The effects of varied boron (B) supply (0.01-50 mu M) and light intensity (100-580 pmol 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 mu M 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.