Effects of Ca2+ on Amino Acid Transport and Accumulation in Roots of Phaseolus vulgaris

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Abstract
Ca2+ stimulates the uptake of α-aminoisobutyric acid (AIB) into excised or intact Phaseolus vulgaris L. roots by a factor of two. In roots depleted of Ca2+ by preincubation with ethylenediaminetetraacetate, ethyleneglycol-bis(β-aminoethyl ether)-N,N′-tetraacetic acid, or streptomycin, the stimulatory effect is 7- to 10-fold. In the presence of Ca2+, roots accumulate AIB more than 100-fold; Ca2+-depleted roots only equilibrate with AIB. Radioautography shows [14C]AIB to be present in all cells after 90 min. Although Ca2+-depleted roots lose accumulated [14C]AIB about 10 times faster than roots supplied with Ca2+, this increased efflux is not the main cause for the decrease in net uptake observed. The latter is rather due to a less negative membrane potential Δψ in Ca2+ depleted roots (−120 mV → −50 mV). The basic feature explaining all the results of Ca2+ deficiency is an increase in general membrane permeability. No indication of a specific regulatory function of Ca2+ in membrane transport of roots has been obtained.