Aluminium (Al) toxicity in rye (Secale cereale L.), an Al-resistant crop, was examined by measuring root elongation and cytoplasmic free activity of calcium ([Ca2+](cyt)) in intact root apical cells. Measurement of [Ca2+](cyt) was achieved by loading a Ca2+-sensitive fluorescent probe, Fluo-3/AM ester, into root apical cells followed by detection of intracellular fluorescence using a confocal laser scanning microscope. After 20 min of exposure to 50 muM Al (pH 4.2) a slight increase in [Ca2+](cyt) of root apical cells was observed, while the response of [Ca2+](cyt) to 100 muM Al (pH 4.2) was faster and larger ([Ca2+](cyt) increased by 46 % in 10 min). Increases in [Ca2+](cyt) were correlated with inhibition of root growth, generally measurable after 2 h. Addition of 400 pm malic acid (pH 4.2) largely ameliorated the effect of 100 muM Al on [Ca2+](cyt) in root apical cells and protected root growth from Al toxicity. These results suggest that an increase in [Ca2+](cyt) in root apical cells in rye is an early effect of Al toxicity and is followed by the secondary effect on root elongation. (C) 2002 Annals of Botany Company.