Lettuce (Lactuca sativa) is a commonly consumed vegetable crop in current food systems. In 2009, the per-capita consumption of lettuce was estimated to be 28 lb annually [U.S. Department of Agriculture (USDA), 2011a]. Lettuce is consumed fresh in the form of salads or as an accompaniment for hamburgers, sandwiches, and tacos. When consumed fresh, it is an excellent source of bulk and fiber (Swiader and Ware, 2002). In the United States, romaine lettuce and leaf lettuce production have increased 192%, from 58,700 acres planted in 1992 to 171,400 acres planted in 2018 (Sen Nag, 2017; USDA, 2011b, 2011c). Crisphead lettuce production decreased in total acreage by 20% from 216,840 acres planted in 1992 to 120,700 acres planted in 2018 (USDA, 2011d). Lettuce is one of the leading fresh-market vegetables in acreage, production, and gross farm value. California and Arizona are the leading producers in the United States, harvesting 213,900 and 75,300 acres, respectively (Sen Nag, 2017). China leads the world production, harvesting 13.5 million tonnes in 2013, followed by the United States and Spain, harvesting 3.6 and 1.1 million tonnes, respectively (Sen Nag, 2017).
Lettuce is classified into four groups: crisphead, butterhead, romaine, and loose leaf. Crisphead lettuce is also referred to as iceberg lettuce. Swiader and Ware (2002) define each of these categories of lettuce by physical characteristics. Crisphead is described as having a large, solid head weighing more than 1.99 lb and measuring more than 5.9 inches in diameter. The leaves are crisp and brittle, with prominent veins and midribs. Crisphead is more tolerant of shipping and handling than all other types and therefore is the leading type of lettuce grown in the United States (USDA, 2011d). The Swiader and Ware (2002) characterization of romaine lettuce is long, narrow foliage; upright growth habit; and loose, elongated heads whereas butterhead lettuce is characterized by smooth, soft, and pliable leaves forming a loose head. The veins and midribs of butterhead types are not as prominent as in crisphead types and are considered to have better table quality and a more delicate flavor than crisphead types. Swiader and Ware (2002) subcategorize butterhead lettuce into two groups: Boston and bibb lettuce. Bibb lettuce is smaller and darker green than Boston lettuce. Loose-leaf lettuce is characterized as producing an open rosette of leaves arranged loosely on the stalk. There is a considerable amount of variation in leaf color within loose-leaf lettuce, ranging from green and purple to red. There is also variation in loose-leaf texture and margin shape (Swiader and Ware, 2002).
Although lettuces vary in physical differences, all subcategories assimilate and concentrate NO3– in their leaf tissue. NO3– is incorporated into proteins and other nitrogenous compounds and is used as a terminal electron receptor in the respiratory chain of chloroplasts (Hill, 1996). NO3– ingestion is a human health concern. Ingestion of NO3– is linked to the formation of N-nitroso compounds, most of which are labeled carcinogens and teratogens. Ingestion is also linked to methemoglobinemia (blue baby syndrome) and adverse pregnancy outcomes (Ward et al., 2018).
Therefore, the objectives of this study were to determine the optimum yielding lettuce cultivars for Louisiana commercial production and to determine whether the greatest yielding cultivars accumulated NO3– at levels that could potentially pose a threat to human health.
Afton, W. 2018 Evaluation of growth characteristics, yield, marketability and nitrate levels of lettuce (Lactuca sativa) cultivars produced in south Louisiana. MS Thesis, Louisiana State Univ., Baton Rouge. 6 June 2020. <https://digitalcommons.lsu.edu/gradschool_theses/4382/>
Brkić, D., Bošnir, J., Bevardi, M., Gross Bošković, A., Miloš, S., Lasić, D., Krivohlavek, A., Racz, A., Mojsović Ćuić, A. & Uršulin Trstenjak, N. 2017 Nitrate in leafy green vegetables and estimated intake Afr. J. Tradit. Complement. Altern. Med. 14 3 632 637
Boudreaux, J. 2009 Louisiana commercial vegetable production recommendations. Louisiana Coop. Ext. Serv. Publ. 2433
Centers for Disease Control 2004 Americans slightly taller, much heavier than 40 years ago. 18 Aug. 2020. <https://www.cdc.gov/media/pressrel/r041027.htm>
Hill, M.J. 1996 Nitrates and nitrites in food and water. Woodhead Publishing, Sawston/Cambridge, UK
Kemble, J., Jennings, K. & Walgenbach, J. 2019 Southeastern vegetable crop handbook. Meister Media Worldwide, Willoughby, OH
Kemble, J., Vinson, E., Akridge, R. & Burkett, J. 2012 Lettuce trials conducted in south and central Alabama, p. 11–12. In: Fall 2011 commercial fruit and vegetable variety trials. Auburn University, Auburn, AL
Maynard, E. 2013 Lettuce cultivar observation trial 2013, p. 41–50. In: Midwest vegetable trial report for 2013. Purdue University, West Lafayette, IN
Maynard, D. & Hochmuth, G. 2007 Knott’s handbook for vegetable growers. 5th ed. Wiley, Hoboken, NJ
Ohio Environmental Protection Agency Division of Drinking and Ground Waters 2014 Nitrate in public drinking water. 6 May 2020. <https://www.epa.state.oh.us/Portals/28/documents/pws/Nitrate_in_Public_Drinking_Water.pdf>
Sen Nag, O. 2017 World leaders in lettuce production. 27 May 2020. <https://www.worldatlas.com/articles/world-leaders-in-lettuce-production.html>
Spalding, D. & Coolong, T. 2008 Romaine lettuce cultivar trial, p. 28–29. In: E. Maynard (ed.). Midwest vegetable trial report for 2008. Purdue University, West Lafayette, IN
Swiader, J. & Ware, G. 2002 Producing vegetable crops. 5th ed. Pearson, London, UK
U.S. Department of Agriculture 2011a U.S. lettuce: Per capita use, 1960-2010. 27 May 2020. <https://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID=1576>
U.S. Department of Agriculture 2011b U.S. romaine lettuce: Acreage, yield, production, and value, 1992-2009. 27 May 2020. <https://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID=1576>
U.S. Department of Agriculture 2011c U.S. leaf lettuce: Acreage, yield, production, and value, 1992-2009. 27 May 2020. <https://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID=1576>
U.S. Department of Agriculture 2011d U.S. head lettuce: Acreage, yield, production, and value, 1950-2009. 27 May 2020. <https://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID=1576>
U.S. Environmental Protection Agency 1991 Nitrate; CASRN 14797-55-8. 10 May 2020. <https://cfpub.epa.gov/ncea/iris/iris_documents/documents/subst/0076_summary.pdf>
U.S. Environmental Protection Agency 1993 Method 351.2, revision 2.0: Determination of total kjeldahl nitrogen by semi-automated colorimetry. 1 July 2017. <http://www.epa.gov/sites/production/files/2015-08/documents/method_351-2_1993.pdf>
Ward, M., Jones, R., Brender, J., de Kok, T., Weyer, P., Nolan, B., Villanueva, C. & van Breda, S. 2018 Drinking water nitrate and human health: An updated review Intl. J. Environ. Res. Health 15 1557