The influence of boron on the morphological development and cell wall composition of Brassica oleracea var. italica

Broccoli (Brassica oleracea var. italica (L.) cv. 'Premium Crop') was grown in perlite and supplied with nutrient solutions containing 0.08, 0.16, 0.32, 0.81, 1.62, or 8.10 mg·L⁻¹ boron. Growth measurements were taken at the 5-, 10-, 15-leaf, and mature (20-leaf) stages of development. These measurements included fresh and dry weights of a mature upper and lower leaf, upper and lower leaf area, percent dry weight of pith and exterior stem samples, and fresh weight of florets and stem. Boron did not have a pronounced effect on growth until the plants had reached the 15-leaf stage. However, at maturity, stem fresh weight increased as boron concentration increased up to 1.62 mg·L⁻¹, then declined at 8.10 mg·L⁻¹ (boron toxicity). Boron deficient plants (0.08 mg·L⁻¹) had higher percent dry leaf weights than plants with sufficient boron (0.81 mg·L⁻¹). Floret weight increased as boron concentration increased from 0.08 to 1.62 mg·L⁻¹. Maximum floret weight was achieved by plants that had leaf and stem boron concentrations of 20 to 100 μg·g⁻¹ and 12 to 20 μg·g⁻¹ , respectively, at maturity. Hollow stem, cell wall content, and mineral analysis measurements were taken at maturity. Mature boron deficient plants had a greater incidence of hollow stem and increased amounts of cell wall in the stem interior and exterior. Increased calcium concentration was also found in stem interior tissue and extracted cell walls of boron deficient plants. Boron deficiency decreased the covalently and ionically-associated pectin fractions of the cell wall and slightly increased the hemicellulosic and cellulosic fractions. This may indicate a relationship between cell wall composition and hollow stem under boron deficient conditions.