In caulonemal filaments of Physcomitrella patens which had been preincubated in the dark for 24h, irradiation with red light (640 nm, fluence rate 85 μmol · m-2 · s-1) evoked (i) the development of side branch initials and (ii) a rapid, but transient, depolarisation of the plasma membrane by 90 ± 13 mV from a resting potential of -178 ± 13 mV. This was followed by a transient hyperpolarisation to a value 21 ± g mV more negative than the original membrane potential. The refractory period for the transient depolarisation was between 12 and 15 min. The fluence rate of red light required to evoke maximal depolarisation was about 80 μmol · m- 2 · s- 1 for a 1-min pulse. At this fluence rate, a depolarising response could be recorded for pulse lengths as small as 7 s. The transient depolarisation was insensitive to 3(3',4'dichlorophenyl)-1,1-dimethyl urea (DCMU) and was unchanged in plants bleached by growth on norflurazon (SAN 9789). Furthermore, the electrical response could be blocked by simultaneous application of far-red light. These results suggest the involvement of the photoreceptor phytochrome in the response. Removing Ca2+ from the external medium or replacing Ca2+ with Mg2+ blocked the depolarisation. The depolarisation could also be blocked by the K+ channel-blocker tetraethylammonium (10 mM) and the Cl channel-blocker niflumic acid (1 μM). Conversely, although calcium channel-antagonists such as nifedipine and lanthanides, applied at a concentration of 100 μM, also altered the response, they did not block it. A possible ionic mechanism for the membrane potential transient is advanced, and the physiological significance discussed in the context of early events in the phytochrome signalling pathway.
- Membrane depolarisation