Decrease in membrane hydraulic conductance of rhizodermal cells under nitrate deficit is related to acidification at the root surface

Abstract

Barley seedlings (Hordeum vulgare L.) were grown on porous plates submerged in Knop medium at pH 6.0 (control) and in a similar nutrient solution where NO3− was replaced with Cl− (treatment); in some treatments Mes buffer (10–20 mM, pH 6.0) was added to the medium. In the absence of buffer, the pH of the medium shifted towards the alkaline region in the presence of NO3− and to the acidic region in the presence of Cl−, with the total shift of no more than 0.3 pH units per day. The replacement of NO3− with Cl− (in a buffer-free medium) decreased the hydraulic membrane conductance of rhizodermal cells (Lp) within a 4-h period; after one day Lp settled at approximately 50% of its initial value observed in untreated plants. When the removal of nitrate from the medium was accompanied by the addition of buffer, no changes in Lp were observed over a 1-day period. The perfusion of external solution (at a rate of 10 mm/s) made it possible to control pH in the proximity to root surface (pHs). These experiments showed that Lp was independent of the surface pH in the pHs range 7.0–5.0, whereas at pHs = 4.5 Lp decreased within 15 min to a steady-state level of about 50% of the control value. It is concluded that the reduction of Lp under nitrate deficit was related to acidification of the medium near the root surface. The acidic pH shift could be caused by the cessation of proton/nitrate symport and by activation of the plasmalemma H+-pump, related to changes in the cytosolic pH-stat.