Physiological effects of foliar-applied β-carotene and carrot root extract on growth, biochemical traits, and yield of two flax varieties grown in sandy soil

Abstract Background Flax ( Linum usitatissimum L.) productivity in sandy soils is often constrained by limited water availability and reduced physiological performance. Although antioxidant-related foliar treatments may support crop performance, direct comparisons of purified β-carotene with carrot root extract, a natural multi-component biostimulant, remain scarce in flax. This field study evaluated the effects of foliar β-carotene and carrot root extract on growth, biochemical traits, and yield performance of two flax varieties, Sakha-2 and Line-4, grown in sandy soil under a field-imposed short-term water-deficit episode. The experiment was arranged in a split-plot design with three replicates. Varieties were assigned to main plots, while foliar treatments were assigned to subplots: untreated control, β-carotene at 25, 50, and 75 mg L⁻¹, and carrot root extract at 5, 10, and 20%. Results Foliar treatments were associated with significant changes in growth, photosynthetic pigments, antioxidant-related traits, and yield attributes, and the magnitude of these responses differed between the two varieties. Across varieties, carrot root extract at 10% and β-carotene at 50 mg L⁻¹ produced the most consistent improvements across the measured traits. Compared with the untreated control, carrot extract at 10% increased shoot length from 45.16 to 64.50 cm, shoot fresh weight from 1.005 to 2.670 g, chlorophyll a from 1.020 to 1.635, total pigments from 1.875 to 2.755, and IAA from 17.51 to 42.34 µg g⁻¹ FW. It also enhanced phenolics, flavonoids, β-carotene content, DPPH radical-scavenging activity, PAL activity, and TAL activity. Yield responses followed the same trend: carrot extract at 10% increased seed yield from 945 to 1748 kg ha⁻¹, biological yield from 5112 to 9157 kg ha⁻¹, 1000-seed weight from 4.165 to 6.025 g, oil percentage from 32.01 to 33.43%, and oil yield from 301 to 584 kg ha⁻¹. β-carotene at 50 mg L⁻¹ also increased seed yield to 1723 kg ha⁻¹ and oil yield to 576 kg ha⁻¹. Conclusions Foliar application of β-carotene and carrot root extract was associated with improvements in several physiological, biochemical, and yield-related traits of flax grown in sandy soil under a field-imposed short-term water-deficit episode. Carrot root extract at 10% produced the most consistent overall response across the measured traits, while β-carotene at 50 mg L⁻¹ was also associated with marked yield improvements. These findings suggest that both treatments may support flax performance under the conditions evaluated in the present study. However, further studies using quantitatively defined irrigation regimes, direct measurements of soil and plant water status, and comprehensive compound-level phytochemical profiling of carrot root extract are required to confirm the underlying mechanisms and optimum application concentrations.