Lettuce is a nutritious plant that contains vitamins such as A, K, C, and folates [1
]. Due to its comparatively large consumption in raw form, lettuce is also a notable source of minerals such as iron (Fe), calcium (Ca), and magnesium (Mg) as well as antioxidants. Red-leaf lettuce cultivars are especially high in phytonutrients such as phenolics and anthocyanins, which have been linked to the reduced incidence of chronic and degenerative diseases [2
]. According to the 2012 United States census of agriculture, lettuce (head, leaf, and romaine combined) ranks first in total production value of fresh market vegetables. In the southern United States, open field and greenhouse lettuce production occur most commonly in the fall and winter seasons. This allows growers to take advantage of cooler temperatures and shorter days, and to maximize economic returns due to fewer overhead requirements and increased product quality. However, the progressing growth of the population and affluent communities coincides with public demand for year-round access to locally sourced, high-quality lettuce.
When grown in adverse environmental conditions, lettuce yield and quality can be severely diminished. For example, subjecting lettuce to high air and root-zone temperatures accelerates the inflorescence initiation (bolting) and results in reduced marketability [3
]. This issue is especially exaggerated during the summer in the southern United States due to consistent exposure to high temperatures and long days, which can increase bolting and lettuce bitterness. Moreover, Jenni reported that crisphead lettuce exposed to heat stress for a 3 or 5-day period, two weeks after heading resulted in 46% of mature lettuce heads with rib discoloration [5
]. Additionally, impacts on yield and quality are heavily associated with fertilization. For example, previous research has reported that increasing nitrogen (N) from 0 to 60 mg·L−1
resulted in greater shoot mass in lettuce grown at deep water culture, and lettuce supplied with 400 mg·L−1
N and supplemental light resulted in accelerated growth and shoot dry matter [6
]. However, Fu et al. [8
] found that leafy lettuce grown under high light (220 mol·m−2
) and low N (7 mmol·L−1
) resulted in increases in shoot biomass and vitamin C. Crisphead lettuce grown with a shortage of Ca resulted in chlorosis at the leaf margins and loss of apical dominance. Additionally, limiting boron (B) caused leaves to become wrinkled, stunted plant growth, and resulted in the loss of apical dominance [9
]. Conversely, Barickman et al. [10
] reported that romaine lettuce grown with increasing potassium (K) concentrations showed improvement in biomass, leaf sucrose, and leaf K content. However, increasing K resulted in decreases in some leaf minerals such as Ca, Mg, and sulfur (S) [10
While K is not a known component of any functional plant molecules or structures, it is essential for growth and required in relatively large quantities. Potassium plays a vital role in protein synthesis, activation of enzymes, and photosynthesis [12
]. Additionally, it is an essential factor in the translocation of photo-assimilates from source to sink tissues, stomatal conductance, and the maintenance of cell turgor pressure [10
]. Potassium also plays a critical role in the survival of plants under stressful environmental conditions by protecting against light-induced cell damage, drought, and enhanced resistance to damage from low temperatures [15
] These abiotic stresses commonly result in the increased production of reactive oxygen species (ROS); however, potassium lowers the production of ROS by maintaining the electron transport chain and reducing NAD(P)H oxidase activity [15
Studies of the influence of K concentration on yield and quality concerning leaf lettuce are lacking, although research on crisphead lettuce and several other crops have examined its effect. For example, crisphead lettuce and Chinese cabbage grown in increasing levels of potassium nitrate (KNO3
) (1, 5, or 10 mM) showed maximum yield at 5 mM, decreased leaf sodium (Na) and chloride (Cl) content, and increased leaf N and K content in both saline and non-saline conditions [16
]. Moreover, Tzortzakis [17
] reported that Ca and K enrichment alleviated salinity-induced stress in hydroponically grown endives. Promising supplemental K work has been conducted in muskmelon (Cucumis melo
). For example, late-season foliar applications of two different K formulations improved fruit marketability attributes such as yield, firmness, and sugar content [12
]. Additionally, increases in human-health related bioactive compounds, ascorbic acid and β-carotene, were observed [12
]. However, research on adequate and elevated levels of K as it relates to lettuce yield and quality is inconclusive. For example, the fresh and dry shoot weight of ‘South Bay’ lettuce plantlets were unaffected by increasing K levels from 15 to 60 mg L−1
]. Bres and Weston [18
] concluded that increasing K or pH levels did not consistently affect tipburn incidence in three lettuce cultivars. Conversely, Fallovo [19
] found that seasonal changes in combination with increasing fertilizer concentrations resulted in plant growth and yield increases in hydroponic leafy lettuce. The current evidence on the effect of K treatments on lettuce yield and quality has produced some promising results. However, further research is needed to understand this dynamic relationship better. Thus, the purpose of this study was to determine the effect of temperature and adequate and elevated K levels on growth chamber produced red-leaf lettuce biomass accumulation, mineral nutrient uptake, and phenolic content.
The current study examined how increasing K concentration in a deep-water culture production system affected the growth and quality of ‘Dark Red’ ‘Lollo’ Rosso lettuce under mild and extreme temperatures. Lettuce is a cool season crop with a preferred temperature range of 7–24 °C. Inflorescence initiation is encouraged by exposure to high temperatures and photoperiods of ≥13 h [23
]. Producing lettuce above these thresholds affects plasma membrane fluidity, microtubules and microfilaments, and the photosynthetic apparatus [24
]. This causes an intracellular signal cascade that ultimately reduces plant yield and quality. Previous studies have examined the effect of elevated temperatures on yield, quality, and growth rate for many vegetables. For example, plants grown at supra-optimal temperatures resulted in smaller tomato fruit, reduced flower and fruit set in bell peppers, and decreased net photosynthesis and chlorophyll content of Chinese cabbage [25
]. Adverse temperatures also altered the form and color of cucumber and eggplant fruit and increased the relative growth rate of lettuce plants by 33% to 85% [28
The results of the current study diverged from the common effects of heat stress on lettuce and other crops. Differences in production temperatures resulted in an inverse relationship between root and shoot development. Plants grown at 33 °C demonstrated greater production of shoot fresh mass, dry mass, and biomass, while plants grown at 25 °C had greater leaf water content, root fresh mass, and root dry mass. The degree to which lettuce can tolerate adverse temperatures is known to vary greatly among cultivars. In at least one variety trial comparing 21 cultivars of red-leaf lettuce, ‘Lollo Rosso’ was among the top four cultivars that exhibited the greatest resistance to heat stress [30
]. Thus, plants produced at the highest temperature in the current study may have withstood deleterious effects on fresh mass, dry mass, and biomass due to heat tolerance inherent in the ‘Lollo Rosso’ cultivar. Moreover, the lack of exogenous stress on plants produced in milder temperatures may have allowed for the greater generation of root fresh and dry mass.
With respect to the production of secondary metabolites, increasing K concentration did not affect lettuce flavonoid and phenolic acid content. The highest concentrations of phenolics were notably present in lettuce plants produced at the highest temperature treatment, which is consistent with the literature demonstrating increased production of phenolics under warm temperatures in lettuce, tomato, and watermelon [31
]. Instead, elevating the temperature to 33 °C resulted in significant differences of flavonoid content in lettuce plants. Unlike amino acids, sugars, and fatty acids, flavonoid and phenolic acid compounds are not required by plants for survival [33
]. However, production of secondary metabolites is an adaptive mechanism by plants to aid in protection from environmental stressors. For example, high light intensity often results in the production of secondary metabolites called anthocyanins in flowering plants. Anthocyanins offer plants additional photoprotection due to the ability to absorb radiation in the photosynthetically active spectrum [34
Previous research has examined the role of flavonoids and phenolic acids in mitigating abiotic stress such as extreme heat. For example, Oh et al. [37
] reported that concentrations of chicoric and chlorogenic acid, quercetin-3-O
-glucoside, and luteolin-7-O
-glucoside increased significantly in five-week-old lettuce plants subjected to heat shock treatments. Additionally, flavonoid content increased in three pigmented baby lettuce cultivars as growing season progressed from February to May [38
]. Conversely, Boo et al. [39
] stated that total polyphenol concentrations decreased with increasing heat treatments. However, another study concluded that lettuce cultivar was a larger contributing factor to the accumulation of phenolic compounds among five tested varieties [40
]. The results of the current study suggest that elevating temperatures were responsible for the increases in flavonoid and phenolic content, since light intensity and photoperiod were equal and held constant in both temperature treatments.
Increasing concentrations of K did not significantly affect the uptake of leaf mineral nutrients except for calcium, which increased among plants treated with 2× (234.6 mg·L−1
) and 8× (938.4 mg·L−1
) levels of K. Although not statistically significant, a visible trend was observed among plants exposed to 2× treatments of K, containing the greatest concentrations Mg, K, Fe, Mn, and Mo. Although K is not a constituent of any functional plant molecules or structures, it is vital to plant growth and development and is known to affect the uptake of mineral nutrients such as Ca and Mg [13
]. Several studies have explored the impact of increasing nutrient solution concentrations on leaf mineral nutrient content. For example, Fallovo et al. [19
] investigated the effects of increasing nutrient solution electrical conductivity (EC) on yield and quality of leafy lettuce. Results suggested that concentrations of N, P, K, and Mg increased concerning increasing nutrient solution EC. Similar results were confirmed by Chiloane [41
], who found that N and P had a tendency to increase with increasing solution EC, and K increased with increasing solution EC before declining with the highest treatments.
Furthermore, Barickman et al. [10
] demonstrated that elevating K in red-romaine lettuce grown in a soilless production medium increased leaf concentration of K by 43% but resulted in decreases in Ca, Mg, and S concentrations. Additionally, results from this study indicated that leaf mineral nutrient content increased before reaching a saturation point and declined with the greatest K treatments [10
]. The results of the current study are consistent with the findings of previous studies. In comparison to control treatments, several nutrients increased with 2× (234.6 mg·L−1
) treatments before content declined at 4× (469.2 mg·L−1
) and 8× (938.4 mg·L−1
). Additionally, concentrations of the macronutrients Mg, K, and Ca and micronutrients Mn and Mo were significantly greater at the lowest temperature treatment, suggesting that lack of exogenous temperature stress supported root productivity and facilitated the uptake and assimilation of minerals better than plants experiencing abiotic stress.
In conclusion, continuing research is needed to develop a better understanding of the dynamic relationship between abiotic stress, such as adverse temperatures and elevated EC, and the deterministic role these factors play in lettuce growth, development, and nutritional quality. While previous research has demonstrated the efficacy of increasing K on positive growth and qualitative characteristics, results vary among crop species and cultivars. In this situation, the application of increasing levels of K produced comparable levels of leaf DM at 117.3 mg·L−1 and 938.4 mg·L−1 and resulted in 14% more leaf calcium content at 234.6 mg·L−1, compared to the control treatment of 117.3 mg·L−1. However, previous research suggested that the beneficial effect of increasing K on some leaf mineral nutrients in lettuce may come with a reduction in others due to competition at the site of root uptake, generating inconsistency in nutritional quality. Ultimately, differences in growth chamber temperatures generated the most significant impact on lettuce biomass, phenolic content, and leaf mineral concentrations. Therefore, the results from this study suggest that temperature is a stronger regulatory factor than increasing K in the determination of lettuce yield and quality; however, increasing K concentration to 234.6 mg·L−1 resulted in higher concentrations of leaf calcium without compromising lettuce yield or quality.