| Summary: | Lettuce harvested at the baby leaf stage is a popular component of mixed salads in ready-to-use packages. Little is known, however, about response of baby leaf lettuce to nitrogen (N), phosphorus (P), and potassium (K) fertilization. Eight lettuce accessions were subjected to five fertilization treatments to investigate genetic differences in reaction to N, P, and K fertilization. The control treatment provided optimal levels of macronutrients for plant growth, while other treatments deprived plants of either N, P, or K. Potassium deprivation had no obvious effect on plant weight or composition, apart from substantially decreased potassium content. Nitrogen and phosphorus deprivations, however, extensively decreased fresh weight and affected plant composition. Phosphorus and nitrogen deprivation considerably increased anthocyanin content in red-colored accessions, but anthocyanin was decreased in dark green-colored accessions, indicating differences in regulation of anthocyanin biosynthesis. Correlations between fresh weight, chlorophyll, anthocyanin, nitrogen, phosphorus, and potassium content were substantially affected by selection of datasets used for analyses; some relationships were revealed when analyzed separately by individual treatments, while others were more likely to be detected when analyzed by individual accessions. Absolute (Δ<sub>ABS</sub>) and relative (2Δ<sub>REL</sub>) parameters described in this study were suitable for detecting over- and underperforming accessions. The Δ<sub>ABS</sub> identified the absolute Lb-fold (logarithm to the base of 2, binary logarithm) change in performance of an accession in a treatment as compared to its performance in control conditions. The 2Δ<sub>REL</sub> parameter showed relative Lb-fold change for an accession as compared to the overall mean of Δ<sub>ABS</sub> values of all accessions tested in control and treatment conditions.
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