The kinetics of calcium and magnesium entry into mycorrhizal spruce roots

The entry of calcium and magnesium from external sources into mycorrhizal r oots of 3-year-old Norway spruce trees (Pic en abies [L.] Karst.) was monit ored. Roots of intact plants were exposed for various periods of time, rang ing from 2 mill to 48 h, to nutrient solutions which contained the stable-i sotope tracers Mg-25 and Ca-44. After labelling, samples of roots were exci sed from the plants, shock-frozen, cryosubstituted and embedded. The result ing isotope composition in this material was analysed by a laser-microprobe -mass-analyser (LAMMA) at relevant positions within cross-sections of the r oots. For both elements, we determined (i) the fractions of the isotopes or iginating From the plant prior to labelling, and (ii) the fraction of isoto pes originating from the corresponding tracer that penetrated into the root . Both divalent cations rapidly penetrated across the cortical apoplast and reached the endodermis. After 2 min of exposure to the labelling solution, an initial transient gradient of the tracers could be observed within the root cortex. Subsequently, calcium as well as magnesium equilibrated betwee n the apoplast of the entire cortex and the external tracer with a half-tim e, t(1/2), of about 3 min. In contrast, the kinetics of radial movement int o the vascular stele showed a delay with a till of 100-120 min. We take thi s as strong evidence that there exists a free apoplastic path for divalent cations in the cortex and that the endodermis is a major barrier to the fur ther passage of Mg and Ca into the xylem. While Mg-25 in the labelling solu tion exchanged rapidly with Mg in the cortical apoplast, the exchange acros s the plasma membrane with ME present in the protoplasm of the same cortica l cells was almost 2 orders of magnitude slower. The kinetics of Ca and Mg entry at +6 degrees C were similar to those obtained at a root temperature of +22 degrees C.