Decrement and amplification of slow wave potentials during their propagation in Helianthus annuus L. shoots

Slow wave potentials (SWPs) are transitory depolarizations occurring in response to treatments that result in a pressure increase in the xylem conduits (P(x)). Here SWPs are induced by excision of the root under water in 40- to 50-cm-tall light-grown sunflower plants in order to determine the effective signal range to a naturally sized pressure signal. The induced slow wave depolarization appears to move up the stem while it is progressively decremented (i.e. the amplitude decreases with increasing distance from the point of excision) with a rate that appears to rise acropetally from 2.5 to 5.5% cm(-1). The decline of the SWP signal, in both amplitude and range, could be experimentally increased (i) when root excision was carried out in air and (ii) when the transpiration of the sunflower shoot was minimized by a preceding removal or coating of the leaves. A further decline of the SW signal was expected to occur when leaves were included in the measured path. However, when the most distant apical electrode was attached to an upper leaf, it showed a considerably larger depolarization than a neighboring stem position. This apparent amplification of the SWP signal is not confined to the leaf blade but includes the petiole as well. The amplification disappeared (i) when the illumination level was lowered to room light, (ii) when the blade was excised either completely or along the remaining midvein and (iii) when the intact leaf blade was submersed in water. These treatments reduce the SWP at the petiole to a small fraction of the signal in the opposite control leaf and specify bright illumination and blade-mediated transpiration as prerequisites of a signal increase that is confined to young, expanding leaves.