electroshock-induced inhibition of phloem translocation

The transient inhibition of phloem translocation which is produced by an electric shock was studied by applying controlled-current stimuli to short lengths of bean stem. Translocation was monitored by observing the accumulation of carbon-11 label into the plant apex. The principal findings are: (i) For constant-current electric shocks whose (current) X (time) product was held constant at 10 mC, those of higher current (and shorter duration) gave longer lasting inhibitions. (ii) Breaking a 5 s pulse into a train of 100 ms pulses slightly shortened the duration of the inhibition, even though the same total charge was passed; however, the separation of the pulses within the train did not seem to matter. But, when the inhibition produced by a pair of 50 ms pulses was studied, the duration of inhibition increased with pulse spacing. (iii) Single pulses as short as 10 ms could produce a detectable inhibition. The duration of inhibition grew rapidly with pulse length for pulse lengths between 10 ms and 100 ms. (iv) When a pair of 2.5 s pulses was applied, a longer inhibition was obtained if the two pulses were of opposite polarity. To explain these phenomena, a qualitative conceptual model is provided in terms of ion channels in the phloem. In the experiments reported here and in all previous pulse-labelling experiments from this laboratory. It has been noted that rapidly increasing stochastic variability in the count data made it extremely difficult to obtain useful results for a period of more than 2 h after loading, even if the number of counts per unit time remained fairly high. A quantitative theory for this limitation is worked out and shown to agree with experimental data. Therefore, since biological variability from plant to plant makes it advisable to compare matched stimuli on the same plant in order to detect trends reliably, the number of challenges which can usefully be applied following a single pulse-labelling with a short-lived isotope is quite limited.