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−1 (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 field-grown muskmelons. They found that late season foliar K applications increased tissue K concentration, fruit sugars, and bioactive compounds such as ascorbic acid and β-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−1 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 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 greenhouse-grown lettuce plant height, biomass accumulation, mineral nutrient uptake, and soluble sugar concentrations.
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