Boron Induces Hyperpolarization of Sunflower Root Cell Membranes and Increases Membrane Permeability to K⁺

Abstract

Although many studies have alluded to a role for boron (B) in membrane function, there is little evidence for a direct effect of B on the plasmalemma of higher plant cells. These studies were conducted to demonstrate, by electrophysiological techniques, a direct effect of B on the membrane potential (Em) of sunflower (Helianthus annuus [L.], cv Mammoth Grey Stripe) root tip cells and to determine if the response to B occurs rapidly enough to account for the previously observed effects of B on ion uptake. By inserting a glass microelectrode into an individual cell in the root tip, the Em of the cell was determined in basal salt medium (BSM), pH 6.0. The perfusion solution surrounding the root tissue was then changed to BSM + 50 micromolar H3BO3, pH 6.0. The exposure to B induced a significant plasmalemma hyperpolarization in sunflower root cells within 20 minutes. After just 3 minutes of exposure to B, the change in Em was already significantly different from the negligible change in Em observed over time in root cells never exposed to B. Membrane hyperpolarization could be caused by a stimulation of the proton pump or by a change in the conductance of one or more permeable ions. Since B has been shown to affect K uptake by plants, the electrophysiological techniques described above were used to determine if B has an effect on membrane permeability to K@, and could thereby lead to an increased diffusion potential. When sunflower root tips were pretreated in 50 micromolar B for 2 hours, cell membranes exhibited a significantly greater depolarization with each 10-fold increase in external [K+] IuM B 1 h prior to the absorption period. Pollard et al. (9) also reported reductions of P0-, Cl-, and Rb+ uptake in Bdeficient maize and faba bean roots tips. They reported 40% restoration of nutrient uptake within 20 min of the addition of B. According to Goldbach (5), both uptake and efflux of 32p were decreased in B-deficient sunflower roots. He also found that the effects of B deficiency could be reversed in less than 1 h by the addition of B to the growth medium.Studies of the effects of B on ion uptake point to a role for B in membrane function. If the primary mechanism of action of B within the plant does occur at the membrane level, then other effects of B observed in earlier work, i.e. sugar translocation (4) and auxin movement (19), could also be explained as secondary effects of the primary effect of B on the cell membrane. However, few experiments have been designed to relate the presence of B to a direct effect on membranes.Vorob'ev and Pelkhanov (20)

Sources and rates of potassium for sweet orange production

“…Such effect was also described for alfalfa (Wallace and Bear, 1949) and avocados (Jaime et al, 1992), even though no specific mechanism at the plant level was proposed to explain such results. Conversely, K uptake by plants is increased with B supply (Cooper et al, 1952;Schon et al, 1990;Obermeyer et al, 1996;Quaggio et al, 2003;Grassi et al, 2004). In this case, B facilitates K uptake probably because of increased hyperpolarization of the plasma membrane (Schon et al, 1990).…”

“…Conversely, K uptake by plants is increased with B supply (Cooper et al, 1952;Schon et al, 1990;Obermeyer et al, 1996;Quaggio et al, 2003;Grassi et al, 2004). In this case, B facilitates K uptake probably because of increased hyperpolarization of the plasma membrane (Schon et al, 1990). …”

Effects of Boron on Proton Transport and Membrane Properties of Sunflower (Helianthus annuus L.) Cell Microsomes

“…Impaired membrane function could underlie many responses to Bdeficient conditions (Parr and Loughman, 1983). In this regard, it was shown that B induced hyperpolarization of sunflower (Helianthus annuus) cell membranes and increased membrane permeability to K+ (Schon et al, 1990). B also affected H+ efflux in young sunflower seedling roots (Roldin et al, 1992) and in carrot cell cultures (Blaser-Grill et al, 1989 (Goldbach et al, 1990;Ferrol and Donaire, 1992).…”

Impact of Potassium-Boron Interaction on Leaf Nutrient Content and Nut Setting of Coconut

“…In the present study, there was synergistic interaction effect between potassium and boron in the case of B 2 K 2 , B 2 K 3 , and B 2 K 1 treatments. Earlier, Schon et al, (1990) and Ranade-Malvi (2011) noticed that boron at an optimum level increased potassium permeability in the cell membrane, though however, an excess of the nutrient causes toxicity in plants.…”