Stimulation of Mitochondrial Calcium Uptake by Light during Growth of Corn Shoots

Ca²⁺ uptake in mitochondrial fractions, isolated on Percoll discontinuous density gradients, from light- and dark-grown corn (Zea mays L. var W64A × W182E) shoots was characterized by dual wavelength spectroscopy and the Ca²⁺-sensitive dye murexide. In light-grown seedlings, the rate of mitochondrial Ca²⁺ uptake was about 40 nanomoles per minute per milligram of mitochondrial protein. A portion of the Ca²⁺ uptake required an exogenous supply of ATP (65%) while the remaining 35% was the respiratory substrate-dependent reaction. Ruthenium red (2 micromolar) completely inhibited both ATP- and substrate-dependent reactions. There was no detectable Ca²⁺ efflux from the mitochondria with the inhibitor. When the mitochondrial fraction was prepared from the dark-grown shoots, the rate of uptake, in particular the ATP-dependent reaction, was greatly reduced. The dark treatment caused a reduction in mitochondrial Ca content which is largely due to the reduction of Ca associated with the mitochondrial membrane rather than to a reduction of Ca in the soluble matrix.