Apoplastic and membrane-associated Ca2+ in leaves and roots as affected by boron deficiency

A combination of fluorescein‐isothiocyanate (FITC), coumarin‐benzothiazol (BTC), and chlorotetracycline (CTC) fluorescence was used to simultaneously monitor apoplastic pH, apoplastic free Ca2+, and plasma membrane‐bound Ca2+. As early boron deficiency reactions supposedly include alterations of plasma membrane‐bound transport processes besides rapid effects on cell wall physical properties, the corresponding changes were followed in leaves and roots of Vicia faba L. cv. Troy. Boron deficiency did not alter the apoplastic pH, but it reduced plasma membrane‐bound Ca2+ in roots at 4 h and leaves at 3 days after starting the deficiency treatment. The decrease in plasma membrane‐bound Ca2+ coincided with an increase in apoplastic free Ca2+ and K+, and occurred before the first visible symptoms were noticed. It is proposed that less Ca2+ is bound to the plasma membrane due to a reduction of specific Ca2+‐binding sites (borate esters with vic‐diols or polyhydroxy‐carboxylates) before plasma membrane integrity deteriorates.