Compartmentation of boron in roots and leaves of sunflower as affected by boron supply

A method for studying the compartmentation of B in different plant organs has been developed. The basic steps of this method are the preparation of cell sap by squeezing the plant material after a freeze-thaw-cycle and the subsequent preparation of the water insoluble residue (WIR) by washing the remaining residue after homogenization. These two fractions represent B pools of different solubility and physiological behaviour. The WIR B seems to be closely bound to cell wall polymers, whereas the cell sap B mainly represents the soluble B in the symplasm which seems to be directly available for possible physiological functions in the cell. Using the described method, the compartmentation of B in roots and leaves of sunflower plants, cultivated with a B supply of 0.1 to 1600 mu mol . L-1, was studied as well as the B concentrations on the pathway from the nutrient solution to the symplasm of the leaf cells which was characterized by analyzing a sequence of liquid compartments (root cell sap, xylem exudate and leaf cell sap). Although the external supply increased by a factor of 16,000, B concentration in root and leaf cell walls increased only by a factor of 2.8 and 22, respectively This indicates that the cell wall does not contribute effectively to the detoxification of excess B taken up by the plant by providing additional binding sites in response to B toxicity. B concentrations in root symplasm and xylem exudate suggest that two mechanisms of B uptake are present in sunflower. At low supply there seems to be a concentration mechanism, which is downregulated in activity when supply increases from 0.1 to 10 mu mol . L-1. At a supply above 100 mu mol . L-1 passive diffusion of boric acid from the external solution across the plasma membranes into the cells of the root cortex seems to be responsible for B uptake. At the passage of the casparian strip into the stele and/or at xylem loading itself a considerable retention of B was observed.