Published May 12, 2014
4 Lettuce and Spinach Ivan Simko, Ryan J. Hayes, Beiquan Mou, and James D. McCreight* A bs t rac t
Lettuce (Lactuca sativa L.) and spinach (Spinacia oleracea L.) are cultivated in many countries around the world. Their production in the United States is concentrated mostly in California and Arizona, where they are grown year-round. U.S. lettuce products are categorized into three market uses: whole heads, bulk harvest (for salad processing, food service, or value-added products), and “baby leaf” or “spring mix.” The three main lettuce types produced in the United States are iceberg (L. sativa L. var. capitata L.), romaine (L. sativa L. var. longifolia L.), and leaf (L. sativa var. crispa L.). Iceberg yield increased 183% from 1950 to 2012, romaine yield increased 12% from 1992 to 2012, and leaf yield increased 14% from 1992 to 2012. Comparative yield trials of old and modern lettuce cultivars have not been done. Yield components were identified for each market use. Spinach market uses are fresh, which may be bunched, bagged [with small (baby) or intermediate sized leaves], or blended into spring mix, and processed (frozen or canned). Spinach yield varies with locations and seasons, but gains in yield have largely been attributed to the development of hybrid cultivars that exhibit heterosis. Fresh market spinach yield in California increased 50% from 1961 to 2011. Increased lettuce and spinach yield due to changes in agronomic practices (e.g., wide beds and high plant density) and more uniform and disease- and pest-resistant cultivars are offset to some extent by shifts in market uses (e.g., reduced proportion of the head harvested for romaine lettuce hearts) and early harvest for baby leaf lettuce and spinach. Sustained and increased lettuce and spinach yield depend on further optimized agronomic practices and productive cultivars with desirable quality, uniformity, and disease and pest resistance that are adaptable to variable environmental conditions and have increased water and nitrogen use efficiency.
Abbreviations: DPP, days post-planting; LIYV, Lettuce infectious yellows virus; QTL, quantitative trait locus; SPWF; Sweet potato whitefly. I. Simko ([email protected]
), R.J. Hayes ([email protected]
), B. Mou (beiquan. [email protected]
), and J.D. McCreight, USDA-ARS, Crop Improvement and Protection Research Unit, 1636 E. Alisal St., Salinas, CA 93905. *Corresponding author ([email protected]
). The authors contributed equally to this review. doi:10.2135/cssaspecpub33.c4 Yield Gains in Major U.S. Field Crops. CSSA Special Publication 33. Stephen Smith, Brian Diers, James Specht, and Brett Carver, editors. © 2014. ASA, CSSA, and SSSA, 5585 Guilford Rd., Madison, WI 53711-5801, USA.
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is a self-fertilizing diploid species (2n = 2x = 18) from the Compositae family. Lettuce is the most popular, commercially produced leafy vegetable and is cultivated mainly in moderate climates in many countries around the world. Over 79% of the world production that totaled 24.3 million tonnes in 2011 (FAOSTAT, 2013) originated from four countries: China, with 55.2% of the total production (by weight), the United States (16.0%), India (4.4%), and Spain (3.6%). Lettuce cultivars are divided into eight horticultural types on the basis of the shape and size of the head; the shape, size, and texture of leaves; stem length; and seed size (Fig. 4–1). The leaves of crisphead (which includes two subtypes, iceberg and Batavia), romaine, butterhead, leaf, and Latin types are typically eaten
Fig. 4–1. Lettuce types frequently grown in the United States: (top row, left) iceberg; (top row, right) Batavia; (second row, left) romaine; (second row, right) speckled romaine; (third row, left) green leaf; (third row, right) red leaf; (bottom row, left) red leaf; and (bottom row, right) butterhead.
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fresh. The stem type is cultivated mainly for edible stems, fresh or cooked. The oil type is used for cooking oil from its relatively large seeds. There are no modern cultivars of the stem or oilseed types commercially grown in the United States. Detailed descriptions of each lettuce type were provided by Rodenburg (1960) and Ryder (1999) and are only summarized here. Crisphead cultivars are known for their thick, crisp leaves that cup and form a spherical head weighing approximately 0.8 to 1 kg (1.8–2.2 lb). Butterhead cultivars produce spherical heads that weigh approximately 0.2 to 0.4 kg (0.44–0.88 lb); the leaves of this type are known for their pliability and oily texture. Romaine cultivars exhibit leaves that are much longer than wide and are typically upright to form an elongated head. Heads of commercially produced romaine weigh approximately 0.75 kg (1.65 lb). The top of the romaine head may or may not close over the inner leaves. Latin types produced quite small heads, less than 0.3 kg (5%. Lettuce processed into salad requires even higher stringencies. Tipburn occurs during a narrow period of time immediately preceding harvest. The occurrence of tipburn is correlated with environmental conditions that either decrease transpiration from the leaf surface or promote rapid growth, both of which impair the plants ability to supply sufficient calcium to growing tissues, which then results in tipburn (Barta and Tibbitts, 2000; Thibodeau and Minotti, 1969). These conditions include low air movement (Goto and Takakura, 1992), artificial or natural enclosure of the leaves (Barta and Tibbitts, 1986), high relative humidity during the daytime (Collier and Tibbitts, 1982), high temperatures (Cox et al., 1976; Misaghi and Grogan, 1978; Yanagi et al., 1983), high radiation levels (Tibbitts and Rao, 1968), long photoperiods (Koontz and Prince, 1986), and high fertilization levels. Growers cannot control all of the environmental factors involved in tipburn, and breeding for tipburn resistance is the best long-term solution. Most breeding for tipburn resistance is conducted by selective culling of plants, families, and lines with symptoms. Using this approach, iceberg lettuce cultivars have been bred with levels of resistance that are adequate for most planting slots (Ryder and Waycott, 1998), although no cultivars are completely resistant (Nagata and Stratton, 1994). Genetic improvements for tipburn resistance are needed most in the romaine gene pool (Jenni and Hayes, 2010). Screening cultivars, breeding lines, and segregating families for tipburn resistance can be performed in the field or in controlled environments, but the results are not always consistent (Cox et al., 1976; Nagata and Stratton, 1994). Tipburn incidence is subject to genotype ´ environment interaction (Jenni and Hayes, 2010), and extensive field testing is needed to fully characterize resistance in cultivars or breeding lines. Little is known regarding the genetics of tipburn resistance. While cultivars exhibited extensive genetic variation for the incidence of the disorder, plant architecture is a major factor influencing the prevalence of the disorder, with open top and slower maturing cultivars expressing lower tipburn incidence (Jenni and Hayes, 2010). The dependence of tipburn incidence on plant architecture makes comparison of resistance across market types virtually impossible. Resistance genes that are independent of morphology are most valuable, as they could improve resistance in all market types. The high level of resistance in iceberg cultivars such as Salinas (Ryder, 1979b) and Tiber (Ryder and Waycott, 1998) have been proposed as sources of resistance to breed resistance in other types (Jenni
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and Hayes, 2010). Six QTLs for tipburn resistance were discovered in two intraspecific recombinant inbred line populations tested in Arizona, and four of the loci cosegregated with head closure, core height, or head maturity (Hayes and Simko, 2010). Global climate change may affect agriculture more through its impact on water availability than on temperature. All lettuce and spinach production in California and Arizona are surface irrigated, but future water availability for growers is expected to decrease. Water is a precious resource and has become increasingly scarce because of population growth, environmental needs, and frequent drought. Climate change has resulted in less predictable precipitation, less snow pack, and earlier snowmelt, leading to periods of water shortages in California (Weare, 2009). Court orders limit the pumping of Northern California water to farms in San Joaquin Valley, severely restricting the leafy vegetable production. Coastal Monterey County faces water shortage and seawater intrusion and has planned the construction of several desalination plants for residential uses. Conversely, ever tighter regulations on farm runoffs call for crop varieties with reduced irrigation requirement and better water use efficiency (WUE). Cultivars with improved WUE will tolerate water deficit stress to the degree that product quality and yield are maintained, significantly reducing growers’ irrigation costs. As the costs of land, labor, fuel, fertilizer, pesticides, seeds, packing material, cooling, transportation, and overhead including food safety continue to rise, it is essential to reduce production costs of leafy vegetables to benefit producers as well as consumers. With the limited water supply, decreases in crop water requirements could result in proportional increases in lettuce and spinach production area. In addition to the conservation of water resources, the decreased irrigation needs of drought-tolerant lettuce and spinach may reduce farm runoff and pesticides leaching into groundwater, helping growers with regulatory compliance and alleviating the pressure on the environment. Research to increase yield in production systems that use limited irrigation is a priority for lettuce breeding but is still in its infancy. Cultivated lettuce is reliant on frequent irrigation and high input of nutrients because of its short taproot and prolific lateral branches in the upper layers of the soil (Jackson, 1995). Contrary to cultivated lettuce, L. serriola, its wild progenitor, develops a long taproot and relies on water from deep soil zones during periods of surface soil drought (Jackson, 1995). The wild L. serriola is, therefore, a potential source of agriculturally important alleles to optimize water and fertilizer use efficiency (Johnson et al., 2000) when integrated with improved irrigation and fertilizer practices. A single QTL was identified in an L. sativa ´ L. serriola population controlling increased taproot length per gram of plant biomass. This QTL was independent of QTL for shoot biomass and the percentage of biomass allocated to roots (Johnson et al., 2000). These results suggest that breeding for a reallocation of root biomass may develop cultivars that can access nutrients from deeper soil depths. Selection for increased root biomass may not be advantageous, as gains are likely to come at the expense of shoot growth and therefore yield (Johnson et al., 2000). Weed problems will become more prominent with global warming, as weeds are expected to adapt to the changing environment better than crops (Clements and DiTommaso, 2012). Weeds in leafy vegetables increase production costs and reduce yields and quality. Weed cover of 25% reduces lettuce yield by 20 to 40%,
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and >25% weed cover may cause complete yield loss. Baby lettuce, a primary component of spring mix, and spinach have zero tolerance for weed contamination, yet high plant densities prevent mechanical cultivation. Herbicides are essential for profitability, but few herbicides are registered for lettuce and spinach in California, and they are subject to cancellation by manufacturers because of their relatively small acreage and low sales volumes, marginal profitability, and high regulatory costs. The lettuce industry has depended on pronamide (Kerb) as the major herbicide. The U.S. Environmental Protection Agency stopped the use of Kerb on leaf lettuce in 2009 because of crop group reclassifications. There is no certainty that Kerb will be reregistered on leaf lettuce, as the herbicide has been classified as a B2 carcinogen and found to leach into groundwater in Virginia (Bruggeman et al., 1995). Herbicide producer Helm Agro suspended in 2008 to 2009 the production of RoNeet, the primary herbicide for spinach. Bayer Co. recently stopped production of Spin-Aid, an herbicide for processing spinach. The prospect of losing effective herbicides is a constant threat for California leafy greens industry. A decade of searching for new lettuce and spinach herbicides has failed, as the crops are very sensitive to herbicide damage. Kerb does not effectively control some weeds, for example, sowthistle (Sonchus spp.) and groundsel (Senecio spp.), and there is no postemergence broadleaf herbicide available for these crops. The loss of Kerb or RoNeet with no adequate herbicide replacement may have a considerable adverse effect on the industry. The loss of Kerb for leaf lettuce has significantly increased production costs by >US$371 per hectare (>US$150 per acre) because of hand weeding (Haar and Fennimore, 2003). Increased hand weeding costs often mark the difference between profit and loss for leafy greens producers. A simple, reliable, and sustainable weed management system might be developed through the use of herbicide-tolerant lettuce and spinach cultivars. A herbicide-tolerant crop is an attractive option, since a broad-spectrum herbicide can remove weeds without injuring the tolerant crop. Herbicide tolerance is a proven approach to weed control as 73% of corn (Zea mays L.), 93% of soybean [Glycine max (L.) Merr.], and 80% of cotton (Gossypium hirsutum L.) crops in the United States were planted with herbicide-tolerant varieties in 2012 (USDA-ERS, 2012). No transgenes are currently used in lettuce and spinach production. On the basis of feedback from the industry, lettuce and spinach cultivars developed by conventional breeding, the same method used successfully in the creation of herbicide-tolerant sunflower (Helianthus annuus L.), rice (Oryza sativa L.), and wheat (Triticum aestivum L.), will be more easily and widely accepted by growers, the industry, and consumers than transgenic crops. Such a breeding program for herbicide-tolerant lettuce and spinach is currently underway in our project by screening mutants in these crops (Fennimore et al., 2012). This project may provide efficient and much more effective weed control tools than growers have now and improve the profitability and sustainability of lettuce and spinach production.
The authors thank K. Nolte for providing photographs of lettuce and spinach production in Arizona, K.V. Subbarao for the figure of the lettuce production areas, and A. Atallah for help with editing figures and the revised version of the manuscript.
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