Action potentials in maize sieve tubes change phloem translocation

The inhibition of phloem translocation caused by electric- and cold-shock was studied by macro- and microautoradiography in mature leaves of maize (Zea mays L.). In addition, both types of stimulation triggered action potentials with amplitudes of more than 50 mV which were transmitted without diminution in sieve tubes with velocities of 3-5 cm s-1. By utilizing X-ray microanalysis ion concentrations of sieve tubes were measured in non-stimulated and stimulated leaves. It was shown that potassium and chloride diminished about 3-fold after stimulation while the amount of cytoplasmic calcium may have increased. These displacements lead to the conclusion that calcium influx as well as potassium and chloride efflux are involved in the propagation of action potentials. A possible relationship between electric signalling and the reduction of phloem translocation is discussed.