The economic value of spinach (Spinacia oleracea L.) continues to grow in the United States as well as globally due to its high nutritive content. Spinach production has been valued over $250 million in the United States in 2014 (USDA, 2015). Spinach is grown for fresh market, freezing, and canning; and 90% of the spinach grown in the United States is for fresh market (Naeve, 2015). California, Arizona, Texas, and New Jersey grow up to 98% of the commercial fresh market spinach (Naeve, 2015). Nearly half of California’s spinach is grown in Monterey County, and although spinach can be grown there nearly year-round, production is limited to the regions and seasons that meet the temperature requirements of spinach (Koike et al., 2011). In Arizona and Texas, production mainly takes place in the winters (CFAITC, 2014).
Spinach is a cool-season vegetable and depending on climate, typically grown in the early spring or late fall when the danger of higher temperatures is not as high (Anderson, 2014). Prior experiments have shown that spinach seeds will germinate in soil temperatures from 5 to 30 °C with germination percentages highest at 20 °C and dropping abruptly between 25 and 30 °C (Atherton and Farooque, 1983). Spinach seed germination has been reported to cease entirely at 35 °C (Leskovar and Esensee, 1999). Substantial seedling root development requires temperatures above 18.9 °C, and top growth will be limited at temperatures below 12.3 °C and above 23.3 °C (Wilcox and Pfeiffer, 1990). Studies have been done on the heat-shock response of spinach, both with whole plants and detached leaf tissue. It has been reported that after being exposed to heat shock (35–50 °C) for 30 min, CO2 assimilation decreases and pigment proteins in thylakoid membranes aggregate, slowing down the plant’s ability to photosynthesize (Tang et al., 2007). In addition, the first heat-shock proteins in spinach leaf tissue are induced when the temperature reaches 28 °C and a full range of heat-shock proteins are produced at 36 °C (Somers et al., 1989). If a spinach genotype has a high germination percentage in high temperature such as 35 °C, it should be a heat-tolerant spinach in germination stage.
Rapid and uniform germination is also necessary for efficient crop production, both in field and greenhouse practices. Although it has been reported that seed treatments may be effective for increasing germination of spinach at higher temperatures (Katzman et al., 2001), managing this trait via selecting heat-tolerant genotypes is a more manageable practice for spinach producers.
Germination under heat stress may also play a significant role in selecting heat-tolerant genotypes. Historically, mass selection was the primary method for developing genotypes, with hybrid breeding becoming popular in recent years, but all are based on field testing (Morelock and Correll, 2008). Although field testing is necessary in many cases, there are numerous environmental effects that contribute to germination performance beyond that of temperature. Therefore, it may be useful to reduce the number of genotypes planted by preliminary testing, improving the statistical approach to reduce error and estimate genotype by environment interaction. Using germination as this pretest has been successful in other crops, such as sorghum (Tiryaki and Andrews, 2001), and would allow quicker and more efficient selections to be made.
The objectives of this study are to determine how temperature affects spinach seed germination and evaluate potential genetic variation for germinating under heat stress.
Anderson, C.R. 2014 Home Gardening series: Spinach. FSA 6077. University of Arkansas Division of Agriculture. 13 Jan. 2014. <http://www.uaex.edu/Other_Areas/publications/PDF/FSA-6077.pdf>
Association of Official Seed Analysts (AOSA) 1993 Rules for testing seeds. Assn. Offic. Seed Analysts, Bozeman, MT
Brandenberger, L.P., Correll, J.C., Morelock, T.E. & McNew, R.W. 1991 Characterization of resistance of spinach to white rust (Albugooccidentalis) and downy mildew (Peronospora farinosa f. sp. spinaciae) Phytopathology 84 431 437
California Foundation for Agriculture in the Classroom (CFAITC) 2014 Commodity fact sheet: Spinach. CFAITC, Sacramento, CA
Correll, J.C., Bluhm, B.H., Feng, C., Lamour, K., du Toit, L.J. & Koike, S.T. 2010 Spinach: Better management of downy mildew and white rust through genetics Eur. J. Plant Pathol. 129 193 205
Hum-Musser, S.M., Morelock, T.E. & Murphy, J.B. 1999 Relation of heat-shock proteins to thermotolerance during spinach seed germination. Horticultural studies Ark. Agr. Expt. Sta. Res. Ser. 466 103 105
Koike, S., Cahn, M., Cantwell, M., Fennimore, S., Lestrange, M., Natwick, E., Smith, R. & Takele, E. 2011 Spinach production in California. Univ. California Agr. Natural Resources. Publ. 7212
Leskovar, D. & Esensee, V. 1999 Pericarp, leachate, and carbohydrate involvement in thermoinhibition of germinating spinach seeds J. Amer. Soc. Hort. Sci. 124 301 306
Machado, J., Souza, M., Oliveira, D., Cargnin, A., Pimentel, A. & Assis, J. 2010 Recurrent selection as breeding strategy for heat tolerance in wheat Crop Breed. Appl. Biotechnol. 10 9 15
Morelock, T.E. & Correll, J.C. 2008 Spinach, p. 189–218. In: J. Prohens and F. Nuez (eds.). Vegetables I: Asteraceae, Brassicaceae, Chenopodiaceae, and Cucurbitaceae. Springer, New York, NY
Naeve, L. 2015 Spinach. Agr. Mktg. Resource Ctr. 22 June 2016. <http://www.agmrc.org/commodities-products/vegetables/spinach/>
Snow Seed Company 2013 Products list: Spinach. 12 Nov. 2013. <http://www.snowseedco.com/vegetables/spinach/>
Somers, D., Cummins, W.R. & Filion, W.G. 1989 Characterization of the heat-shock response in spinach (Spinacia oleracea L.) Biochem. Cell Biol. 67 113 120
Swallow Tail Garden Seeds 2015 Spinach seeds. 22 June 2016. <http://www.swallowtailgardenseeds.com/veggies/spinach.html>
Tang, Y., Wen, X., Lu, Q., Yang, Z., Cheng, Z. & Lu, C. 2007 Heat stress induces and aggregation of the light-harvesting complex of photosystem II in spinach plants Plant Physiol. 143 629 638
Tiryaki, I. & Andrews, D. 2001 Germination and seedling cold tolerance in sorghum: I. Evaluation of rapid screening methods Agron. J. 93 1386 1391
U.S. Climate Data 2015 Climate Fort Smith—Arkansas. 22 June 2016. <http://www.usclimatedata.com/climate/fort-smith/arkansas/united-states/usar0197>
U.S. Department of Agriculture (USDA), National Agriculture Statistics Service 2015 Vegetables 2014 Summary. U.S. Dept. Agr. Natl. Agr. Stat. Serv., Washington, DC
Waters, E.R., Garrett, J.L. & Vierling, E. 1996 Evolution, structure, and function of the small heat-shock proteins in plants J. Expt. Bot. 47 325 338
Wilcox, G.E. & Pfeiffer, C.L. 1990 Temperature effect on seed germination, seedling root development, and growth of several vegetables J. Plant Nutr. 13 11 1393 1403