The effects of varied boron (B) supply (0.01–50 μM and light intensity (100–580 μpmol m−2 s−1) on plant growth, leaf symptoms and membrane permeability of leaves were studied in sunflower plants over a 10‐day‐period of growth in nutrient solution. Membrane permeability was measured by incubating isolated young leaves in aerated distilled water. Additional experiments showed the effect of short‐term supply (20 min to 2 h) of B and germanium (Ge) on membrane permeability of B‐deficient leaves incubated in distilled water. Low supply of B decreased shoot and root growth and caused brown‐purple pigmentation of young leaves, particularly under high light intensities. Leakage of K‐ from leaves increased with severity of B deficiency whereas, in B‐sufficient leaves, leakage of solutes was low. Also leakage of phenolics, amino acids and sucrose was enhanced by B deficiency. Compared to B‐sufficient leaves, the leakage from B‐deficient leaves was 35‐fold higher for K+, 45‐fold higher for sucrose and 7‐fold higher for phenolics and amino acids. Increases in solute leakage were marked in leaf areas with brown‐purple pigmentation and particularly pronounced by increasing light intensity or by exposure of leaves to continuous light. In severely B‐deficient leaves grown under high light intensity, treatment with B and Ge up to 1 000 μMM for 20 min resulted in an immediate decrease in K+ efflux to about the same levels as in B‐sufficient leaves. Similar rapid decreases in K+ efflux were also found in B‐deficient leaves when exposed to darkness for 16 h or 30 h before the leakage measurement. The results demonstrate a particular role for B in maintaining the integrity of plasma membranes. Boron presumably stabilizes the structure of the plasma membrane by complexing membrane constituents. It also has a protective effect on membrane constituents by complexing phenolics, so that oxidation of phenolics to highly toxic quinones and oxygen free radicals is prevented or limited. In these functions, boron seems to be to a large extent replaceable by germanium.

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