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Lettuce is one of themajor crops of theUnitedStates and canprovide a large portion of income for small to medium size growers. Growing lettuce in adverse environmental conditions can have negative effects on quality. Elevated levels of potassium (K) have been shown to positively influence quality in various fruits and vegetables, such as tomato, pepper, and strawberry. However, research is lacking on the effects of elevated levels of K on leafy vegetables such as lettuce. Therefore, seeds of ‘Cimmaron’ lettuce were sown into a soillessmediumand grown in greenhouse conditions at 25/20 8C (day/night). At 27 days after seeding, the plantlets were transferred to 3.8-L plastic nursery pots. Plants were grown under increasing K treatments of 98 (control), 185 (23), 370 (33), and 740 (83) kg·ha. Plants were harvested 56 days after seeding. Application of elevated levels of K fertilizer treatments in red romaine lettuce had a positive quadratic effect on plant height increasing 7.0% from the control. Fresh weight (FW) increased 13.0% from the control and dry weight (DW) increased 15.5%. There was linear increase of 30.0% in sucrose concentrations in lettuce leaf tissue. In addition, the increase in K treatments caused an increase of 43.3% in K concentrations in the leaf tissue. In other nutrients, such as Calcium (Ca), Magnesium (Mg), and Sulfur (S), there was a decrease in the leaf tissue of 61%, 52%, and 46%when compared with the control treatment, respectively. The results of the current study suggest that increasing K fertilizer to 185 kg·ha has the best results for plant height, FW and DW, and mineral nutrient concentrations. This study may initiate research that could examine the effects of increasing K fertilizer levels in lettuce or other leafy green vegetables on antioxidant levels and postharvest storability. Open field and greenhouse production of lettuce (Lactuca sativa) in southern United States occurs predominately in the fall and winter seasons due to lower temperatures and shorter days. In most southern states, lettuce provides a large amount of income. Romaine type lettuce is the main lettuce grown by small to medium producers. Depending on plant spacing, the average yields of romaine lettuce can be around 30,000 kg·ha (Mossler and Dunn, 2005). Romaine type lettuce is a preferred leafy green grown in these operations because of consumer preferences due to its higher nutritional value compared with head type lettuce. In adverse environmental conditions, such as high temperatures and inadequate fertilization, lettuce can decrease in quality by having long internodes, becoming bitter in taste, and having increased incidences of tipburn (Bres and Weston, 1992; Eskins et al., 1995). For example, lettuce grown under limited Ca had a chlorotic appearance and reduced growth. Lack of boron (B) also caused lettuce to have wrinkled leaf tissue, lose apical dominance, and exhibit limited plant growth (Petrazzini et al., 2014). In addition, Luo et al. (2012) found that increasing K in lettuce plants elevated soluble sugar, starch, and leaf K concentrations when grown under different root-zone temperatures. On the other hand, effects of different temperature regimes resulted in differences in chlorophyll and anthocyanin content in greenhouse-grown red leaf lettuce (Kleinhenz et al., 2003). Thus, growing lettuce under adverse environmental conditions can have an unfavorable effect on growth, yield, and quality. Potassium is vital to plant growth, yield, and quality even though it is not a constituent of any functional molecules or plant structures (Marschner, 2012). In addition, it helps to regulate stomatal conductance and photosynthesis. Potassium is also involved in photophosphorylation, transport of photoassimilates from source to sink tissues via the phloem, enzyme activation, turgor maintenance, and stress tolerance (Marschner, 2012). However, K may be best known to positively influence many qualities of fruits and vegetables. Previous research has demonstrated that K fertilizer applied to the soil enhanced color, increased fruit tissue firmness, and elevated soluble sugar concentrations in apple (Malus ·domestica) fruit tissue (Nava et al., 2008). However, contrasting results have been indicated in previous research. For instance, Fallovo et al. (2009a) examined how different nutrition solution concentrations affected lettuce yield and quality. The results indicated that the effect of the growing season on yield and quality was more prominent than that of the nutrient solution composition. Thus, results from K research studies are variable in different plant species for growth, yield, and quality. Research has indicated that supplemental K nutrition has been associated with increased fruit size and higher soluble solid and ascorbic acid concentrations (Lester et al., 2005). Studies have shown that supplemental K increases yield and carotenoids in vegetables such as tomato (Solanum lycopersicum) (Fanasca et al., 2006; Taber et al., 2008) and pepper (Capsicum annuum) (Anathi et al., 2004), and size, color, firmness, and sugar content in tree fruits, such as apple (Wojcik, 2005) and citrus (Dutta et al., 2003; Srivastava et al., 2001). Some of the most successful supplemental K research has been conducted on muskmelon (Cucumis melo). For example, Lester et al. (2005, 2006) found that supplemental applications of K increased fruit firmness, vitamins, sugars, and yield. In addition, Jifon and Lester (2009) examined different forms of K fertilization to improve quality of fieldgrown muskmelons. They found that late season foliar K applications increased tissue K concentration, fruit sugars, and bioactive compounds such as ascorbic acid and b-carotene. Research associated with adequate and elevated levels of K on lettuce yield and quality is limited and inconclusive. Some studies found that K in nutrient solution did not affect lettuce yield and quality. For example, Fallovo et al. (2009b) investigated the effects of macro-anion and cation ratios in hydroponic lettuce in two different seasons. The results indicated that the change in season from spring to summer and increasing the fertilizer concentrations increase plant growth and yield. Soundy et al. (2001) demonstrated that increasing levels of K in the nutrient solution from 15 to 60 mg·L increased fresh and dry root weight at 28 d after transplant. Other studies found inconclusive results on the effects of K applications. Bres and Weston (1992) demonstrated that increasing pH levels at two different K concentrations did not affect tipburn Received for publication 19 Nov. 2015. Accepted for publication 29 Feb. 2016. This research was made possible through support from USDA NIFA Hatch S-294 project MIS 149160 andMississippi State University, Mississippi Agriculture and Forestry Experiment Station. Research Associate. Corresponding author. E-mail: t.c.barickman@ msstate.edu. 504 HORTSCIENCE VOL. 51(5) MAY 2016 incidence in hydroponically grown lettuce. In addition, Hoque et al. (2010) applied different combinations of nitrogen (N), phosphorus (P), and K but concluded that K did not affect quality compared with N and P. The limited research for K treatment effects on lettuce yield and quality demonstrate some positive results; however, more research is needed to explain variable results. The purpose of this study was to determine the effect of adequate and elevated levels of K on greenhousegrown lettuce plant height, biomass accumulation, mineral nutrient uptake, and soluble sugar concentrations. Materials and Methods Plant material, growing conditions, and treatments. Seeds of ‘Cimmaron’ lettuce (Harris Seeds, Rochester, NY) were sown into Pro-Mix BX soilless medium (Premier Tech Horticulture, Qu ebec, Canada) and germinated in greenhouse conditions at 25/ 20 C (day/night). At 27 d after seeding, the plantlets were transferred to 3.8-L plastic nursery pots filled with Pro-Mix BX soilless medium. Plants were fertilized with 28 g (Osmocote; Scotts Miracle-Gro, Marysville, OH) of 15N–3.9P–9.9K. The N, P, and Kwere then calculated on a kg·ha basis. Elemental concentrations of nutrients applied to the plants were (kg·ha): nitrogen (N; 139) and phosphorus (P; 36). Plants were grown under increasing K treatments of 98 (control), 185 (2·), 370 (3·), and 740 (8·) kg·ha. Elevated K treatments were applied as potash (0N–0P–49.8K). The experiment was carried out on 16 Sept. 2014 and on 2 Mar. 2015. Natural photoperiod and intensity of sunlight averaged 350 mmol·m·s over the entire photoperiod. Light intensity readings were taken at 1.22 m off the ground. Experimental design was a randomized complete block with four K treatments, four replications, and five individual pots representing an experimental unit with one lettuce plant per pot. Treatments of elevated K were sidedressed at three different times during the experiment at 1/3 the amount of the full treatment total. Plants were harvested at 54 d after seeding. Subsequently, lettuce plants from each treatment were separated by replication and weighed for biomass. At least three leaves from each experimental unit were subsampled, frozen, and prepared for elemental nutrient and soluble sugars. Samples were stored at –20 C before analysis. Mineral composition. Nutrient analysis was conducted according to Barickman et al. (2013) with slight modifications. In brief, leaves were collected and dried for 48 h in a forced air oven (model large; Fisher Scientific, Atlanta, GA) at 65 C. Dried samples were ground to homogeneity using liquid nitrogen, and 0.5-g subsamples were combined with 10 mL of 70% HNO3 and digested in a microwave digestion unit (Model: Ethos; Milestone Inc., Shelton, CT). Nutrient analysis was conducted using an inductively coupled plasma mass spectrometer (ICP-MS; Agilent Technologies, Inc., Wilmington, DE). The ICP-MS system was equipped with an octapole collision/reaction cell, Agilent 7500 ICP-MSChemStation

Elevated Levels of Potassium in Greenhouse-grown Red Romaine Lettuce Impacts Mineral Nutrient and Soluble Sugar Concentrations