THE EFFECT OF CALCIUM STARVATION ON ASSIMILATE PARTITIONING AND MINERAL DISTRIBUTION OF THE PHLOEM

Populus plants were grown in a medium lacking calcium and exposed to ( CO2)-C-14. In contrast to plants in the complete nutrient medium, the percentage amount of C-14-assimilates increased in the leaves of calci um-deficient plants and decreased in the stem and the roots. When plan ts were grown without potassium or magnesium no differences in the amo unt of C-14-label occurred in comparison with plants in the complete n utrient medium. Translocation was recorded by microautoradiography. It was observed that considerable amounts of labelled photoassimilates w ere unloaded from the phloem in the middle part of the stem in plants of the complete nutrient medium. In contrast, during calcium starvatio n C-14-label was restricted to the phloem of the stem. In addition, th e concentrations of magnesium and phosphorus showed a remarkable incre ase in stem sieve tubes of calcium-deficient plants. When sieve tubes of source leaves from Populus, barley and maize were compared with tho se of sink leaves, the latter showed higher calcium concentrations. Th e results suggest that calcium is a necessary factor in the regulation of phloem translocation.