Elevated Levels of Potassium in Greenhouse Red Romaine Lettuce Impacts Mineral Nutrient Content and Nutritional Quality

Potassium (K) is an essential plant mineral nutrient that can have significant influence on increasing many phytonutrients compounds in fruits and vegetables.  However, K is not a constituent of organic or structural compounds in the plant.  The involvement of K in many physiological and biochemical processes leads to plant growth, yield, and quality attributes.  Among the plant mineral nutrients, potassium (K) has been shown to have a strong impact on fruit quality attributes that determine marketability, consumer preference, and critically important human-health phytonutrients.  In recent years, research has demonstrated that flavonoids and vitamin C intake has many benefits in human diets due to antioxidant activity.  Consumer preference for more flavorful produce has also sparked interest to increase these quality characteristics.  Although K has been examined for quality parameter in many fruits and vegetables, research in lettuce (Lactuca sativa) production is lacking.  Thus, the purpose of this study was to determine the impact of K on yield, biomass, mineral elements, soluble sugars, and phytonutrients in red romaine lettuce.  Seeds of ‘Cimmarone’ romaine lettuce were grown in 1.8-L containers in the greenhouse at 21/17 °C (day/night) under a 10-h photoperiod.  Lettuce plants were arranged in a randomized complete-block design with 4 replications.  Plants were treated with provided K as K₂O at the rates of 3.36, 6.72, 13.44, and 26.88 g per 1.8 L container.  The K treatments were applied at three different times throughout the study.  Plants were harvested at 55 days after seeding and weighed for fresh mass and subsamples taken for analytical analysis.  In red romaine lettuce, there were differences in mineral nutrients.  Concentrations of soluble sugars, vitamin C, and flavonoids will also be discussed.  These results demonstrate that elevated levels of K can have a significant impact on nutritional quality in greenhouse grown red romaine lettuce.