California Cooperative Extension (UCCE). Dr. Wang explains in the 30-minute video the current knowledge about efficient management of nitrogen in processing tomatoes by following the 4Rs: Right Rate, Time, Place and Product.

    UCCE research supported by the California Tomato Research Institute shows that each ton of tomatoes requires approximately 4.6 pounds of nitrogen for efficient production. Nitrogen fertilizer injections into drip systems are best scheduled in the latter half of an irrigation set. Commercial and organic nitrogen fertilizers have varying levels of potential to move past the crop root zone. These and other agronomic and irrigation pointers are covered in the video, available in both English and Spanish. For those who want “just the facts,” 5-minute condensed versions of the video are also available.

    Visit the BMP section of https://www.curesworks.org/nitrogen-management/ ; see Nitrogen Fertilizer 4R videos on canning tomatoes as well as walnuts. Coming soon in the CURES video series on the “4Rs of Nitrogen Management”: pistachios, almonds, citrus, high tonnage wine grapes, strawberries and lettuce. Sign up to be notified for all new video releases at www.curesworks.org “Contact Us.”

    CURES was founded in 1997 to support educational efforts for agricultural and urban communities focusing on the proper and judicious use of pesticides and plant nutrients. Since its formation, the organization has focused its efforts on pesticide and nitrogen fertilizer stewardship and research projects, including studies on the effectiveness of management practices to minimize movement of farm inputs and sediment into surface and groundwater. A key goal is to implement educational programs, coordinate research and provide information and professional expertise to users and applicators of pesticides and nutrients to enhance and protect the environment, as well as public and worker health and safety. All its projects are implemented either by CURES staff or through partnerships with organizations such as commodity groups, water quality coalitions, private companies and the University of California.

    For more information, contact Parry Klassen at 559-288-8125 or klassenparry@gmail.com or visit CURES website at www.curesworks.org.

  • Largest CA Processing Tomato Crop Forecasted Since 2015

    The USDA National Agricultural Statistics Service has forecasted contracted production for California processing tomatoes at 12.9 million tons for 2023, averaging 50.8 tons per acre. The current production forecast is 23% above last year’s contracted production of 10.5 million tons, and 2% above the May forecast. The projected harvested acreage of tomatoes grown under contract is 254,000 acres, up 13% from 2022.

    Unseasonably wet weather through winter and spring delayed planting by weeks, but with record-high prices and ample water, contracted acreage increased significantly. Harvest began with a slow start in mid-July, a couple weeks behind average, and is expected to continue well through October if the weather stays dry. Now entering peak harvest, canneries are busy managing the logistics of consistently delivering ripe tomatoes to the plant.

    The Processing Tomato Advisory Board published shipments through August 26, 2023, showing a 16% decrease compared to the end of August 2022. However, due to the late crop, shipments are expected to catch up and exceed the past 5 years.

    This processing tomato estimate is funded by the California League of Food Producers.

  • USDA Extends Comment Period on Regulatory Status of Tomato Brown Rugose Fruit Virus (ToBRFV)

    USDA’s Animal and Plant Health Inspection Service (APHIS) is issuing a Stakeholder Registry announcement extending the public comment on two pathway risk assessments and three potential regulatory policy options for Tomato brown rugose fruit virus (ToBRFV) in fresh tomato and pepper fruit for consumption and plant propagative material (including seeds). The public will have an extra 30 days to provide comments to the documents in response to requests to extend the comment period beyond August 30, 2023.  APHIS will now consider all comments received by September 29, 2023.

    To safeguard U.S. agriculture against the introduction of the virus into the United States, APHIS issued a Federal Order effective on November 22, 2019, to restrict the importation of tomato and pepper fruit and plant propagative material, such as plants intended for planting, plant parts and cuttings, and seeds. On June 3, 2020, APHIS amended the Federal Order to clarify the requirements for fruit, specifically requiring an additional declaration for tomatoes and/or pepper fruit from countries that already must provide a phytosanitary certificate for these commodities. All other requirements remained unchanged.

    The distribution of ToBRFV is rapidly changing across the world and given numerous detections of the virus in the United States over the past several years, APHIS is reevaluating the Agency’s policies for regulating fresh tomato and pepper fruit for consumption and plant propagative material (including seeds) for ToBRFV.

    APHIS developed pathway risk assessments to update and inform its policy decisions related to the virus. APHIS drafted the following documents: (1) a tomato and pepper fruit for consumption pathway risk assessment, (2) a propagative plant materials pathway risk assessment, and (3) a document that describes three potential regulatory responses considering the pathway risk assessments.

    APHIS is seeking scientific information that would provide a greater understanding of the distribution of ToBRFV in the United States and of best management practices, including the development of resistant tomato varieties for this virus. APHIS is also seeking information about the economic impacts of the potential regulatory options.

    APHIS is considering three regulatory policy options:

    1. Continue to regulate Tomato brown rugose fruit virus as a quarantine pest but remove those import restrictions for tomato and pepper fruit for consumption. APHIS would continue to regulate propagative materials, including plants and seed, as described in the existing import Federal Order. APHIS would continue to take action on domestic detections of the virus in seeds and other propagative material.
    2. Categorize Tomato brown rugose fruit virus as a non-quarantine pest and rescind the import Federal Order. This action would remove all import restrictions for this virus for fruit for consumption and for propagative materials. APHIS would no longer take action on domestic detections of the virus.
    3. Continue to regulate Tomato brown rugose fruit virus as a quarantine pest and implement the requirements in the existing import Federal Order without change. APHIS would continue to take action on domestic detections of the virus in fruit, seeds and other propagative material for consumption.

    APHIS will accept public feedback of the documents on the Stakeholder Risk Assessment Consultation web page: https://www.aphis.usda.gov/plant-health/risk-assessment-consultation.

    The current ToBRFV Federal Order is available on APHIS’ website here: https://www.aphis.usda.gov/aphis/ourfocus/planthealth/import-information/federal-import-orders/. If you have any questions, please direct them to PPQPRAComments@usda.gov; please indicate ToBRFV in the subject line.

  • Pathway Risk Assessments and Potential Regulatory Options for Tomato Brown Rugose Fruit Virus (ToBRFV)

    USDA’s Animal and Plant Health Inspection Service (APHIS) is issuing a Stakeholder Registry announcement to solicit public comment on two pathway risk assessments and three potential regulatory policy options for Tomato brown rugose fruit virus (ToBRFV) in fresh tomato and pepper fruit for consumption and plant propagative material (including seeds).

    To safeguard U.S. agriculture against the introduction of the virus into the United States, APHIS issued a Federal Order effective on November 22, 2019, to restrict the importation of tomato and pepper fruit and plant propagative material, such as plants intended for planting, plant parts and cuttings, and seeds. On June 3, 2020, APHIS amended the Federal Order to clarify the requirements for fruit, specifically requiring an additional declaration for tomatoes and/or pepper fruit from countries that already must provide a phytosanitary certificate for these commodities. All other requirements remained unchanged.

    The distribution of ToBRFV is rapidly changing across the world and given numerous detections of the virus in the United States over the past several years, APHIS is reevaluating the Agency’s policies for regulating fresh tomato and pepper fruit for consumption and plant propagative material (including seeds) for ToBRFV.

    APHIS developed pathway risk assessments to update and inform its policy decisions related to the virus. APHIS drafted the following documents: (1) a tomato and pepper fruit for consumption pathway risk assessment, (2) a propagative plant materials pathway risk assessment, and (3) a document that describes three potential regulatory responses considering the pathway risk assessments.

    APHIS is seeking scientific information that would provide a greater understanding of the distribution of ToBRFV in the United States and of best management practices, including the development of resistant tomato varieties for this virus. APHIS is also seeking information about the economic impacts of the potential regulatory options.

    APHIS is considering three regulatory policy options:

    1. Continue to regulate Tomato brown rugose fruit virus as a quarantine pest but remove those import restrictions for tomato and pepper fruit for consumption. APHIS would continue to regulate propagative materials, including plants and seed, as described in the existing import Federal Order. APHIS would continue to take action on domestic detections of the virus in seeds and other propagative material.
    2. Categorize Tomato brown rugose fruit virus as a non-quarantine pest and rescind the import Federal Order. This action would remove all import restrictions for this virus for fruit for consumption and for propagative materials. APHIS would no longer take action on domestic detections of the virus.
    3. Continue to regulate Tomato brown rugose fruit virus as a quarantine pest and implement the requirements in the existing import Federal Order without change. APHIS would continue to take action on domestic detections of the virus in fruit, seeds and other propagative material for consumption.

    APHIS will accept public feedback for 30 days following the publication of the documents on the Stakeholder Risk Assessment Consultation web page: https://www.aphis.usda.gov/plant-health/risk-assessment-consultation. APHIS will consider all comments received by August 30, 2023.

    The current ToBRFV Federal Order is available on APHIS’ website here: https://www.aphis.usda.gov/aphis/ourfocus/planthealth/import-information/federal-import-orders/. If you have any questions, please direct them to PPQPRAComments@usda.gov; please indicate ToBRFV in the subject line.

  • Processing Tomato Variety Evaluation for Tolerance/Susceptibility to New Soilborne Pathogen

    In recent years, there has been an increase in crown rot/vine decline problems in processing tomatoes. Many tolerant, as well as several susceptible tomato cultivars, have been identified from previous efforts funded by CTRI. The number of fields in the Sacramento Valley with confirmed Fusarium falciforme vine decline has been increasing each year. In 2022, eight new tomato fields were diagnosed in Sutter and Colusa counties, for a total of 20 fields since 2019. In 2021 and 2022, we worked with AgSeeds and evaluated processing tomato varieties in field trials for their tolerance or susceptibility to Fusarium falciforme by rating disease severity and sending diagnostic samples to the Swett lab at UC Davis for confirmations. Unfortunately, in 2021, we were unable to confirm F. falciforme in the particular fields we evaluated, however, in 2022, all three fields in Sutter County where we evaluated trials were positive for F. falciforme, along with Fusarium crown and root rot in 2/3 fields. We evaluated fields for advanced decline and whole plant collapse. Yield data was provided by AgSeeds. Below is a table with the average number of declined plants at our three field sites in Sutter County. The table also includes percent decline and yield for two of the fields. Also worth noting, field #2 was evaluated the day before harvest, whereas field #1 was evaluated a couple weeks before harvest and field # 3 was not harvested due to September rain events in 2022.

    The varieties with the highest decline are similar to results we have seen at other sites over the years and a more comprehensive table (developed by UCCE Vegetable Crops Advisor, Brenna Aegerter, in San Joaquin County) which can be found in my December 2022 newsletter. If you are interested in more results and information from other regions of the state, you can see Brenna’s article on pages 4-5 in the UCCE San Joaquin County Field Notes newsletter from February 2023. — By Brenna Aegerter, UCCE San Joaquin; Co-PIs: Tom Turini, UCCE Fresno; Amber Vinchesi-Vahl, UCCE Colusa/Sutter/Yuba; Cassandra Swett, UC Davis Extension Pathology Specialist; Collaborators: AgSeeds (Funded by California Tomato Research Institute)

  • Researchers Study How Plants Respond to Changing Climate

    Farmers know that heat stress and drought are hard on plants. But what, exactly, is happening to the plants under these conditions, and what can we do about it? These questions are becoming ever more urgent as we move toward a future with more carbon dioxide (CO2) in the air, more heat, and more erratic weather.

    At ARS, researchers set out to understand in detail how plants respond to these stresses, using tomatoes as their first test subject. Tomato plants are often grown in greenhouses because they are viewed as a high-cost, high-value crop. Jennifer Boldt is a research horticulturist at the Application Technology Research Unit in Wooster and Toledo, OH, who specializes in studying crops grown in controlled environments, such as high tunnels, greenhouses, indoor vertical farms in warehouses, or shipping containers.

    “The idea behind this research is that we are asking, ‘How do different environmental conditions potentially affect how nutrients are taken up by the roots of the plant, and what implication does that have for plant growth or the quality of the plants that are being grown?’” she said.

    In response to elevated temperature and carbon dioxide (right), tomato leaves shift to a more vertical orientation and soybean leaves exhibit increased leaf cupping. (Photo courtesy of Dileepa Jayawardena, University of Toledo)

    Previous research showed that conditions like heat, elevated CO2, and drought change the ways that plants function, but there was little research on how the combination of those conditions affects them.

    “Under my purview, we were able to say, ‘let’s not look at one factor individually, but what happens when we have multiple factors that are changing at the same time – high temperature and high CO2, or drought and high CO2,’” explained Boldt. “Under that combination of drought and CO2, the plants weren’t able to take up nutrients as much, so we had lower levels of nitrogen and phosphorus in the leaves. Nitrogen is a key component in proteins, and so if you have less nitrogen, then the plant is not going to be as green, and it’s going to grow more slowly, and any edible portion of the plant is going to be less nutritious.’”

    The reasons for that reaction are a function of the plant’s response to changed conditions. Boldt explained that, under current CO2 conditions, plants grow and take up nitrogen at a certain rate, creating a balance. However, under high CO2, the plants grow more, but they’re not necessarily taking up more nitrogen at that same elevated rate, so the end result is a higher ratio of carbon to nitrogen in the plant.

    Tomato plants await the start of a research study that will examine plant responses to elevated temperature and carbon dioxide. (Photo courtesy of Dileepa Jayawardena, University of Toledo)

    “It appears that the nitrogen concentration has gone down, but it’s just because the plant has put on more bulk more quickly,” she said, “because it’s taken that CO2 and turned it into sugars and structural components in the plant.”

    To perform the research, Boldt collaborated with colleagues from the University of Toledo. The team wanted to understand more about why they were seeing lower rates of nitrogen uptake. They knew that nutrients in the plant tissues could be affected by environmental conditions, but didn’t know whether it was because the plants were taking up fewer nutrients from soil, or because there were things going on within the plant that prevented the nutrients from moving up to the top part of the plant where they could be utilized.

    They found that under the combination of elevated CO2 and heat, the leaves of their tomato plants were oriented much more vertically than under current conditions. Typically, the horizontal leaf position helps plants capture sunlight well. When, in response to stress, the leaves turn up vertically, they capture less light, and ultimately, said Boldt, “aren’t able to utilize all of their systems as efficiently.” One result of that decreased efficiency: relatively less nitrogen.

    Measuring the rate of photosynthesis in a tomato leaf. (Photo courtesy of Dileepa Jayawardena, University of Toledo).

    A natural next question was whether or how these results might translate to other plants, especially commodity crop plants. To find out, Boldt decided to look at plants that are typically grown as field-based crops. She examined the effects of changed conditions on barley, corn, and big bluestem (a type of prairie grass). In all cases, she found that the combination of conditions inhibited nutrient uptake, with some variation by type of plant.

    Given these findings, what can farmers do to support their crop plants? Boldt’s research showed that the problem was not in the plant’s ability to take nutrients up from the soil, so adding more fertilizer is unlikely to help. Instead, the issue is what the plant does with nutrients as it captures them. To address that, farmers would need plants that operate differently; Boldt suggests that looking to breeders may be the best approach. The ultimate goal, then, is that her research could lead to the development of plants that are better able to manage the distribution of nutrients as plants face hotter growing conditions with more CO2 and drought. — By Kathryn Markham, USDA-ARS Office of Communications