Electrical signaling and gas exchange in maize plants of drying soil

The involvement of electrical signaling in the regulation of gas exchange was checked with maize plants subjected to a drying cycle of 5 days in a controlled environment. Gas exchange and extracellular electrical potential measurements were made on attached leaves. The CO2 uptake and transpiration rate decreased in drying soil and increased to their original values after irrigation. Continuous records of the electrical potential difference between two surface points were made on the same leaf on which the gas measurements were performed. A daily rhythm was clearly visible and seemed to be correlated with the soil water status. After soil drying the plants were watered and increases in CO2 and H2O exchange have been demonstrated to follow the arrival of an electrical signal in the leaves. When a dye solution was applied to the roots its uptake and movement to the leaves was observed continuously by microscopy showing that the increase of gas exchange 12-15 min after irrigation could not be triggered by water ascent. By using severed aphid stylets it was shown that sieve tubes served as a pathway for electrical signal transmission. Furthermore, roots were water-stressed by addition of non-penetrating osmolyte to the root medium. The gas exchange of the leaves decreased clearly 6 min after application of polyethylene glycol 6000 (PEG, -0.5 MPa water potential). Comparative measurements of the sieve tube electrical potential indicated that PEG-induced water stress evoked a propagating depolarization of the potential. Other osmolytes (100 mM NaCl) caused similar results showing that the leaf responses are not related to any specific toxic effect of PEG. Thus, the results strongly support the view that electrical signaling plays an important role in root to shoot communication of water-stressed plants. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.