When following the membrane potential of Elodea densa leaf cells during a dark-light regime and analysing the different phases of the cycle, the pattern under boron deficiency resembled the one reported to occur after 3-(3,4-dichlorophenyl)-1,1-dimethylurea application. The potential in the dark slowly decreased (1,12,14,17,18,20). It is still a matter of controversy whether these differences in net uptake are the consequence of a reduced active transport system such as H+-pumping ATPases (14) or of a more or less general effect on membrane structure (13). The latter assumption is supported by the fact that in several cases of B deficiency lower flux rates for the ions K+, Rb+, and H2P04-were found (4,17,18,20), although a higher net efflux of K+ (23) ary events of B deficiency and lead to a better understanding of boron's role in plant metabolism. MATERIALS AND METHODS Plant Cultivation and Nutrient SolutionsElodea densa was grown in a 0.1 strength nutrient solution according to Hoagland and Arnon (7). The B supply as B(OH)3 was modified as stated in the tables and figures. Helianthus annus was raised hydroponically in a 0.2 N Hoagland solution (see above) for about 6 to 8 weeks after germination. Suspension-cultured cells ofDaucus carota were raised as described earlier (4). The B-deficiency treatments were started either by changing the solution during the potential measurements or by preincubating plant parts (Elodea), whole plants (Helianthus), or thoroughly washed cells (Daucus) in an essentially B-free solution. The nutrient solution employed for the experiments was either a 0.2 strength Hoagland solution (Elodea, Helianthus) or a modified Murashige and Skoog medium (without Fe for ferricyanide reduction to avoid the formation of blue-colored cyanide-complexes). For continuously monitoring HW-release of Daucus cells, a simplified solution, containing only 3% (w/v) saccharose, 0.1 mM CaSO4, and 10 mM K2SO4 was used. To minimize B contamination, highest purity grade chemicals and ultrapure water (Seradest Up) were used, and the storage of solution in glassware and low quality plastics was avoided throughout. If required, solutions were treated additionally with the boratespecific Amberlite IRA 743 ion exchanger (analytical grade, prewashed in ultrapure water).K was determined by flame photometry (Elex 6361, Eppendorf) after wet ashing in HNO3/HCl04/H2SO4 (20:2.5:1, v/ v).