APOPLASTIC AND MEMBRANE-ASSOCIATED CA2+ IN LEAVES AND ROOTS AS AFFECTED BY BORON DEFICIENCY

A combination of fluorescein-isothiocyanate (FITC), coumarin-benzothia zol (ETC), and chlorotetracycline (CTC) fluorescence was used to simul taneously monitor apoplastic pH, apoplastic free Ca2+, and plasma memb rane-bound Ca2+. As early boron deficiency reactions supposedly includ e alterations of plasma membrane-bound transport processes besides rap id effects on cell wail physical proper:ies, the corresponding changes were followed in leaves and roots of Vicia faba L. cv. Troy. Boron de ficiency did not alter the apoplastic pH, but it reduced plasma membra ne-bound Ca2+ in roots at 4 h and leaves at 3 days after starting the deficiency treatment. The decrease in plasma membrane-bound Ca2+ coinc ided with an increase in apoplastic free Ca2+ and K+, and occurred bef ore the first visible symptoms were noticed. It is proposed that less Ca2+ is bound to the plasma membrane due to a reduction of specific Ca 2+-binding sites (berate esters with vic-diols or polyhydroxy-carboxyl ates) before plasma membrane integrity deteriorates.