One-Time Foliar Application and Continuous Resupply via Roots Equally Improved the Growth and Physiological Response of B-Deficient Oilseed Rape

Abstract

Oilseed rape (Brassica napus L.) is a high-boron (B)-demanding crop, and initially, normal growing plants might show B deficiency at advanced growth stages on soils with marginal B availability. Hence, we compared the effects of B resupply via roots and leaves on growth and physiological response, and relative expression of B transporters in B-deficient oilseed rape plants. Four-week-old plants initially grown with inadequate B (1 µM B for the first two weeks and 0.25 µM B for the next two weeks) were later grown either as such with 0.25 µM B, with 25 µM B in nutrient solution or foliar sprayed with 7 mL of 30, 60 and 150 mM B solution plant^-1 as boric acid. Plants grown with 25 µM B in the nutrient solution from the beginning were included as adequate B treatment. Results showed that B resupply to B-deficient plants via roots and leaves (60 mM B) equally improved root and shoot dry matter, but not to the level of plants grown with adequate B supply. Foliar-applied 150 mM B proved toxic, causing leaf burn but not affecting dry matter. Resupply of B via roots increased B concentration in roots and leaves, while leaf-applied B did so only in leaves. Net carbon assimilation had a positive relationship with dry matter accumulation. Except for the highest foliar B level, B resupply via roots and leaves increased the accumulation of glucose, fructose and sucrose in leaves. Boron-deficient plants showed significant upregulation of BnaNIP5;1 in leaves and roots and of BnaBOR1;2 in roots. Boron resupply via roots reversed the B-deficiency-induced upregulation of BnaNIP5;1 in roots, whereas the expression of BnaBOR1;2 was reversed by both root and foliar B resupply. In leaves, B resupply by both methods reversed the expression of BnaNIP5;1 to the level of B-adequate plants. It is concluded that B resupply to B-deficient plants via roots and leaves equally but partially corrected B deficiency in B. napus grown in hydroponics.

Keywords: B resupply; B transporters; Brassica napus; boron; foliar application.

Figure 1

Response of B. napus to different B treatments after 42 days of transplantation in the nutrient solution. +B, 25 µM B in NS; −B, 0.25 µM B in NS; −B+RR, 0.25 µM B in NS for the first two weeks and then 25 µM B for the latter two weeks; −B+LR₃₀, 0.25 µM B in NS and foliar-applied 30 mM B at two weeks; −B+LR₆₀, 0.25 µM B in NS and foliar-applied 60 mM B at two weeks; −B+LR₁₅₀, 0.25 µM B in NS and foliar-applied 150 mM B at two weeks.

Figure 2

Root (A, C) and shoot (B, D) dry matters and their responses to different B treatments in nutrient solution grown Brassica napus for 42 days. +B, 25 µM B in NS; −B, 0.25 µM B in NS; −B+RR, 0.25 µM B in NS for the first two weeks and then 25 µM B for the latter two weeks; −B+LR₃₀, 0.25 µM B in NS and foliar-applied 30 mM B at two weeks; −B+LR₆₀, 0.25 µM B in NS and foliar-applied 60 mM B at two weeks; −B+LR₁₅₀, 0.25 µM B in NS and foliar-applied 150 mM B at two weeks. The data ± SE are means of five independent pot replicates. Different letters on bars indicate significant differences between treatments (ANOVA with Duncan’s multiple range test p ≤ 0.05).

Figure 3

Boron concentration in young leaves (A), mature leaves (B) and roots (C) of Brassica napus grown in nutrient solution under different B treatments for 42 days. +B, 25 µM B in NS; −B, 0.25 µM B in NS; −B+RR, 0.25 µM B in NS for the first two weeks and then 25 µM B for the latter two weeks; −B+LR₃₀, 0.25 µM B in NS and foliar-applied 30 mM B at two weeks; −B+LR₆₀, 0.25 µM B in NS and foliar-applied 60 mM B at two weeks; −B+LR₁₅₀, 0.25 µM B in NS and foliar-applied 150 mM B at two weeks. The data ± SE are means of five independent pot replicates. Different letters on bars indicate significant differences between treatments (ANOVA with Duncan’s multiple range test p ≤ 0.05).

Figure 4

Changes in net CO₂ assimilation rate (A; µmol CO₂ m⁻² s⁻¹) and transpiration rate (B; µmol H₂O m⁻² s⁻¹) of Brassica napus grown in nutrient solution under different B treatments for 42 days. +B, 25 µM B in NS; −B, 0.25 µM B in NS; −B+RR, 0.25 µM B in NS for the first two weeks and then 25 µM B for the latter two weeks; −B+LR₃₀, 0.25 µM B in NS and foliar-applied 30 mM B at two weeks; −B+LR₆₀, 0.25 µM B in NS and foliar-applied 60 mM B at two weeks; −B+LR₁₅₀, 0.25 µM B in NS and foliar-applied 150 mM B at two weeks. The data ± SE are means of four independent pot replicates. Different letters on bars indicate significant differences between treatments (ANOVA with Duncan’s multiple range test p ≤ 0.05).

Figure 5

Glucose, fructose and sucrose concentrations in the roots (A), mature leaves (B) and young leaves (C) of Brassica napus grown in nutrient solution under different B treatments for 42 days. +B, 25 µM B in NS; −B, 0.25 µM B in NS; −B+RR, 0.25 µM B in NS for the first two weeks and then 25 µM B for the latter two weeks; −B+LR₃₀, 0.25 µM B in NS and foliar-applied 30 mM B at two weeks; −B+LR₆₀, 0.25 µM B in NS and foliar-applied 60 mM B at two weeks; −B+LR₁₅₀, 0.25 µM B in NS and foliar-applied 150 mM B at two weeks. The data ± SE are means of five independent pot replicates. Different letters on bars indicate significant differences between treatments (ANOVA with Duncan’s multiple range test p ≤ 0.05).

Figure 6

The fold change in the relative expression of BnaBOR1;2 and BnaNIP5;1 gene in roots (A,C) and leaves (B,D) of Brassica napus grown in nutrient solution under different B treatments for 42 days. +B, 25 µM B in NS; −B, 0.25 µM B in NS; −B+RR, 0.25 µM B in NS for the first two weeks and then 25 µM B for the latter two weeks; −B+LR₃₀, 0.25 µM B in NS and foliar-applied 30 mM B at two weeks; −B+LR₆₀, 0.25 µM B in NS and foliar-applied 60 mM B at two weeks; −B+LR₁₅₀, 0.25 µM B in NS and foliar-applied 150 mM B at two weeks. Bars represent means of three biological replicates, technically replicated two times ± SE. Different letters on bars indicate significant differences between treatments (ANOVA with Duncan’s multiple range test p ≤ 0.05).