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Simple Summary: Potassium deficiency is a constraint that causes numerous dysfunctions at the plant level, notably a reduction in leaf growth, photosynthesis and production. Among the multiple physiological disturbances induced by potassium deficiency, some are manifested very early and others only when the deficiency is severe. Therefore, we studied the effect of insufficient potassium supply on physiological, biochemical and anatomical parameters in basil, Ocimum basilicum. Results revealed a decrease in root and leaf biomass production in K+-deficient plants in association with an inhibition of root elongation and a reduction in leaf expansion. Similarly, a decrease in photosynthetic pigments and variability in the response of phenolic compounds content was recorded, depending on the organ and the K+ concentration in the medium. Analysis of the anatomical structure of stems showed that exposure to potassium deficiency in basil decreased the diameter of these organs and the amount of lignin produced. The aim of this study was to investigate the effect of a variable supply of potassium to culture medium on physiological and anatomical parameters (histological sections at the third internode) in basil, Ocimum basilicum. Thirty-four-day-old plants grown on basic nutrient medium were divided into four batches and grown on media with varying doses of potassium: 0.375 mM, 0.250 mM, 0.125 mM and 0 mM K+. After 64 days of culture, a final harvest was performed. The results showed that root and shoot growth in basil was decreased with decreased K+ concentration. This restriction was associated with a reduction in root elongation and leaf expansion, which was coupled with a decrease in chlorophyll and carotenoid contents. The estimation of electrolyte leakage reveals that this parameter was increased by potassium deficiency. With respect to total polyphenol and flavonoid contents, only the third leaf-stage extracts exhibited a decrease under low-K+ conditions. However, variability in response of phenolic compounds was recorded depending on the organ and the K+ concentration in the medium. Stem cross sections of potassium-deficient basil plants revealed a decrease in the diameter of these organs, which can be attributed to a restriction of the extent of different tissue territories (cortex and medulla), as well as by a reduction in cell size. These effects were associated with a decrease in the number of conducting vessels and an increase in the number of woody fibers. [ABSTRACT FROM AUTHOR]