Isolation of soluble boron complexes and their determination together with free boric acid in higher plants

Isolation of soluble boron complexes and their quantitative determination together with free boric acid in higher plants was carried out by gel permeation chromatography in the liquid compartment of plant leaves and roots. The described method is suitable not only for the isolation of B complexes from plant cell sap, but also allows for their quantitative determination together with free boric acid, which is possible for samples with a B concentration of at least 15 mu mol L-1 (Table 2). This gel permeation chromatography method therefore complements methods that allow characterization, but not quantitative determination of B complexes in plant samples. Soluble B complexes have been isolated from sunflower (Helianthus annuus L.) grown with a marginal to toxic B supply and curly kale (Brassica oleracea L.). In sunflower root symplasm there was only a small concentration of B complexes that was not dependent on variations in B supply from the marginal to the toxic level. The resulting large variations in total symplasmic B were therefore only reflected in the concentrations of free boric acid. In contrast, the concentration of B complexes in sunflower leaf symplasm was about 50-fold larger than in the root symplasm and made up 31% of the total symplasmic B. In curly kale leaves even 59 % of the total symplasmic B was present as B complexes. Further experiments are necessary to characterize the B complexes in sunflower and curly kale to investigate their physiological role in plant metabolism.