Uptake of calcium in wheat and cucumber roots

Uptake of Ca2+(45Ca) was investigated in plants of wheat (Triticum aestivum L. var. Svenno) and cucumber (Cucumis sativus L. var. Cilla) cultivated in a nutrient solution with various Ca2+ concentrations. The adsorption of Ca2+ was higher in cucumber roots than in wheat roots especially at lower Ca2+ levels in the external medium. The intracellular fraction of Ca2+ was less than 20% of the total Ca2+ in wheat roots and less than 10% of the total Ca2+ in cucumber roots. The uptake of Ca2+ in cucumber was about 40 times higher than in wheat. Transport of Ca2+ in the roots towards the endodermis is suggested to take place mainly in the apoplastic pathway regulated by the availability of negatively charged binding sites along the cell wall continuum. Further transport of Ca2+ towards the stele may involve diffusion of Ca2+ into the symplasm in the vicinity of the endodermis. An active extrusion of Ca2+ towards the stele or towards the external medium is suggested to play a role in the regulation of Ca2+ uptake.