A field experiment commenced during the winter of 2020–2021 at the Al-Hamdhyia Research Station, College of Agriculture, University of Anbar, Anbar, Iraq. The research evaluated the growth, yield, and quality traits of four flax (Linum usitatissimum L.) cultivars (Sakha-1, Sakha-4, Sakha-6, and Local) under the influence of three levels of potassium fertilizer (0, 25, and 50 kg ha^-1). The experiment, in a randomized complete block design (RCBD), had a factorial arrangement and three replications. The results showed the flax cultivar Sakha-1 was significantly superior in the fruiting branches per plant, capsules per plant, seeds per capsule, seed yield (1.297 t ha^-1), and seed oil percentage. However, cultivar Sakha-4 excelled in plant height, straw yield, fiber length, and fiber yield (1.869 t ha^-1). Moreover, the increased level of potassium fertilizer (50 kg ha^-1) led to a significant increase in its plant height, fruiting branches per plant, seeds per capsule (8.01 seed capsule^-1), and oil percentage. Meanwhile, the potassium fertilizer increase at 25 kg ha^-1 was substantially superior in the capsules per plant and seed yield (1.261 t ha^-1) for the Sakha-4 cultivar.

Keywords: Flax (L. usitatissimum L.), potassium levels, growth and morphological traits, seed yield, seed oil percentage, fiber yield

Key findings: The flax (L. usitatissimum L.) cultivars significantly differed for morphological traits. Flax genotypes showed varied values for various agronomic and quality traits with different levels of potassium. For improving the flax seed oil and lint, cultivars Sakha-1 and Sakha-4 could benefit as a breeding source for further improvement.