Effect of pH and boron concentration in the nutrient solution on translocation of boron in the xylem of sunflower

Abstract

The influence of the pH in the nutrient solution (pH 3 to 10) on uptake and translocation of boron (B) in the xylem of young sunflower plants was investigated over a three hour treatment. Two B concentrations were supplied during the whole growth period of seven days: a marginal (1 μM) and a sufficient supply (100 μM). Boron concentrations in the xylem exudate responded both to B and pH treatments. When B supply was sufficient, B concentration in the xylem exudate was the same or lower than that in the nutrient solution and inversely related to the exudation rate. In contrast at marginal supply, B concentration in the xylem exudate was up to 25-fold higher than in the nutrient solution and decreased in direct proportion to the pH dependent concentration of undissociated boric acid. In conclusion, the results indicate that there might be two different mechanisms for uptake and translocation of B in sunflower. At sufficient supply, it seems that a passive diffusion process of boric acid, partly restricted by permeability of membranes, is responsible for uptake. At marginal supply, a concentration mechanism for boric acid builds up a gradient against the nutrient solution, obviously to satisfy the B demand of the plant.