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Electrotactic Response of Zoospores of Seven Species of Phytophthora. K. L. Khew, Former Research Assistant, Department of Plant Pathology, University of California, Riverside 92502, Present address of senior author: School of Biological Sciences, Universiti Sains Malaysia, Penang, Malaysia; G. A. Zentmyer, Professor, Department of Plant Pathology, University of California, Riverside 92502. Phytopathology 64:500-507. Accepted for publication 25 October 1973. DOI: 10.1094/Phyto-64-500.

Electrotaxis of zoospores of seven species of Phytophthora (P. cactorum, P. capsici, P. cinnamomi, P. citrophthora, P. megasperma var. sojae, P. palmivora, and P. parasitica) was studied under standardized conditions. In deionized water, zoospores exhibited three basic types of electrotactic response to currents of 0-5 µA. Type-A (attraction) was observed at the anode at currents usually of <0.5 µA (<1.2 V/cm). Zoospores exhibited an active, oriented attraction to the anode, followed by encystment and germination. No positive orientation of the germ tubes to the electrode occurred. Type-B (repulsion) was observed at the anode at currents usually >0.5 µA. Attraction of zoospores at the boundary to the electrode was active and oriented as in the case of Type-A. Type-C (immoblization) occurred at the cathode at currents usually >0.5 µA. Responses of zoospores ranged from decreasing swimming velocity and rotation to cessation of motion and bursting. All three types of electrotactic responses followed the equipotential lines very closely when the current was flowing. No significant difference was observed in the basic patterns of electrotactic responses among different species of Phytophthora. The presence of various organic acids, sugars, metabolic inhibitors and surface-active agents in the zoospore suspension did not alter or prevent electrotaxis at chemical concentrations that did not affect motility of zoospores. Basic patterns of electrotaxis did not change among zoospores of various intermediate physiological stages before encystment. These results suggest that there might not be a direct relationship between electrotaxis and metabolic activity of the zoospores. Microelectrophoresis and staining behavior of both motile and encysted zoospores of Phytophthora indicated that they were negatively charged. Electrokinetic properties of zoospores suggest the presence of a preponderance of acidic surface groups. In nature, along with many complex factors of soil, tactic response of zoospores (both chemotaxis and electrotaxis) may serve as an important way to cause accumulation of zoospores on plant roots.